Scientific Brochure ELI Beamlines

Chapter 1 page 010 - 043






Introduction page 004 - 009

Facts and Figures page 168 - 176

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Plasma and high energy density physics

Applications in molecular, biomedical, and material science

Particle acceleration by laser

X-ray sources driven by ultrashort laser pulses

Lasers

Introduction

Exotic physics and theory

Facts and Figures

Introduction

Introduction

Extreme Light Infrastructure (ELI) is one of the 35 large-scale European projects identified on the ESFRI Roadmap published in 2006 and updated in 2008 (Roadmap of 44 large-scale research infrastructure projects of high priority identified by the European Strategic Forum on Research Infrastructures).

Officially launched in Paris on February 21st and 22nd 2008, the Preparatory Phase of ELI involves nearly 40 research and academic insti-tutions from 13 EU Member States. With an end scheduled in December 2010, this 36-month phase aims at bringing the project to the level of legal, organisational, financial and scientific maturity. The Preparatory Phase of ELI is orga-nized around nine work packages coordinated by 15 participating institutions. The Institute of Physics of the Academy of Sciences of the

Czech Republic is involved in this process as the coordinator for the strategic work package on lasers.

The main objectives of the ELI Project include the construction of a modern, cutting-edge la-ser facility and realization of many research and application projects involving interaction of light with matter at intensities that are 100 to 1000 times greater than the values achieved at pres-ent. ELI will be delivering ultrashort laser pulses lasting typically a few femtoseconds (10-15 fs) and its peak output will approximate 200 PW.

In September 2008, the Czech Republic sub-mitted its bid along with France, Hungary, Ro-mania and the United Kingdom for hosting ELI. The decision about location of ELI was made in October 2009 and the Czech Republic, Hungary and Romania jointly received from the ELI-PP Consortium the mandate to proceed towards the construction of ELI. Fully supported by the European Commission, the decision to imple-ment ELI as a distributed infrastructure will lead

to the construction by end 2015 of three facili-ties dedicated to three of the scientific pillars of the project (attosecond science, beamline gen-eration of secondary sources and laser-driven nuclear physics). The location of the fourth one – the ultra-high intensity pillar – will be decided in 2012.

The Extreme Light Infrastructure might be man-aged in accordance with the new governance model designed for the European Research Infrastructure Consortium (ERIC). The ELI-ERIC will involve the founder members, the Czech Republic, Hungary and Romania, as well as the major partners of the ELI-Preparatory Phase Project, Germany, United Kingdom, France and potentially others.

Introduction

7www.eli-beams.euIntroduction

Introduction

ELI: Extreme Light Infrastructure

The primary mission of the ELI Beamlines Fa-cility will consist of producing an entirely new generation of secondary sources driven by ul-tra-intense lasers. These secondary sources will produce pulses of radiation and particles such as flashes of X-rays and gamma-rays, bunches of accelerated electrons, protons and ions, etc., exploitable as unprecedented research tools in many research disciplines and in the develop-ment of new technologies. The research agenda using the ultrashort and ultra intense pulses de-livered by the ELI laser will focus on the follow-ing main activities which are described in detail in next chapters:

• X-ray sources driven by ultrashort laser pulses

• Particle acceleration by lasers

• Applications in molecular, biomedical, and material sciences

• Physics of plasmas, physics of high energy densities and of warm dense matter

• Exotic and frontier physics

The ultrashort and ultra-intense pulses of light and particles will find many applications in fun-damental research and in chemistry, biology, medical technologies, and development of new materials. In fundamental research for the first time it will be possible to study the phenomena connected to e.g. quantum electrodynamics, space-time dependent radiation fields, struc-ture of vacuum, and many others, in the lab. Furthermore, it will help understanding the as-trophysical phenomena involved in mechanisms of radiation emitted by pulsars, brown dwarfs, and giant planets for example. In the field of practical applications, the new laser-driven sources will enable significant improvements in screening techniques in medical diagnos-

tics, and the capability of ultrashort pulses to provide high-resolution snapshots of molecules will contribute to better understanding of com-plex diseases such as cancer, and will enable development of personalized medicaments and medical treatments. The capability of ultrashort light pulses to obtain snapshots that have been thus far inaccessible and “frozen in time” of physical materials and chemical molecules will help the development of new materials for elec-tronics, optoelectronics, nanotechnologies, and many others.

ELI will bring short-, medium- and long-term economic opportunities associated with the construction and particularly with the develop-ment of specific technologies required for R&D facilities such as high-repetition lasers, control systems, etc. This in turn will present a unique opportunity for laser, optical and vacuum indus-try. By participating in building of such major fa-cility, these industries will be able to extend their current portfolio of products that will find use in

ELI in theCzech Republic

Introduction

industry, in small- and medium-scale research laboratories, and will participate on the develop-ment of qualitatively new technologies.

The most positive impact will be seen on a long-term time horizon. Amongst the technologies and derivative applications that are expected to bring most long term economic impacts are the new techniques used in medical imaging and diagnostics, radiation therapy, tools for develop-ment and testing of new materials, new tech-niques of disposal of radioactive waste, new X-ray optics, etc. It is also expected that ELI will contribute to tackling selected problems related to thermonuclear fusion.

The ELI facility will also create an attractive plat-form for bringing up a new generation of PhD students, scientists and engineers. This will increase significantly the visibility of the host country of such cutting edge research facility and the Czech Republic will attract further in-vestments in advanced technologies.

The project is managed in the Czech Republic by a dedicated team of the Institute of Physics of the Academy of Sciences of the Czech Re-public and supported by the Ministry of Educa-tion, Youth and Sports, the Academy of Sciences and the Central Bohemia Region. The Consor-tium ELI-CZ, which already includes 14 Czech universities and research institutions, is another key player that demonstrates the strong support of the Czech scientific community to the project.

The site that will host the ELI Beamlines Facility is a 6-hectare lot located in the town of Dolní Břežany, in the Central Bohemia Region.Thislocation is accessible from downtown Prague by public transportation within less than half an hour. It is in close proximity to the nearly com-pleted Prague motorway ring, which directly connects to the European motorway network

and provides direct communication with the Prague International Airport.

Dolní Břežany is a developing and ambitiouscommunity showing a clear and coherent concept of local and regional development. Employees and visitors will enjoy a pleasant working environment with sufficient services, newly built accommodation capacities and op-portunities for relaxation, leisure and other out-side activities.

Situated within an area reserved by the town for the development of a technological park, the ELI site itself already offers enough room for potential upgrades of the facility, but also for spin-off companies or future industrial activities related to laser and optical science. This cluster approach will certainly foster the scientific and economic impact of the future facility.

Management and location

9www.eli-beams.euIntroduction

Introduction


Architectonic design was made by Hamiltons Architects.

Chapter 1Lasers generating repetition-rate ultrashort pulses and multi-petawatt peak powers

Author: Ing. Bedřich Rus, Ph.D.

Lasers generating repetition-rate ultrashort pulses and multi-petawatt peak powers

The ELI laser system will be a complex facil-ity that will be providing laser pulses for all research activities. The baseline design of the ELI laser system features multipurpose mission of this facility in fundamental and applied re-search, as well as versatility of its operation and of user (access) research. The laser pulses with various levels of energy will be delivered into the individual experimental areas by a versatile switching system.

The ELI laser, as designed by the ELI-Prepara-tory Phase consortium, is a complex instrument consisting of three main sections (pillars), as shown in Figure 2:

a) Attosecond facility

b) Beamlines facility

c) High-intensity facility

These facilities will be optimized to provide spe-cific combination of laser pulse energy, repeti-tion rate, and peak and/or average power. Each of these facilities represents a major advance-ment compared to the currently available laser facilities in the world; implementation of each of these facilities has also its own technology challenge. Following recommendation of the ELI-Preparatory Phase consortium, implemen-

tation of the attosecond and beamline facilities will be a priority of the project with delivery date 2015; simultaneously, vigorous technology de-velopment shall be carried out in international collaboration with the goal to prototype and test critical components for the high-intensity facility before 2015. The project of ELI facility in the Czech Republic reflects all these recom-mendations of the international ELI-Preparatory

Introduction

Figure 1: Relationship between the individual Research Activities of this project. The laser systems that are subject of the Re-search Activity 1 will, as a backbone instrument of the ELI facility, deliver high-intensity pulses for the Research Activities 2 to 6 and will enable their realization.

13www.eli-beams.euLasers generating repetition-rate ultrashort pulses and multi-petawatt peak powers

Chapter 1


Phase consortium.

The attosecond facility will be largely based on the Petawatt Field Synthesizer (PFS, description see below) technique optimized for delivering ultrashort pulses with duration of ~5 fs and operating at a repetition rate of typically kHz. This facility will be composed from a single (or

several) petawatt-class systems, and will have a specific mission in generation of coherent XUV and X-ray pulses in the attosecond time domain for fundamental research and applications.

The ELI Beamlines Facility will equally exploit the PFS technology in the front end up to a few

100 mJ but will use other amplification tech-niques (especially cryogenic multislab amplifi-ers) to provide ultrashort petawatt-class pulses with up to 50 J of energy, at a repetition rate of up to 10 Hz. As emphasized by the ELI-PP con-sortium, ideally all the beamlines should run at the 10 Hz repetition rate, in order to enable ELI to become, amongst other, a highly competitive source of accelerated electrons or protons for applications.

The high-intensity facility is designed to deliver a sum peak power of about 200 PW (0.2 EW), with nominal repetition rate 0.1 Hz or less. The primary goal of this facility will be to provide pulses for fundamental research in frontier (ex-otic) physics, using focused intensities of ≥1024

Wcm-2 which i s approximately 100x larger val-ue than those available at current cutting-edge laser facilities and entering the ultrarelativistic regime. The high-intensity facility will consist of laser blocks providing 10 to 30 PW. The design

Figure 2: Baseline scheme of the laser ELI laser facility, as resulted from works and analysis carried out within the ELI-Preparatory Phase project, and as approved in the ELI-Preparatory Phase Mid-Term Report (MTR) submitted to EC in July 2009.


options for laser systems, as well as techniques of coherent combination of their pulses, will be prototyped and tested within this project.

Another part of ELI considered since recently and not represented yet in the Mid-Term Report scheme as shown in Figure 2, is the photonu-clear facility using a 10-PW class laser system to explore nuclei with high energy photons and for studies and development of new techniques of gamma-ray generation. This facility will likely use a flashlamp-pumped laser with repetition rate about 0.1 Hz, and will use some technolo-gies common to those that will be implemented in the high-intensity facility.

The schematic layout of the laser system of Beamlines Facility is in Figure 3. The laser tech-nologies of the individual sections of this system are described below.

The laser front end will consist of the oscillator section (three optically synchronized identical

oscillators) delivering ~5 fs pulses with >300 nm equivalent bandwidth, pre-amplified by the PFS technique to approximately 10 mJ level. The front end will supply pulses into booster

Figure 3: Schematic layout of the ELI laser of the Czech Republic’s facility. Its primary mission will be to operate as the ELI Beamlines Facility. The two 10-PW laser blocks will serve to development of the high-intensity section and to testing of selected

technologies. Pulse compressors are not represented in this scheme.


Chapter 1


repetition-rate amplifiers based on the PFS technique and pumped by thin disk lasers cur-rently prototyped at the Max-Planck Institute for Quantum Optics (MPQ) in Garching and at the Max-Born Institute (MBI) in Berlin. The system will involve two types of the booster amplifiers: the first type running at 1 kHz and providing equivalent energy of 200 mJ in the compressed pulse, the second type running at 100 Hz and delivering equivalent energy of about 1 J in the compressed pulse. The first type will de-liver pulses directly to the experimental areas, whereas the second type will feed the beam-line power amplifiers and the 10-PW test laser blocks.

The beamline power amplifiers will be based on the OPCPA technique, driven by repetition rate diode pumped solid-state lasers (DPSSL) at a frequency of 10 Hz. The OPCPA technique is preferred as nominal solution due to its large bandwidth and a possibility to provide high out-

put energy at a significant repetition rate. The DPSSL pump systems for the beamline power amplifiers will be a critical component for suc-cessful delivery of this research activity. These pump systems will be based on a multi-slab cryogenic amplifiers and will represent today’s cutting-edge technology. Development and op-timization of these lasers and associated tech-nologies is to be accomplished, partially within this project. Given that 2015 is the commission-ing date of the facility.

The designed laser system will involve two types of repetition-rate beamline power am-plifiers, both operating at 10 Hz. Two identical beamlines of each type will be installed in the facility. The first type will deliver approximately 10 J of compressed energy, implying require-ment for ~100 J (at the fundamental wave-length 1 µm) pump pulses. The second type will provide about 50 J of compressed energy and will thus require ~500 J pump pulses. One 10

J and one 50 J beamline will be dedicated to the target area for electron and acceleration, and will be compressed to provide flexible pulse lengths between about 50 and 200 fs.

The fallback solution for the beamline power amplifiers will be Ti:Sapphire technology using as pump systems conventional Nd:YAG flash-lamp lasers (or Nd:GGG heat capacity flashlamp pumped lasers). This technology matured and is commercially available. If used here, the beam-lines will provide repetition rate of 0.1 Hz.

The designed ELI laser involves two blocks of the future high-intensity end. Each of these nominally 10-PW (potentially 20-30 PW) blocks will provide at their output compressed pulses with energy of 200-300 J and pulse duration of 20-30 fs, with repetition rate of approximately 0.1 Hz. Two amplification technology options, currently examined by ELI-PP partners, exist for these blocks: OPCPA architecture that will


be tested at the Rutherford Appleton Labora-tory (10 PW Vulcan project) and Ti:Sapphire architecture that will be tested at the Institut de Lumiere Extreme (Apollon 10 PW project). The pumping system in both cases will be flash-lamp pumped lasers (with the active material of Nd:glass, Nd:GGG, or Nd:YAG ceramics). The mission of the 2x10 PW laser section will be twofold: to develop and test technologies of multi-10-PW lasers such as coherent beam combination, and to deliver focused intensity of 1023 Wcm-2 or higher to perform experiments in the field of exotic physics with adaptable beam geometries as required for specific high-field experiments.

The architecture of the designed ELI facility al-lows straightforward upgrade of the high-inten-sity section by providing space for implemen-tation of up to 6 high-intensity laser units and compressors (see also Figure 4). This upgrade of specifications of the high-intensity section

will be possible operatively anytime both during implementation of the designed facility before 2015, or later.

The layout of the laser shown in Figure 3 is reflected in the structure of the budget of this project see Chapter 3 of ESOP. The budget of the laser consists of four parts:

a) Front end including the oscillator section and the three booster amplifiers;

b) Beamlines with output energy 10 J, includ-ing pulse compressors;

c) Beamlines with output energy 50 J, includ-ing pulse compressors;

d) High intensity 2x10 PW section, including pulse compressors.

Scheme of implementation of the laser in the designed building in the district of Dolní Břežany

is shown in Figure 4. The individual laser sys-tems are located in the ground floor of a mono-lithic structure ensuring high level of mechanical and thermal stability. The laser area is divided into four subsections, hosting the front end a booster amplifiers, 10-J repetition rate beam-lines, 50-J repetition rate beamlines, and high-intensity amplifiers. The laser front end (block of oscillators and preamplifiers) will be located in an enclosed compartment in the first sub-section. The layout features easy maintenance and economy of operation, and possibility for future upgrades. Overhead the laser systems in the first elevated floor are pump and support-ing systems (drivers of diode lasers, flashlamp pump lasers, capacitor banks, cryogenic sys-tems, etc.) The final compressors of the 10-PW laser blocks are located below the amplifiers in the underground floor which provides flexibility of delivery of the large-aperture high-intensity beams into the Exotic physics and Particle ac-celeration experimental areas.


Chapter 1


Figure 4: Designed implementation of the ELI Beamlines Facility in the Czech Republic, with indicated locations of the research activities exploiting the laser. The laser systems, including oscillator & front end, repetition rate beamlines and a test high intensity system are located in the ground floor, the driving and pumping systems (DPSSL drivers, pump lasers, capacitor banks, etc.) are placed in the 1st floor.


The general goal here is to implement the de-signed ELI laser facility with primary mission consisting in beamline applications and in test-ing key elements of the future high-intensity facility (with 200 PW projected peak power). The particular key research topics will consist in development, optimization and implementation of those individual components of the ELI laser which cannot be obtained commercially off the shelf or as a custom solution.

Delivery of the designed ELI facility represents a significant research and engineering task, with proportion of development and engineering works varying according to a particular part of the laser system. From the point of view of laser development and the scheduled delivery date 2015, the most challenging part of the laser are

the diode-pumped solid-state lasers which will be the backbone of all the projected repetition-rate beamlines. International cooperation in de-velopment of these systems will be of crucial importance for success of this project.

Key research topics which are necessary for de-livery of the designed ELI laser include:

1. Implementation and optimization of the femtosecond high-contrast front end

This part of the laser chain includes broadband oscillators (5 fs) and preamplifiers (about 10 mJ). The oscillators will largely employ com-mercially available technology; the preampli-fiers will use the Petawatt Field Synthesizer amplification technology currently prototyped at the Max Planck Institute in Garching (Germany).

2. Booster amplifiers: femtosecond OPCPA high rep-rate systems pumped by thin disk lasers

The booster amplifiers will provide peak power of typically 200 TW and will amplify the front end signal at a repetition rate of 100 Hz (two units providing input signal for the beamlines and the high-intensity block) and 1 kHz (one unit provid-ing pulses directly for applications in molecular, biomedical, and material sciences). The booster amplifiers will be based on the PFS amplifica-tion technique and will be pumped by thin disk lasers providing 1-2 ps long pulses. These thin disk pump lasers for both repetition rates will be based on the architecture currently developed at the Max Planck Institute in Garching and at the Max Born Institute in Berlin, constituting to-day’s cutting-edge technology; collaboration on prototyping and optimization of these lasers will be the subject of this ELI project.

Key research topics


Chapter 1


3. Repetition-rate multi-10-J beamlines pumped by slab DPSSL systems

The laser design involves two types of repeti-tion beamlines, providing respectively 10 J and 50 J in the compressed pulse, both at repeti-tion rate 10 Hz. The amplification technique will be OPCPA. The most critical element of the beamlines will be DPSSL pump systems provid-ing respectively 100 J and 500 J at the fun-damental harmonics (1.05 microns for Yb:YAG ceramic). These pump lasers have been con-ceptually designed at the Rutherford Appleton Laboratory (UK), based on the technology dem-onstrated by the Mercury laser at the Lawrence Livermore National Laboratory (60 J with 10 Hz rep-rate). The architecture uses a stack of vari-ably doped cryo-cooled Yb:YAG ceramic slabs, face-pumped by laser diode arrays. Extensive international collaboration, especially with the Rutherford Appleton Laboratory, on prototyping and optimization of these pump lasers will be one of the key points.

4. High-intensity beamlines (10 PW and multi-10-PW)

Two options for the ELI high-intensity pillar are suggested: OPCPA amplification and Ti:Sapphire amplification techniques, providing peak power of at least 10 PW per beam, i.e. 200-300 J in 15-20 fs pulses. Both options will use as the driver Nd:glass flashlamp-pumped lasers (or, alternatively. Nd:GGG heat capacity flashlamp pumped lasers). As the primary option, OPCPA technology will be used to build two nominally 10-PW units within this project, in collaboration with the Rutherford Appleton Laboratory which is currently establishing a 10 PW OPCPA-based laser facility. The secondary option for these 10-PW units are Ti:Sapphire amplifiers. The option that will ultimately be selected for the designed laser will be made by end 2012. Amongst the research topics that will be carried out in con-nection to these lasers will be beam diagnostics and diagnostics of PW and multi-10-PW pulses, which currently do not exist.

5. Control systems of the laser chains and of pulse distribution

Control systems of the lasers and of delivery of the laser pulses to the required experimental areas will be a major element of efficient opera-tion of the facility and of its flexibility. The laser operations will be controlled and supervised from control rooms that will feature efficient interfacing with control rooms of the individual experimental areas. Works on this research topic will involve mostly engineering develop-ment activities, involving system engineering due to complexity of the facility. The control sys-tems will be based on state of the art industrial standards in automation engineering, ensuring low susceptibility to faults and high operation reliability. Modular PLC (programmable logic controller) systems and distributed input-output peripheral systems located in the vicinity of as-sociated sensors and actuators will be used. Data acquisition and graphical user interface will be based e.g. on the LabVIEW platform.


6. Compression of multi-10-PW ultrashort pulses

A specific research and development task that has to be addressed in implementation of the ELI laser is compression of 10-PW and multi-10-PW laser pulses delivered by beams with apertures as large as 400 to 600 mm (or even larger). In order to compress broadband few hundred picosecond to nanosecond pulses down to 10-15 fs, close to their bandwidth limi-tation with low pedestal, the required bandwidth of the compressor has to be at least 150 nm. Compressors for such large beams and oper-ating with such broadband constitute a unique technological and engineering task. Due to the beam size, large compression gratings (~1 m diagonal) are required, while due to the band-width the gratings must have relatively low line density (~900 lines/mm). Compressor for 10-PW large aperture beams has been designed at the Rutherford Appleton Laboratory (UK), and a supplier of the required grating (with gold layer)

was identified. Another type 10-PW compres-sor will be developed at the Institut Lumiere Extreme (France), which is furthermore involved in development of new high damage threshold (~J/cm²) large bandwidth gratings using metal-dielectric layers. An international cooperation with the two above-mentioned institutions and other partners in Germany (University of Jena) will be an important element for successful de-livery of the compression units at the facility.

7. Techniques of coherent superposition of multi-100-J femtosecond pulses

Implementation of the full scale high-intensity ELI pillar (6 to 12 multi-10-PW beams) will re-quire to coherently superimpose the individual ultrashort pulses in the time domain in order to generate through their constructive interference the 200 PW design power. Another technique that has to be developed for full scale high-intensity ELI pillar is the accurate spatial super-

position of the individual beam foci. Although coherent temporal superposition of low-energy femtosecond pulses has been demonstrated, superposition of large beams with multi-10-PW power will require developing new techniques. The systems for phase control and temporal superposition of the compressed beams will exploit the existing well-proven technology of adaptive optics in astronomy, especially from technology of large push-pull/tip-tilt mirrors and actuators. The large adaptive optics will operate in closed loop with sensors of coherent com-bination of the pulses. A leading candidate for measurement of the temporal and spatial su-perposition of femtosecond beams is interfer-ence pattern detection.


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As mentioned above, the oscillator section is designed to involve three optically synchronized oscillators providing effectively 5 fs equivalent bandwidth. The oscillator signal will be pre-amplified at 1 kHz first in a Ti:Sapphire pre-amplifier (regenerative or multipass type) and subsequently in PFS preamplifiers. Oscillators providing ~300 nm bandwidth are nowadays available commercially (e.g. Octavius-85M 6 fs Ti:Sapphire oscillator, Menlo Systems Inc.); another alternative of a 5-fs oscillator is the ap-proach developed at MPQ and described below.

Directions of implementation at ELI

Implementation and optimization of the femtosecond high-contrast front end

Figure 5: Nominal scheme of the oscillator unit for the designed ELI facility. The output produced by a commercially available broadband oscillator (∆λ>300 nm) is pre-amplified to 10-mJ level and is used to produce mutually synchronized seed and pump input pulses of the booster amplifiers, as well as 1 kHz optical synchronization signal. The oscillator section will consist of three identical units optically synchronized.


The oscillator / front end solution currently de-veloped at MPQ where the required ~300 nm bandwidth is generated by frequency broaden-ing the output of commercial Ti:Sapphire oscil-lator in a photonic crystal fiber (PCF), is shown in Figures 6 and 7. The wavelength shifting, accomplished by soliton self-frequency shift in the fiber, serves to adjust the wavelength of the pulse that feeds the thin-disk DPSSL pumping laser chain (see Figure 7). The current system operates at frequency of 70 MHz, with energy between 10-20 nJ per pulse.

The Petawatt Field Synthesizer technique (PFS) is OPCPA-based system using high-repetition rate DPSSL pumping. The pump laser, which is based on the thin-disk architecture, produces pulses of the order of ~1ps, with typically kilo-hertz repetition rate. These pulses are used to pump KDP or DKDP crystals acting as OPCPA amplifiers. Due to the thin OPCPA amplifiers, the PFS technique possesses inherently large

bandwidth while the ps pumping allows achiev-ing high pump intensities with low sensitivity to damage issues, and reduces also contribution of the fluorescence in the output signal, thereby ensuring a high contrast.

PFS represents an advanced approach of the OPCPA technology. It benefits from availability of large diameter KDP or DKDP crystals and en-

sures high-fidelity re-compression of the ampli-fied pulse, as the higher order dispersion terms introduced by the stretcher and the compressor can be better matched for low-compression ra-tio (ps to fs, rather than ns to fs as for the “con-ventional” OPCPA). The PFS beamlines have to propagate in vacuum in order to avoid deterio-ration of the B-integral by picosecond pulses in the air).

Figure 6: Scheme of the oscillator unit applicable for ELI, developed at MPQ Garching. The pulses produced by Ti:Sapphire oscil-lator are spectrally broadened and wavelength shifted in a photonic-crystal fiber (PCF), upon which the signal is amplified in a commercial Yb:glass fiber amplifier (14 nJ pulses, 1 W at 70 MHz).


Chapter 1


Figure 7: Scheme of the ELI oscillator and front end section as being currently prototyped at MPQ Garching within the ELI-Preparatory Phase project. The current frequency of 10 Hz will be boosted to 1 kHz by using thin-disk systems that are currently developed (courtesy of Stefan Karsch, MPQ).


The PFS technique is due to its bandwidth a prime candidate both for the ELI booster ampli-fiers. The key issue for achieving the designed output energies is the pumping system operat-ing at 100-200 Hz or at 1 kHz, based on the thin-disk technology. Collaboration on develop-ment of these systems with MPQ Garching and MBI Berlin will constitute a major activity of this key research topic.

Principle of the thin-disk DPSSL systems is in Figure 9. One of the currently demonstrated

Figure 8: Generic scheme of the PFS technique using OPCPA with the amplified pulses stretched to typically 1-2 ps.

Booster amplifiers: femtosecond OPCPA high rep-rate sys-tems pumped by thin disk lasers


Chapter 1


systems (MPQ) provides 25 mJ in compressed pulses, at repetition rate 3 kHz (see Figure 10). The system operates in the regime of a regen-erative amplifier. This system exhibits very high stability both pulse-to-pulse (0.7% rms) and in long term operation.

An up-scaled system that will operate as mul-tipass amplifier (20 passes) is being presently implemented at MPQ. It will provide about 300

mJ in the compressed pulses at repetition rate 10 kHz and will be pumped by 3 kW of CW pump power. The pump region will be increased to 8.2 mm. This thin-disk system, after its completion and testing in 2010, will be directly relevant to the considered booster amplifiers of the de-signed ELI facility. By straightforward, low-risk scaling of this system, thin-disk delivering 2 J at repetition rate 1 kHz will be obtained (see Figure 11). Participation on development of this

system, in collaboration with MPQ Garching and MBI Berlin, will be undertaken as a major part of this key research topic.

Figure 9: Principle of the think disk repetition rate amplifier (courtesy of P.Nickles, MBI Berlin). The pumping diodes delivering ~1 ms pulses operate either at the required frequency (for 100-200 Hz systems) or in CW regime (for operation at 1 kHz or higher frequencies). Figure 10: Thin-disk based laser system prototype de-

veloped in MPQ Garching for pumping PFS amplification chains (courtesy of T. Metzger and G. Korn, MPQ Garch-ing). The system provides 25 mJ in compressed pulses (1.6 ps) at repetition rate 3 kHz and is CW pumped by laser diodes delivering 280 W (5.2 kW/cm²). The size of the output beam is 2.5 mm. The system exhibits excellent stability of operation and diffraction limited beam quality (M2=1.1).


DPSSL technology is a prime choice for the realization of the designed ELI repetition-rate beamlines. The beamline mission of ELI re-quires a laser source delivering energies of 10 J and 50 J on target in a sub-20 fs pulse at a repetition rate of up to 10 Hz. The final ampli-fier of this source, be it Ti:Sapphire or OPCPA, will require a pump laser delivering multi-100J pulses of few-ns duration at around 500 nm. These demands can be best met by a diode pumped solid state laser delivering around 500 J at the fundamental wavelength of ~1µm.

Different options for various aspects of the final

Figure 11: Scheme of thin-disk head providing 2 J at the frequency 1 kHz (left), with optical pump homogenizer for 15 kW pump power (right). The pumped region of the crystal is 18.6 mm. Courtesy of G. Korn and S. Karsch, MPQ Garching.

Repetition-rate multi-10-J beam-lines pumped by slab DPSSL systems


Chapter 1


amplifier, the most challenging part of such a la-ser, have been evaluated by Rutherford Appleton Laboratory and form a part of the ELI-Prepara-tory Phase Mid-Term Report, submitted in July 2009. Following these considerations a concept for an amplifier is described that is thought to present the best compromise between the dif-ferent constraints and requirements.

a) Gain Material

The requirements for the gain material are:

• Availability in large size to handle high en-ergy.

• Good thermo-optical and thermo mechani-cal properties: to handle high average pow-er.

• Sufficiently low saturation fluence: to facili-tate energy extraction.

• Long fluorescence lifetime: to minimize number of pump diodes required.

Materials that have been suggested are Nd:Glass, Yb:CaF

2 and ceramic Yb:YAG. Any

glass-based material is not suitable due to the very poor thermal properties. In the case of Nd:Glass, the short fluorescence lifetime is another disadvantage. Yb:CaF

2 suffers from a

very high saturation fluence which makes both pumping and efficient energy extraction while avoiding optical damage very difficult. Also, the growth of sufficiently large crystals with good optical quality remains yet to be demonstrated. This leaves ceramic Yb:YAG as the most prom-ising candidate. The required pump and extrac-tion fluence levels for efficient operation are still quite high at room temperature, but this can be overcome by cooling the medium to cryogenic temperatures around 150 K.

b) Amplifier geometry and cooling

The two following considerations apply with re-gards to amplifier geometry and cooling:

• For effective cooling, the amplifier medium needs to have a high surface-to-volume ratio

• To minimize wave front distortion caused be temperature gradients, heat flow must not be perpendicular to the beam propagation direction; ideally it should be in the same direction.

This leaves us with a face-cooled thin slab ge-ometry, of which there are currently two vari-ants: the active mirror concept and the gas-cooled slab-stack architecture as demonstrated in the Mercury laser system (see Figure 12).


The active mirror (or thin disk) concept has been very successful for CW and high repetition rate (repetition period << fluorescence lifetime) ap-plications where tremendous average power and efficiency levels have been achieved. In these applications both the amount of energy stored in the gain medium (and hence the gain) and the peak intensity are not very high. This is not the case for high energy amplifiers and this leads to two severe problems:

• Because of the high aspect ratio of the gain medium, the (parasitic) transverse gain is very much higher than the (useful) longi-tudinal gain. To avoid high amplified spon-taneous emission (ASE) losses, such an amplifier can only be operated at a very low gain, which in turn means that many am-plifier heads are required to reach required output pulse energy. This leads to a techni-cally very complex system and to increased optical losses. Due to the quasi three level

nature of Yb:YAG, efficient operation with low gain is also only possible at very low temperatures.

• Since the input and output pulse overlap with each other in the gain medium and at the surface, the optical intensity is in-creased up to a factor of four, with the ac-cording consequences for optical damage and nonlinear effects.

This leaves the Mercury-type architecture as the most promising one, since the aspect ratio of the gain medium can be chosen freely with-out compromising cooling. Also, the concept should be applicable to any temperature and one does not need to find a medium that is liq-uid in the chosen temperature range, as would be the case for the active mirror concept.

Figure 12: Mercury DPSSL repetition rate demonstrator built at the Lawrence Livermore National Laboratory (USA). The system provides 62 J at the repetition rate 10 Hz, in ~10 ns pulses, and provides beam quality of 4x of the diffraction limit.


Chapter 1


c) Amplifier Concept

The basic structure of the amplifier is outlined in Figure 13. The amplifier medium consists of a stack of quadratic ceramic Yb:YAG slabs. Cold helium gas is forced through the gaps between the slabs for cooling. The amplifier is end or face-pumped from both sides. Employing slabs with increasing doping level towards the cen-tre of the amplifier reduces the required overall thickness for a given maximum gain coefficient and also equalizes the heat load for all slabs.

Figure 13: Schematic of the design of DPSSL high-energy amplifiers studied at the Rutherford Appleton Laboratory (courtesy of J. Collier and K. Ertel, RAL).


d) Design parameters

Size-independent parameters

Pumping of the amplifier has been simulated with a spectrally and temporally resolved 1-D code. The parameters used are listed in Table 1.

The pump intensity was chosen as it seems to represent a value that can realistically be achieved and maintained over a certain depth of field with the brightness of currently avail-

able high power diodes. The same applies to the pump spectral width. The duration was chosen equal to the fluorescence lifetime which offers a reasonable compromise between fluorescence losses and required diode peak power. The op-erating temperature was chosen because spec-trally resolved absorption data is available for this value.

The main quantity calculated by the numeri-cal model is the pump efficiency which is ex-tractable fluence divided by pump fluence. The losses considered are fluorescence, quantum defect, un-absorbed pump and minimum popu-lation needed (due to quasi 3-level nature). For the given parameters quantum defect and fluo-rescence limit the efficiency to 58%. The other two loss mechanisms need to be balanced off against each other and an optimum is found for a certain optical depth (doping concentration times thickness) of the amplifier medium.

Using absorption data published in [2] and as-suming a lower laser level population of 0.64%, the results listed in Table 2 are obtained.

Despite the non-ideal pump spectrum and modest pump intensity, an efficiency quite close to the theoretical maximum is achieved. As-suming a stimulated emission cross section of 5.2x10-20 cm2, the resulting small-signal gain is 3.8.

Pump intensity (each side) 5 kW/cm2

λpump ~ 940 nm

∆λpump 5 nm FWHM

Pump duration 1 ms

Temperature 175K

Table 1: Pump modelling parameters for 1-kJ amplifier DPSSL multislab head.

Maximum pump efficiency 50.2%

Optimum pump wavelength 939 nm

Optimum optical depth (doping concentration times)

3.15 % cm

Table 2: Modelling results for 1-kJ amplifier DPSSL mul-tislab head.


Chapter 1


Aperture scaling

Nothing is said about transverse size and as-pect ratio of the gain medium in the previous section. The aspect ratio is governed by ASE management requirements and is independent of aperture size: if the aperture is increased, thickness needs to be increased by the same factor and the doping concentration lowered accordingly in order to maintain the optimum optical depth. This way the transverse gain-length product (small signal gain coefficient times aperture size, called transverse gain in the following) remains constant. A rule of thumb says the transverse gain should be kept below 3 for all possible paths. For an amplifier with a square cross section, the highest transverse gain is found along the diagonal across the outer face of the amplifier.

For a constant doping level and a maximum

transverse gain, the required aspect ratio is 0.74, meaning the gain medium (excluding cooling channels) has to be thicker than the beam width. The thickness can be reduced by using different doping levels for the individual slabs in the amplifier. Using ten slabs of equal thickness with an optimised doping profile, the aspect ratio can be increased to 1.4. If a higher maximum transverse gain turned out to be ac-ceptable, the aspect ratio would increase pro-portionally.

The required aperture size for a given output energy is mainly governed by the stored energy. The required aperture size for 1 kJ output is 14x14 cm2 and the required total thickness 10 cm. It is unlikely, however, that the total stored energy can be extracted without losses, hence it is either necessary to increase the amount of stored fluence by stronger pumping or to em-ploy more than one amplifier head in series.

Refinement of model

The parameters presented in the previous sec-tion present a point design that may not be ac-curate (due to incorrect spectroscopic data) and is probably not optimum either. It is also possible that with progress in diode laser technology, the parameters listed in Table 1 can be improved upon. Further numerical and especially experi-mental investigations are therefore required.


Risks and disadvantages

Currently the apparent risks and drawbacks of the presented concept are:

• Cooling system and concept has unknown

Technical complexity

Cost

Effect on transmitted wave front

• YAG has a high nonlinear refractive index, resulting in high B-integral in large aperture amplifiers (as amplifier thickness scales lin-early with aperture size). For a 1 kJ system however, a tentative calculation of the am-plification process yields a B-integral value of 0.7 for a 4 ns pulse, which is quite ac-ceptable.

As mentioned above, OPCPA architecture op-tion is considered as nominal for the two de-signed 10-PW blocks; the second solution is the architecture option based on Ti:Sapphire. The final decision on implementation of par-ticular technology will be made by end 2012 following prototyping works carried out at the Rutherford Appleton Laboratory (Vulcan 10 PW OPCPA project) and at the Institute Lumiere Ex-treme (Apollon 10 PW Ti:Sapphire project). The repetition rate will be at least 1 shot per min, ideally 0.1 Hz.

The design based on the 10 PW Vulcan OPCPA project proposes to use 6 beam lines for full-scale ELI to deliver a total of 200 PW to target. The architecture proposed by Rutherford Apple-

ton Laboratory is based on optical parametric chirped amplification. OPCPA has already been demonstrated at significant power levels. A number of different combinations of pulse du-ration and energy that could be envisioned to achieve this ranges from 10 fs and 300 J to 30 fs and 900 J. Shorter pulse duration has impli-cations for the seed source and the longer pulse durations conversely have implications for the pump laser.

The design to achieve these peak powers is based on the parameters in the Table 3 below.

The proposed seed generation scheme is shown in Figure 14. As can be seen for the seed gen-eration scheme, OPA is used to generate the pulses at 900 nm, and is seeded and pumped by the output of a single oscillator ensuring good optical synchronicity. The architecture for the power amplification stages is shown in Fig-ure 15.

High-intensity beam-lines (10 PW and multi-10-PW)


Chapter 1


Parameter Value Assumption

N [mm-1] 900 900 line / mm gratings available: avoid -1st order over BW

Size [mm, DxND] ~530 x ~530 Grating availability

Pump Energy [J] 1200 green

Ein [J] 480 Assumes 40% OPCPA efficiency

O/P Fluence [mJcm-1] 170 Long pulse LIDT > Short pulse LIDT

θin [deg] 16.9 ∆θ of 14° is ok for efficiency

Separation [mm] 3670 To separate i/p and o/p beams

∆λ out [nm] 123 145 nm FWHM 3rd order super Gaussian input. Grating band-width supports this

O/P Fluence [mJcm-1] 102 No difference between gold and di-electric in short LIDT

ηnet

[%] 60 Assumes 4 Gold Gratings @ 90 % efficient per grating (65%)

Eout [J] 290

t compressed [fs] 15 Fourier Transform Limit

Power [PW] ~19.2

Table 3: Key parameters of the design of 10-PW block for the ELI high-intensity (from ELI-PP Mid Term Report).


The choice of DKDP for the power amplifica-tion stages makes the requirement on the seed that it must be cantered at 900 nm. There are a number of schemes that can be used: direct seeding at 900 nm, although a suitable seed

source has not been found, or using nonlinear process. Two alternative schemes have been demonstrated of generating ultra-short puls-es at 910 nm for amplification in DKDP. One scheme uses a Cr:Fosterite laser operating at

1250 nm to seed an OPA pumped at 527 nm to generate 910 nm. This scheme has demon-strated sufficient bandwidth for 45 fs pulses. The alternative is to use a chirp compensated OPA scheme seeded at 710 nm and pumped at 400 nm. This scheme has the advantage that the seed and pump pulses can be derived from the same source a broad bandwidth Ti:Sapphire laser; consequently, both pulses are highly optically synchronized. This scheme has been demonstrated to have sufficient bandwidth to generate a <15 fs pulse. The chirp on the pump beam enables the broadband amplification by relaxing the phase matching requirement that the pump and signal beam must be phase matched for all wavelengths simultaneously, because it is chirped and the instantaneous phase matching at the pump wavelength is relatively narrow, the fluorescence in the idler beam will also be chirped. Since the chirp of the idler and the chirp of the fluorescence is in the same direction when the pulse is compressed, the fluorescence will be as well.

Figure 14: Schematic for the architecture to generate the seed pulses at the wavelength 900 nm, with >150 nm bandwidth.

Figure 15: Architecture of the 10-PW OPCPA chain designed at the Rutherford Appleton Laboratory, showing the power ampli-fication stages.


Chapter 1


We see this as an essentially solved problem with a suitable seed source now available. If greater bandwidth is required then white light generation in a PCF might be employed to gen-erate a large bandwidth.

a) Energy Requirement

To achieve the required energy onto target re-quires relatively large beams in the OPA stages; consequently, this limits the choice of nonlinear crystal. There are three potential candidates that can be grown in sufficient size: KDP, its deuter-ated isomorph DKDP and YCOB. Of the existing high peak power designs DKDP has been the most studied and pursued. Calculations have demonstrated that an OPA system based on a LBO preamplifier system and DKDP power am-plifier stages will have sufficient bandwidth to support 15 fs pulses.

As the size of the gain crystal is increased,

care has to be taken that the transverse gain is insufficient to lead to parasitic lasing or sig-nificant gain depletion due to ASE. This prob-lem is one that Ti:Sapphire is prone to. On the contrary, due to the phase matching conditions of the OPA process there is no transverse gain and consequently no depletion of the gain due to ASE. Parasitic OPO does become a problem for gain stages with large small signal gains, but this can be mitigated by using crystals that have wedged faces preventing reflections experienc-ing multiple passes during the gain duration of the pump pulse.

b) Pump Beam Requirement

The principal requirements of the pump beam are to have an approximately Top-Hat profile in the temporal and spatial domains within the non-linear crystal. Optical Parametric Amplification is relatively intolerant to the pump wavelength. This means that conventional Nd:glass lasers

(λ=1053 nm/ 527 nm) can be used to pump large aperture OPAs or with sufficient develop-ment Yb:X (λ=1030 nm/ 515 nm). High energy Nd:glass lasers with sufficient energy to achieve the required pump power have been developed and demonstrated, though with the current and projected technology they have repetition rates on the minute timescales. Nevertheless, recent development of diode pumped Yb:Host and even Nd:Host laser has made significant advances in laser pulse energy generation. At present they have been limited to the ~100J region, but it is expected with the added interest from other la-ser development programmes that systems will be designed to be capable of delivering 1kJ of energy at the fundamental on a repetition rate measured in seconds or a fraction thereof. The calculated phase matching angle for DKDP for these two pump wavelengths is shown in Figure 16. As can be seen, both pumping wavelengths enable a broad bandwidth to be achieved, with the phase matching decreasing more rapidly for the longer wavelengths. The gain centre for


the 515 nm pump also appears to be shifted slightly to shorter wavelength with respect to the 527 nm.

c) Contrast Limitations

On the nanosecond timescale the contrast is limited by the amount of parasitic parametric fluorescence (PPF) that is generated in the am-plifier stages. This will be generated for the du-ration of the pump pulse within the OPA stages. PPF can be reduced by ensuring that as much gain is achieved for shorter stretched pulse du-rations and injecting a clean seed pulse into the power amplification stages. The amount of PPF that might impinge onto target can be estimated by:

The impact of temporal modulations on the pump pulse has recently come to the attention

of a number of authors. Temporal modulations of the pump beam impinge onto the spectral amplitude of the seed pulse being amplified. The cause of the temporal modulations has been attributed to ASE within the pump laser

leading to a coherent noise modulation of the laser output. On compression this spectral modulation will lead to a plateau on the pico-second timescale that has a contrast ratio of the order of 104 to 106 depending on the pump

Figure 16: The calculated phase matching angle for non-collinear OPA pumping at 515 nm (solid) and 527 nm (dashed). From ELI-PP Mid-Term Report.


Chapter 1


laser properties. The temporal modulations of the pump beam can be appropriately reduced if the OPA amplifiers are run in saturation, which is likely at the expected energies of the system. Operating the OPA stages in satura-tion will also reduce the impact of any modu-lations on the spectral amplitude modulations.

Appropriately designed control systems are one of the key factors in reliable operation of the whole facility and in providing effective environ-ment for its mission in research and develop-ment. Therefore the control systems will be one of the first systems that will be engineered since the beginning of this project, followed by design and engineering of particular components of the ELI laser.

The systems will be based on robust industrial controllers using modular PLC systems and dis-tributed input-output peripheral systems (most likely based on PLC Siemens industrial stan-dard); similar systems are now used in cutting-edge automotive and aerospace applications. The baseline scheme of the control system is displayed in Figure 17. The core of the system consists of an array of PLCs (programmable logic controllers). The master PLC and the slave PLCs are linked by a robust, interference-re-sistant, system bus (e.g. Profibus). Each PLC is located near a specific technological unit/node (e.g. amplifier head, pulse compressor, cryo-genic unit, vacuum chamber, etc.) that has to be individually controlled. The PLC units provide hardware warning signals, which ensures maxi-mum robustness and operation reliability.

The PLC units will be equipped with HMI (hu-man-machine interface, e.g. touchscreens, keyboards) for local manual control, and will

also make use of e.g. Labview for data visual-ization. The local manual control (local peripher-ies) will be exploited especially for the start-up period, system testing, and maintenance pur-poses. The local peripheries are linked to the local PLC by means of standard I/O links or standard interfaces. The sensors and actuators (e.g. beam sensors, valve coils, position sen-sors, limit switches) of the monitored and/or controlled components are linked to the local PLC via distributed I/O units and local bus. This concept ensures robustness of the system and resistance to interferences, minimizes the need of cabling, and allows for expansion of the sys-tem. All important data (configuration, working stages, faults, accesses, etc.) are stored in the main data storage unit.

The whole control system will be entirely sep-arated from other local control subsystems, LANs, personal computers, etc., and will have a dedicated UPS power backup. This will ensure

Control systems of the laser chains and of pulse distribution


maximum operational reliability of the control system. In order to facilitate interaction with the control of particular experiments, privileged users will be able to enter the system through a sole external access point. This exclusive ac-cess point/gate using dedicated HW and SW tools will protect the control system against ex-ternal environment.

Figure 18 shows the layout of the beam deliv-ery architecture. Two beam/pulse switchyards will be located along the ground floor, while one switchyard system to distribute additionally large-energy pulses will be located in the under-ground experimental floor. This pulse delivery is one the facility systems that will be remotely controlled and fully automated, and will inte-grate fundamental beam diagnostic systems.

Figure 17: Baseline concept of a system for the control of the laser chains and of pulse distribution into the experimental areas. It is based on robust, state-of-the-art, modular PLC units and distributed input-output peripheral systems ensuring transmission of individual signals to the PLC via interference-resistant serial bus. The system is expandable; selected components can be commanded remotely by e.g. TCP/IP communication.


Chapter 1


The limitation on the minimum stretched pulse duration is governed by the pump laser. The limit is set by the B-integral of the pump pulse as it is amplified; consequently for pump pulse energies of the low kilojoules the stretched pulse will need to be ~3 ns long. The size of the bandwidth requires that the stretcher is all reflective to prevent chromatic aberration. For the proposed bandwidth the line density of the gratings and the beam polarization also play an important role. Figure 19 shows the calcu-lated diffraction efficiency for different grating possibilities. As can be seen the 900 lines/mm gratings demonstrate a broader bandwidth than the 1100 lines/mm option and the Littrow out-of-plane design has the broadest and flattest diffraction efficiency predicting a bandwidth of

Figure 18: Designed beam delivery implementation of the ELI facility (preliminary version). Appearing in red are two beam switch-yards along the laser areas in the ground floor, and one L-shape beam switchyard in the underground floor.

Compression of multi-10-PW ultra-short pulses


160 nm with diffraction efficiency greater than 95%.

Different designs for achromatic stretcher sys-tems have been demonstrated that they have the possibility of being extended to incorporate the full bandwidth of the proposed system. It is expected that a cavity design will be needed to

generate the required stretch.

The energy that it will be possible to deliver to target is limited by the gratings in the com-pressor. The current proposal is based upon gold gratings; however, it is envisioned that the development of the MLD gratings might have achieved sufficient maturity to provide a better

solution. Although in the short pulse duration laser induced damage threshold gold and MLD gratings demonstrate similar damage threshold levels. The design uses gratings that are 530 mm x 530 mm. A manufacturer capable of pro-ducing gratings of this dimension and at the re-quired line density has been identified.

Figure 19: Calculated diffraction efficiency for three different grating options: 900 lines/mm at out-of plane Littrow angle (left), 1100 lines/mm in plane non-Littrow (centre), and 900 lines/mm in plane non-Littrow angle (right).


Chapter 1


However we envisage that larger gratings will become available in the near future. With a size of 920 x 530 mm this should allow an approxi-mately 70% increase in the amount of energy that the compressor is capable of withstanding.

If a MLD or hybrid MLD/gold grating solution can be found then the efficiency of the throughput of the compressor could be increased as well. If we assume 98% diffraction efficiency for the MLD gratings then the throughput will increase

to 92%. Were these two improvements in grat-ing technology to be made then this would in-crease the potential output power to 50 PW.

Figure 20: Visualization of the 10-PW compressor design that will be built at as a part of the 10 PW Vulcan project at the Rutherford Appleton Laboratory. The compressor is designed for 900 lines/mm gratings and for out-of diffraction plane Littrow configuration (courtesy of J.Collier, RAL).


To attain focused intensities exceeding signifi-cantly 1023 Wcm-2 it will be typically required to superimpose several laser beams. Since the duration of the laser pulses is several femto-seconds, temporal superposition of the pulses delivered to the focus will have to be better than one micron in order to achieve their construc-tive interference. The goal of this key research topic will be to develop and demonstrate tech-niques for accurate on-line measurement of the mutual phase between several focused femto-second laser pulses, and to demonstrate tech-niques of active on-line phase control of 10-PW and multi-10-PW beams. The developed diag-nostics will make possible accurate spatial and

temporal characterization of the focus of pro-duced by several multi-10-PW laser beams.

The leading candidate for the detection of su-perposition of femtosecond pulses in both tem-poral and spatial domain are interferometric techniques. A specific challenge of develop-ment in the context of the proposed ELI facility is the need of a technique that will allow ac-curate phase detection of non-collinear beam wavefronts delivered at low-repetition rate (0.1 Hz) at a precisely defined target plane. The su-perposition detector will likely use low-energy high repetition rate (100 Hz to 1 kHz) track la-sers propagating along the optical path of the 10-PW beams. The output from the superposi-tion detector will provide signal to the adaptive optics (push-pull/tip-tilt mirrors) that will form the closed loop and will provide the required phase corrections. The developed techniques of coherent superposition of multi-100-J fem-tosecond pulses will also be potentially used for

Techniques of coher-ent superposition of multi-100-J femto-second pulses

controlling the temporal and spatial field distri-bution in the focus.


Chapter 1


Chapter 2X-ray sources driven by ultrashort laser pulses

Authors: Ing. Tomáš Mocek, Ph.D. / Ing. Michaela Kozlová


One of the main goals within the ELI scien-tific community is to produce ultrashort X-ray beamlines, both coherent as well and non-co-herent, which can open an entirely new field of research in multidisciplinary fields. In fact, the discovery of X-rays by W.C. Roentgen in 1895 [1] has been one of the most important driving forces to push forward understanding and knowledge in numerous scientific areas. Applications range from structure analysis in solid-state, atomic physics and molecular chemistry via imaging applications in medicine and the life sciences to the discovery of the basic building blocks of life, in particular the DNA, and more generally the structure of proteins and other macromo-lecules. Having access to the spatial resolution of molecular structure and electron orbitals is only one side of the coin to be explored by X-rays. It took about one century to flip the coin to the other side showing the temporal resolu-tion of the atomic and molecular motion, mak-ing it possible to monitor the dynamics of mol-ecules and electrons on their natural time scale,

which is now in the attosecond range. The early X-ray generation devices and techniques such as X-ray tubes, electrical discharges or the first synchrotron sources have not been able to deliver X-ray pulses that had a duration of less than several nanoseconds and thus could not be used to gather both types of information, spatial and temporal, simultaneously. An insight into the temporal dynamics of quantum systems was first gained by using probes of much larger wavelength than that of X-rays: namely infrared (IR), visible (VIS) and ultraviolet (UV) laser pulses. Today, the fundamental limit of ultrashort laser pulses is one single optical cycle, lasting about 1 fs in the UV to several femtoseconds in the IR spectral region [2-3]. Having at hand the possi-bility of monitoring molecular and electronic mo-tion, unfortunately it is not possible to use these lasers to directly image molecular structure at atomic resolution owing to the large wavelength of the laser photons. The principal solution to this problem is to combine X-rays and lasers to take advantage of both the short wavelength

and the temporal coherence properties to cre-ate ultrashort pulses of X-ray radiation.

Ultrashort, high-intensity lasers can produce very bright bursts of X-rays through the pro-duction of very short lived, high temperature laser plasma. Such X-ray bursts may be used to study ultrafast structural dynamics in solids and complex molecules like proteins. In fact, intense attosecond X-ray pulses will make it possible to the movement of electrons in an atom or molecule as it undergoes a quantum or chemical transition. In particular, coherent X-rays present a very important secondary source perfectly suitable for the nano-scale metrology because of its very short wavelength combined with cohe rence. X-ray interferometry with X-ray lasers can be an excellent method to measure the surface properties with nm-scale resolu-tion, thanks to the fact that X-ray lasers are extremelly monochromatic sources. Possible applications thus include X-ray holography, co-

Introduction

47www.eli-beams.euX-ray sources driven by ultrashort laser pulses

Chapter 2


herent imaging or measurement of fine struc-tures on any solid surface. It may be also used for inspection of defects in lithography masks having thus great impact on the industry. The high-energy/high-peak power lasers developed at the ELI Beamlines Facility will be able to gen-erate complementary X-ray sources such as X-ray lasers, high-order harmonics, X-ray free-electron-laser (XFEL), K-alpha source, betatron radiation, with extreme parameters far beyond current limits. From the user´s point of view, high repetition rate and high average power will be particularly important.

The main goal of this research activity is to pro-vide intense, brilliant, ultra-short X-ray beams for multidisciplinary applications. Since there is no “ideal” photon source for all currently imaginable applications, complementary X-ray sources will be developed and optimized. These include new injection-seeded, plasma-based X-ray lasers operating in the water window,

XFEL, advanced K-alpha sources, betatron radiation, and ultra-high-order harmonic gen-eration in keV region. The output parameters of X-ray sources at ELI Beamlines Facility should be comparable and sometimes even better than the parameters of large-scale XFEL facili-ties planned world-wide, but on a much smaller scale. The particular advantages of these ad-vanced X-ray sources include: ultrashort pulse duration, beam collimation, spatial and temporal coherence, full synchronization with an ultrafast IR/VIS laser pulses for pump-probe investiga-tions, and extremelly high brightness.


Since the first demonstration of lasing action at optical frequencies in 1960 [4], there has been considerable theoretical and experimental effort aimed at the extension of lasing into the X-ray spectral region. To date, lasing at wavelengths between 3 nm and 50 nm has been observed in numerous ionic species [5] and X-ray lasers have the potential to extend current optical laser applications to much shorter space and time-scales, while probing much deeper into the core of matter. X-ray time-resolved imaging of bio-logical samples, nanolithography, or laboratory astrophysics are typical examples of applica-tions that are recently starting to be explored as X-ray lasers become closer to reality. The use of X-ray lasers in these novel applications is ob-viously limited by the parameters of the driver creating population inversion. For the first X-ray laser demonstration nearly 20 years ago, kilo-

joules of laser pump energy were required to generate a laser at ~20 nm [6]. However, year after year, the required pump energy has been dramatically reduced (nevertheless at the ex-pense of output pulse energy) thanks to the im-provement of plasma and X-ray laser dyna mics. To date, the most energetic collisional X-ray la-ser [7] has been demonstrated at the kJ-class laser facility PALS [8] in the Czech Republic. It operates in a quasi steady state regime where the driving sequence consists of a weak pre-pulse followed by a strong main pulse, both be-ing linearly focused on solid Zn target (Fig. 1). The double-pass zinc X-ray laser at 21.2 nm (58.5 eV) delivers up to 10 mJ (1015 photons), 150 ps pulses in a narrowly collimated beam with divergence of about 4×6 mrad2.

To reduce the pumping energy and increase the repetition rate, transient collisional excita-tion scheme was proposed and demonstrated [9]. In this scheme, a longer (ns) pulse, which

ge nerates plasma and the required closed-shell ionization balance, is followed by a much short-er (ps) pulse, which rapidly heats the performed plasma and generates a transient population inversion. In 2005, an extension of the transient pumping scheme was demonstrated using a grazing-incidence angle for pumping (GRIP), achieving lasing at 18.9 nm close to saturation

Current Status of X-ray Source Development and Applications

Figure 1: Typical geometry of quasi steady state X-ray laser using half cavity.

X-ray lasers


Chapter 2


[10]. Similarly to the original prepulse pumping mentioned above, a long pulse (200-600 ps) producing 1012 to 1013 Wcm-2 is first applied to the target, generating plasma with the required (Ni-like) ionization balance. Subsequently, a short pulse (1-5 ps) is injected into the plasma under a small angle, sampling a specific elec-tron density region where it instantaneously raises the electron temperature and efficiently creates population inversion (Fig. 2). Using this approach, strong laser emission at wavelengths down to 10.9 nm was demonstrated [11].

In 2004, an optical field ionization X-ray laser amplifier produced by femtosecond laser exci-tation of a krypton gas cell was seeded with the 25th harmonic of a Ti:sapphire laser to generate saturated amplification in the 32.8 nm laser line of Ni-like krypton [12]. This seeded X-ray laser provides about 1011 (0.8 µJ) photons per pulse in a narrowly collimated beam with divergence of as low as 1 mrad. Also the amplification of

19th harmonic of a Ti:Sapphire laser in Pd-like xenon laser at 41.8 nm was demonstrated [13]. By further elaboration of this approach, the in-jection-seeded X-ray amplifier using a solid tar-get (Ne-like Ti) was demonstrated at 32.6 nm in 2006 [14]. Using the same experimental setup, seeded X-ray lasers at wavelengths down to 13.2 nm [15] have been achieved so far, de-livering coherent X-ray beams with excellent parameters (aberration-free, Fourier-limited, polarized) but still with low output pulse energy on a sub-microjoule level only.

The use of extremely powerful lasers deve loped at the ELI Beamlines Facility as a driver for in-jection-seeded X-ray lasers presents an excel-lent opportunity to boost significantly the X-ray laser output up to the mJ level and even more, using the concept of “slab amplifier” consisting in increasing the active surface of the amplifier. This scheme is purely pump dependent, and so there is no principal limit for the output X-ray laser pulse energy. At ELI Beamlines Facility, seeding technique will be extended down to the “water window” (2.2 – 4.4 nm) through pump-

Figure 2: Generic scheme of the grazing incidence pumping.


ing energy scaling laws. In particular, energetic Ni-like Au X-ray laser at 3.6 nm operating at high repetition rate can finally become a real-ity, being an ideal source for X-ray imaging and holography of living cells (Fig. 3) as suggested long time ago [16] but never accomplished. As the seeding technique matures, the X-ray laser beamlines may actually overcome the perfor-mances of current and even projected XFEL facilities, i.e. ultra-high X-ray intensities above 1020 Wcm-2 can be available.

Regarding X-ray applications, coherent radia-tion is necessary for interferometry, holography, diffractive imaging or nanopatterning. For such applications free electron lasers or plasma X-ray lasers are the best sources. For example, using a 46.9 nm wavelength EUV radiation from a capillary discharge laser a series of experi-ments was performed [17]. An interferometric lithographic technique was implemented using a Lloyd’s mirror interferometer to pattern a cone

shaped, 58-nm FWHM diameter nano-dots over the area of 500 x 500 µm2 [18]. The scheme al-lows also to pattern lines, holes and oval shaped features in PMMA and HSQ photoresists. Holo-graphic imaging in Gabor’s in-line scheme was performed using 50-80 nm diameter carbon nanotubes CNTs as an object. After a numeri-cal reconstruction of the hologram the images

of CNTs were retrieved with a spatial resolution of 46 +/- 2 nm, comparable to the illumina-tion wavelength. Finally, the diffractive “lens-less” imaging with the spatial resolution of 70 nm was achieved by storing and processing a single diffraction pattern from the object using a phase-retrieval algorithm.

Apart from the above-mentioned applications, sufficiently strong X-ray lasers are the only sources allowing for dense plasma probing and interferometry. X-ray laser interferometry can be used for measurements in material science, metrology and especially dense plasma diag-nostic. Two types of X-ray laser interferometry have been applied in practice: amplitude divi-sion interferometry (Mach-Zehnder interferom-eter [19]) and wavefront division interferometry (Fresnel double-mirror [20] or Lloyd’s mirror [21]). Amplitude division interferometry requires high temporal coherence, while wavefront di-vision interferometry requires high transverse coherence. Injection-seeded X-ray lasers elab-

Figure 3: The use of X-ray lasers for 3-D imaging of living cells may become a reality at ELI Beamlines Facility [16].


Chapter 2


orated at ELI Beamlines Facility will possess both of these features. X-ray laser holography offers the potential to obtain high-resolution 3-D images of biological and other specimens. High-resolution images can be obtained thanks to the high coherence combined with the high brightness of X-ray lasers that enables a single shot record. Exposure times can be reduced to < 1 ns, which eliminates motion blur in bio-logical samples as the dynamic processes in biology have time scales of typically 1 msec. Note that synchrotron X-ray holography and un-dulator X-ray holography typically needs about 1000 second exposure times from many shots. The first demonstration of X-ray laser hologra-phy was carried out using a Gabor in-line ge-ometry and a hologram of a 3-D structure of a gold test pattern was recorded by using the Ne-like Se X-ray laser at 20.6 nm [22]. Since then, unfortunately, no big progress has been done due to the lack of sufficiently strong X-ray laser. One of our challenging missions will be to revive this topic.

X-ray laser microscopy offers the opportunity to observe biological specimens in an aqueous environment [23]. The main goal here is to have lasing at wavelengths near or inside the water window (2.3 - 4.5 nm) where the high contrast in absorption cross section between carbon in biological specimens and oxygen in water can be obtained. However, as current X-ray lasers cannot be routinely produced at wavelengths much shorter than ~10 nm, other wavelength ranges have been employed for X-ray micros-copy to date. The high brightness and short duration of X-ray lasers again allow eliminating the of problems associated with motion blur and radiation-induced chemical decomposition of any biological specimen. Researchers at LLNL demonstrated X-ray laser microscopy using a Fresnel zone plate lens and imaged a test pat-tern with a resolution of 75 nm with a Ni-like Ta X-ray laser at the wavelength of 4.5 nm in 1992 [24], and subsequently rat sperm nuclei were also observed [25]. In this proof-of-prin-ciple experiment the X-ray laser had an energy

of 10 µJ in 400 ps and structures with size of about 50 nm were resolved. Since then, few demonstrations of X-ray laser microscopy have been reported until now. One reason is that no saturated X-ray laser at a wavelength inside the water window has been achieved. Moreover, in this particular case the coherence character-istics of X-ray lasers are not so beneficial for microscopy because of the problem of fringes and/or speckle. To stimulate further progress in this field, X-ray laser saturation at a wavelength in the water window is required.

X-ray laser radiography is useful for measur-ing the plasma parameters. The short pulse duration of X-ray laser is of the same scale as the hydrodynamic timescale of laser-produced plasma. The high brightness of X-ray lasers al-lows them to be used as a backlighter for the im-aging bright sources such as high temperature plasmas. The very short wavelength enables the X-ray laser to probe large and near-solid den-


sity plasmas. Measurement of the early laser imprint and subsequent Rayleigh-Taylor insta-bility is of great interest in studying direct drive inertial confinement fusion [26].

An important method to measure densities and temperature of plasmas is the Thomson scatter-ing. X-ray laser Thomson scattering has a great potential for probing plasmas at much higher density than optical probing, even above the solid density [27]. A laser entering a plasma is scattered by electrons. The plasma temperature can be measured by observing the spectrum of scattered light and the density of plasma can be measured by observing the amount of scattered light. As the Thomson scattering cross-section is very small, it is difficult to gain a signal above the noise level caused by plasma emission. Be-cause of this fact, X-ray laser Thomson scatter-ing experiments have not succeeded in observ-ing the scattered spectrum so far. To improve the situation, X-ray lasers of higher brightness

and at a shorter wavelength are absolutely nec-essary.

Recent advances in laser wakefield accelera-tors have allowed the production of electron beams with energies ranging from tens of MeV to more than 1 GeV within a centimeter scale, and with pulse durations of several femtosec-onds. The enormous progress in improving the beam quality and stability makes them seri-ous candidates for driving the next generation of compact light sources. The versatile laser-based radiation sources range from infrared to X-ray energies. They attract a large user community because they are extremely reliable sources of short radiation pulses tunable up to keV photon energies, making them useful for structural analysis of matter. The development of Free-Electron Lasers has led to an enormous increase of brilliance and coherence. However, accelerator technology currently limits the pulse duration of synchrotron sources just under a

X-ray free electron lasers


Chapter 2


picosecond and demands large and expensive facilities due to the accelerating field gradients of conventional accelerators, which are restrict-ed to 20–100 MV/m by radiofrequency-cavity electrical breakdown. In contrast, a plasma, which is already fully broken down, can sustain electric fields that are 3–4 orders of magnitude higher, exceeding 100 GV/m.

The first combination of a laser-plasma wake-field accelerator (producing 55–75 MeV elec-tron bunches) with an undulator in order to gen-erate visible synchrotron radiation, has been successfully demonstrated [28]. This experi-ment, aimed to demonstrate the production of undulator synchrotron radiation by laser-accel-erated electrons, has been carried out using the high-intensity Ti:sapphire laser JETI at Jena. The set-up (see Fig. 4) consists of a laser wakefield accelerator as a source of relativistic electrons, driven by an 80 fs laser pulse with an intensity of 5×1018 W/cm2. The electrons traverse a 50

periods, 1-m-long undulator with a deflection parameter K = 0.6, and are then analyzed by a calibrated electron energy spectrometer. The measured electron spectrum showed a distinct maximum at 64 MeV with a width of 3.4 MeV (FWHM), and a total charge of 28 pC. The cor-responding undulator radiation spectrum was centered at 740 nm and contained 2.84 x 105 photons. Moreover the wavelength scaling with energy and the narrow-bandwidth spectra had been demonstrated.

XFEL generated by using wakefield electrons could provide orders of magnitude brighter X-ray radiation (1012 photons/pulse/0.1% BW) than conventional sources. In this scheme, the laser-accelerated electron beam is injected into permanent magnet undulators, shaped into microbunches separated by the resonance wavelength of the magnetic structure; then, as it propagates into the undulator, a burst of bright X-rays is produced from the coherent emission of all microbunches. Owing to the high electron

Figure 4: The principle of laser-plasma based XFEL [28].


beam peak current generated from laser-wake-field accelerators, saturation lengths for X-ray emission of a few meters can be obtained, of-fering the opportunity to develop very compact XFEL devices. This is in contrast to the exist-ing huge scale facilities world-wide, which are based on radiofrequency technology accelera-tion requiring hundred-meter-long undulators and kilometer-long accelerating lines.

Laser driven K-shell thermal and Kα sources are currently the most widely applied laser-plasma X-ray (defined as photons with energies that exceed 1 keV) sources. These sources are bright, short-lived, quasi-monochromatic and may be well synchronised to a laser. This ful-fils the basic requirements for radiography and X-ray scattering, thus the area that has seen widest application for these sources is dense plasma physics. These sources are often used in point projection and area backlighter con-figurations. Kα sources are monochromatic (∆λ/ λ<10-4) and available with photon ener-gies between 1.48 keV to 68.8 keV from alu-minium and gold targets respectively. The emis-sion is fast electron driven. These fast electrons are created during the laser-target interaction, and propagate into the target material and slow down rapidly through bremsstrahlung losses and

collisions. Collisions result in atomic inner-shell ionisation and subsequent de-excitation can re-sult in the emission of a Kα photon of specific and material dependent, wavelength. Brems-strahlung losses results in continuum emission and ultimately degrade the contrast between the monochromatic source and background. The resulting X-ray pulse is short, lasting a few picoseconds longer than the laser pulse [28]. Sub-20 ps temporal resolution is readily achieved. The conversion efficiency from laser energy to Kα energy is good at around 10−4 al-beit into a solid angle of 4π. In general, higher laser intensity yields brighter K-alpha sources as more laser energy couples into these non-thermal electrons. However, inner-shell ion-ization probability increases and then falls off as the energy of the fast electrons increases. Source optimisation is target material depen-dent and is possible by controlling laser inten-sity and prepulse characteristics.

K-alpha


Chapter 2


Non-coherent laser-plasma X-ray sources equipped with collecting optics offer relatively high fluence reaching 1 J/cm2 in a wide or nar-row spectral range depending on the optics. Ra-diation of such sources has been used for many applications related to X-ray imaging, material investigations, material processing and micro-machining. Non-coherent X-ray imaging can be performed using imaging mirrors, zone plates or combination of both types. The most interesting wavelength range is again the “water window” and few pilot experiments were performed with laser-plasma sources emitting radiation in this range. Concerning material investigations there were some attempts to perform L-shell fluo-rescence and Resonant Inelastic X-ray Scat-tering (RIXS) investigations using laser-plasma EUV source. The average intensity with existing sources is until now too low for such investiga-

tions to give comparable results with synchro-trons; however, this will completely change with the extremelly powerful and highly energetic ELI lasers. There have been measurements of gas density distribution in gas targets using X-ray backlighting method. Laser-plasma sources were also used for micro- and nanomachin-ing by photo-etching. The experiments were performed employing contact masks or by di-rect writing with a proper demagnifying optical system. Relatively high aspect ratio (about 10) of the structures created in PTFE and FEP was obtained. Higher values are in principle possible but they would require a special optical system giving a parallel beam with sufficient fluence. Laser-plasma EUV source was also used for modification of polymer surface and creation of self-organized micro- and nanostructures. Irradiation of polymer surface with EUV radia-tion offers new possibilities and modification of a very thin near surface layer.

Other X-ray sources


Current X-ray lasers are mainly based on elec-tron collisional pumping scheme. At the pro-posed ELI Beamlines Facility, in order to make the X-ray laser sources fully competitive with current and projected state-of-the-art XFEL, major effort will be devoted to improve the beam quality, shorten the pulse duration, and boost the output pulse energy. At this moment, the most promising way is to elaborate the concept of injection-seeded X-ray laser which ensures excellent beam quality in terms of spa-tial homogenity [30], aberration-free wavefront [31], high degree of coherence, polarization, and short pulse duration limited only by the la-ser transition bandwidth.

We will start by investigating the physical phe-nomena constituting a base is for understand-

ing and development of novel X-ray lasers with a special emphasis on the medium excitation and the consecutive generation, amplification and propagation of the X-ray radiation. There are two possibilities of the amplifier architec-ture: normal incidence (Fig. 5(a)), and grazing incidence pumping geometry (GRIP, Fig. 5(b)). In the normal incidence scheme, a weak long prepulse (ns, 100 mJ level) followed by a much shorter stronger pump pulse (ps, multi-J level) are sent perpendicularly onto a bulk solid tar-

get. In the GRIP scheme, only prepulse is sent at normal incidence to the target whilst the en-ergetic pump beam is injected under an angle. The latter case has an advantage of reduced pump beam energy. In both cases we will seed the X-ray laser plasma amplifier with a high-order harmonic beam to improve the output beam quality (divergence, coherence, photon number).

Directions of implementation at ELI Beamline Facility

X-ray lasers

Figure 5: Basic geometries for seeded X-ray lasers: (a) normal incidence, and (b) grazing incidence.


Chapter 2


With a pulse of sufficiently high energy avail-able at ELI Beamlines Facility, it will be possible to create multi-stage X-ray laser chains (Fig. 6) that will deliver beams with excellent quality of wavefront and energy on the multi-mJ level in sub-ps pulses. Considering the nominal pump laser parameters (50 J / 15 fs operating at 10 Hz), several X-ray laser beamlines at various wavelengths may be constructed, all of them being synchronized to each other. With appro-priate focusing, X-ray intensities on the order of 1020 Wcm-2 could be reached [12]. The high-energy, high-power ELI laser driver will allow to realize novel X-ray laser schemes in the “water window” region that are desired for investiga-tion of biological species in their natural envi-ronment with a hundreds time higher resolution than with any existing optical microscope.

The alternative schemes such as “recombi-nation scheme” and “inner-shell ionization scheme” will be also explored as they have in-

herent potential to overcome the wavelength-limits of collisionally pumped X-ray lasers. Achieving high gain in a fast recombining plasma is a very desirable in the pursuit of X-ray lasers. Compared to collisional X-ray laser schemes, where a very high degree of ioniza-tion is needed, recombination schemes require relatively low pumping power. This, combined with the high quantum efficiency achieved by using the transition to the ground state, makes the creation of a compact X-ray laser in “wa-ter window” feasible. However, rather stringent experimental conditions are required in order

to achieve recombination gain. X-ray lasers based on inner shell transitions in atoms and ions are also very attractive for the possibility of generating gain at wavelengths below 1 nm. Although inner shell transitions in ions may lead to shorter wavelengths, the accompanying plas-ma electrons can accelerate undesired process of quenching population inversion and gain due to fast Auger process. While the physics of re-combination X-ray lasers is based on a process of plasma relaxation which follows very rapid heating of cold matter, lasing in the inner shell transitions of ions should occur near the plasma

Figure 6: Principal scheme of the XRL beamline at ELI Beamline Facility.


peak temperature. The very short heating phase generates free electrons associated with a dis-tribution of inner-shell vacancies formed in ions. So, laser transitions can occur between highly populated upper levels and inner shell vacan-cies before equilibrium restoration. The idea of inner shell transition for lasing in sodium at 37 nm was first proposed about 40 years ago [32] but never realized. It is based on the fact that cross section for photo-ionization of atom or ion in deeper states can be larger than for more shallow states. More challenging experiments are expected for X-ray laser gain generations using several very innovative approaches to ob-tain lasing in inner shell transitions, which may be very attractive for wavelengths significantly below 1 nm. However, it will require pumping pulses of very high intensities and very short durations in the order of 100 as. With the ad-vent of the ELI project, such pulses are already on lasers “horizon”.

Regarding the implementation of this X-ray source at the ELI Beamlines Facility, a good starting place is to consider the provision from future XFEL sources which are supposed to produce exceptionally high quality X-ray beams. As a guide we can assume that these sources will deliver photons with 10 keV energy, 1012

photons per pulse with 100 fs duration at 100 Hz, with excellent transverse coherence, poor temporal coherence, and be highly collimated in sub-mm diameter beam at the target sta-tion. Though these are quite challenging source characteristics, it may be possible to compete, but excitement is likely found by providing a simultaneous mix of synchronized sources (photon Terahertz to gamma, electron, proton, and neutron) to an experiment. The features that distinguish Kα sources from XFEL and potentially make them unique are high diver-

gence and high photon energy (68.8 keV). To attract the widest science base it is important to develop Kα sources with high-average and high-peak brightness sources at approximately 10 keV and above. Ultimately, this demands a source repetition rate of 100 Hz.

The next step in Kα source development should be to optimize the yield, enhance the source to background contrast, and reduce the source size from ~10 µm down to ~2 µm. Optimisa-tion and contrast improvement are closely re-lated. This part of work will require comprehen-sive experimental and computation studies to assess optimum laser wavelength, and accept-able laser contrast, optimal laser pulse duration and shape. A predictive capability is needed to determine optimal laser characteristics for materials of different atomic number. Progress demands precise measurements using high-energy and high-performance laser systems.

K-alpha


Chapter 2


Contrast and photon flux may be improved by using polycapillary micro-lenses, although these devices temporally stretch the X-ray pulse. The need for high throughput target manufacture, target and debris handling in high repetition sources is well known and needs to be addressed. As a part of this we should at-tempt to exploit the coherent properties of the Kα source. Spatial resolution is a major incen-tive to reduce the effective source size to and below the few micron scale.


As an international large-scale research infra-structure, ELI Beamlines Facility will offer sec-ondary X-ray sources with excellent parameters and complementary characteristics allowing ex-perimental setup that is not feasible in any other existing laser laboratory. In practice, it means enough flexibility in manipulation and timing of several X-ray pulses. Taking the advantage of inherent synchronization of all ELI lasers which originate from one master oscillator, it will be possible to perform pump-probe experiments with multiple, ultra-intense X-ray beams such as X-ray lasers, high-order harmonics, and/or non-coherent X-rays (e.g. K-alpha). This is extremelly difficult with current XFEL facilities which provide intense but merely one single X-ray beam. Second, despite the typical pico-second time duration, laser-plasma based X-ray lasers are still unbeatable in terms of photon flux and thus are capable of probing near-solid density plasmas. Considering 50 J / 15 fs laser operating at 10 Hz, a typical user at ELI may re-quire 2-3 mutually synchronized energetic (e.g.

X-ray laser) and/or ultrashort (e.g. high-order harmonic) X-ray beams along with several high power, auxiliary NIR laser beamlines. This will make possible e.g. complex investigation of fu-sion relevant plasmas via X-ray laser Thomson scattering and X-ray interferometry at the same time, as well as various pump-probe configura-tions with multiple directional quantum beams.

The X-ray sources developed within this Re-search Programme should deliver both coherent and non-coherent radiation at much higher in-tensities than any other existing plasma source of that type. Concerning coherent sources the most promising are X-ray lasers seeded with high-order harmonics. There are however also some ideas for X-ray lasers based on inner-shell excitation. Two possibilities are considered: di-rect excitation of inner shells by ultra-high pow-er laser pulse or indirect excitation with a high power non-coherent laser-plasma X-ray source. Coherent radiation, however, is generally not

necessary for excitation. Instead of wavelength matching a sufficient power density is needed. Efficient focusing of non-coherent radiation should deliver the proper conditions. A flexible advanced target arrangement should be de-signed and constructed for the investigations of X-ray laser itself. It should allow to use targets of different parameters and experimental condi-tions for the source optimization. As a result of these investigations, coherent X-ray beams at different wavelengths and reliable parameters will be generated. These X-ray beams will be then manipulated (wavefront correction, spatial filtering, focusing) and delivered to the users.

Source Development and Advanced Source Use


Chapter 2


The possibility of using X-ray lasers in combi-nation with microscopy and holography for the study of cell biology is very attractive even at 10 nm. Of course, X-ray lasers will have a much broader application potential in biology when they become operational in the “water window” where cells in their natural environment (water, atmospheric pressure) can be observed with a high resolution X-ray microscope. Structures of biomolecules such as protein with atomic reso-lution can be also determined by X-ray diffrac-tion. The high brightness of X-ray lasers should produce enough scattering to record diffraction pattern of very small samples such as single particles of large macromolecular assemblies without the need for samples to be crystalline where large signals are obtained because of coherent addition of the electric field of the X-rays by Bragg reflection. Femtosecond pulses

are short enough to avoid image blurring due to the vibrational motion of nuclei, thus atomic structure can be obtained without averaging over any periodic atomic motions. X-ray lasers will be suitable for observing biological speci-mens in their natural wet environment. Though radiation damage may become a problem, the very short period (femtosecond) can be expect-ed to allow the collection of diffraction patterns before radiation damage that distorts or de-stroys the biomolecule structure occurs. A large degree of spatial coherence of an X-ray laser is just perfect to record a 3-D image by holo-graphic technique. Imaging at the atomic scale can be obtained by advanced X-ray microscopy making it possible to observe amorphous and disordered materials, including polymers, crys-tals with strains and defects, inorganic struc-tures such as nanotubes, and bio-molecules that are difficult to crystallize.

Femtochemistry will be one of the key applica-

tions of X-ray lasers at ELI Beamlines Facility. Femtochemistry by pump-probe spectroscopy aims at real-time observation of transient mo-lecular behaviours including the atomic posi-tions, bond lengths and angles during chemi-cal reactions. As chemical reactions occur at timescales of order of femtoseconds or pico-seconds and structural changes occur on an atomic scale, high spatial and temporal resolu-tion corresponding to these scales are required to observe molecular motion in real time. The advanced X-ray lasers should have sufficient spatial and temporal resolution for this purpose. Even for such a short pulse, the photon flux per pulse can be enough to allow spectroscopic observations with a single pulse. Short pulse X-ray lasers can provide an opportunity to ob-serve various nanoscale dynamics spectroscop-ically, such as viscoelastic flow, protein folding, crystalline phase transitions, and magnetization dynamics of nanostructures over a wide range of time scales.

X-ray lasers


Finally, some fundamental scientific experi-ments that have not been able to be carried out so far with other light sources such as synchro-tron radiation can be realized with X-ray lasers at ELI Beamlines Facility. They include study-ing of plasma, warm dense matter and atomic physics, e.g. determination of energies, ion-ization cross sections, lifetimes and transition rates. The high X-ray intensity will make it pos-sible to study nonlinear effects that lead to mul-tiphoton processes in the keV region. Thanks to the expected high repetition rate of the source, X-ray lasers may be utilized to create controlled structures in the nanometre range which is a big challenge for the semiconductor industry (nanolithography).

With future multi-petawatt lasers like ELI, ultra-high beam currents of about 100 kA in 10 fs can be expected, allowing a drastic reduction in the undulator length of free-electron lasers. However, the very extreme electron beam prop-erties required to produce an XFEL, or at least X-ray synchrotron light, are not available yet. A crucial parameter will be the charge contained in the 0.1% spectral bandwidth: 1 nC at 1 GeV energy will be required to produce an efficient XFEL. FELs operating in the ultraviolet or X-ray region rely on self-amplified spontaneous emission, which requires electron beams with extremely small emittance and long and highly engineered undulators. However, the self-amplified spontaneous emission process also depends on the current density of the electron

beam, which could be very high for wakefield accelerators. Therefore, with future reduction in energy spread, laser-plasma accelerators may be optimal compact drivers for XFEL.

Very recent experiments by Leemans et al. (LBL, U.S.A.), utilizing only 40-TW laser pulse of 38 fs duration and a gas density of 4.3×1018 cm-3, clearly showed that 1 GeV electron beams can be produced by means of capillary discharge. However, the measured charge of 30 pC is sig-nificantly below the goal of 1 nC. In order to fur-ther improve the resulting current, an alternative scheme has been proposed [34]. The number of bubble electrons can be increased if high density plasmas are used, because the feed-ing process is more efficient if a high amount of plasma electrons is available. An increased gas density requires shorter laser-pulses (sub-10-fs), such that the laser pulse length fits into one plasma period. In this case, the entire laser pulse energy can be used, while longer pulses

X-ray free electron-lasers


Chapter 2


lose energy during self-shortening for enter-ing the bubble regime. Therefore, in contrast to standard laser-acceleration experiments, a different driver laser, namely a 5 fs-Petawatt la-ser (like the Petawatt-field-synthesizer at MPQ, Germany), has to be used. PIC simulations show that such ultrashort lasers can capture more than 1 nC charge inside the bubble. The smaller plasma period leads to smaller bubbles, hence shorter bubble stems and thus higher currents (1 nC in 10 fs). Recent analytical estimates and simulations have shown that laser-plasma accelerator-based FELs can be operated with meter-scale undulators. The key parameter is the ultra-high electron peak current, which sig-nificantly reduces the gain length and increases the tolerance with respect to the energy spread and emittance.

Other non-coherent laser-plasma X-ray sources at present time utilize mainly nanosecond la-sers. In this case plasma is thermalized what means that its spectrum is defined by its tem-perature and atomic number of the excited ma-terial. Ultra-high intensity femtosecond pulses will generate X-rays with essentially different spectrum because of the non-thermal excita-tion. It will allow obtaining intense X-ray ra-diation with controllable spectrum. The other important parameter of X-ray radiation is time duration of the pulse. Because of non-thermal excitation the resulting X-ray pulses can be very short comparing to current EUV pulses cre-ated with nanosecond laser pulses. Assuming relatively high conversion efficiency, high power X-ray pulses can be obtained this way. Apart from that short duration of the pulse would al-low to perform measurements of high speed

processes requiring X-rays. The non-coherent X-ray source will be equipped with separate chambers with different target systems (solid, liquid, gas targets) to assure experiments with different spectral distributions. The chambers for solid and liquid targets must be equipped with a debris mitigation system. Effective utiliza-tion of the intense, non-coherent X-ray radiation will require novel and highly durable X-ray col-lecting optics. Every chamber should be nomi-nally equipped with such optics which can be, depending on application, either broad-band or wavelength-selective optics. Because the non-coherent X-rays will be emitted in a wide solid angle, each chamber should employ at least two collectors: one grazing incidence mirror for wide band collection and one multilayer mir-ror for narrow band collection. The collecting X-ray mirrors can be paraboloids to give paral-lel beams propagating in specified directions. In this way, the X-ray sources at ELI Beamlines Fa-cility will have beamlines for different purposes similarly to synchrotrons.

Other X-ray sources


The development of the short wavelengths sources for the scientific and technological use in the next 30 years is absolutely crucial. In particular, the 2007 report of US Department of Energy explicitly states that: “During the half-century since the laser was conceived, the use of coherent light has become an indispensable part of our world, with applications such as tele-communications and optical data storage play-ing a trillion-dollar role in the global economy. In today’s world, the development of tools and techniques that use coherent light at ever-shorter wavelengths – in the extreme ultravio-let and soft X-ray regions of the spectrum – is assuming increasing importance”. Considering the size and future needs of X-ray sources for research, academia and industry, it is not sur-

prising, that huge effort is being devoted to de-velop these sources of coherent X-ray radiation as well as methods for their application.

The main advantages of ultrashort and intense X-ray sources are threefold: first, the spatial resolution information to be extracted from a system is proportional to the wavelength of the probing electromagnetic field. Sufficiently strong X-ray sources will greatly help science and technology to move from the micrometer to the nanometer scale range and beyond. Co-herence and polarization of such sources can further improve the quality of the obtained infor-mations. Second, the intensity of X-ray sources to be elaborated at the ELI Beamlines Facility, will reach the highest intensities ever at these wavelengths. As a consequence, transforma-tions in the matter can be initiated on a nano-meter scale range in a controlled way, with tem-poral resolution on the order of femtoseconds. Last but not least, the spectral quality of these X-ray sources can reach a very high spectral

purity, below 10-5 relative bandwidth, making them suitable for very precise investigation of the dynamics of sample structure and spectral properties. The above aspects can specifically impact the following R&D areas with certain commercial potential:

1. Material science where nanotechnologies are quickly developing: magnetism, phase transitions, smart materials, liquids and disordered systems, cluster vibrations and reactions, phase transitions and cluster melting, warm dense matter generation, and imaging plasmas are waiting for the ultra-high spatial resolution to be achieved with the advanced X-ray sources at ELI.

2. Time-resolved studies aimed at bet-ter understanding of ionization dynamics of atoms, molecules, clusters and highly charged ions, multiple core hole forma-tion, nonlinear effects in the X-ray domain,

Potential for Applica-tions, Business and Technology Transfer


Chapter 2


Coulomb explosion of clusters, generation of plasmas at solid density, hydrodynamic response to X-ray pulses, non-equilibrium plasmas generation and the study of mate-rials under extreme conditions.

3. High-resolution atomic physics: excitation of highly charged ions, study of X-ray non-linear effects, resonant elastic scattering, plasma spectroscopic time-resolved diag-nosis, application of X-ray lasers in nuclear spectroscopy.

As mentioned earlier, the high peak current of laser–plasma electron beams could lead to compact XFEL facilities and underpin a wide range of applications. High dissemination to-wards multidisciplinary users is then foreseen in fundamental science (time-resolved X-ray dif-fraction, photoelectron spectroscopy, ultrafast studies of material damage, plasma physics, ra-diography of dense materials, X-ray microscopy

and X-ray imaging) but also in more societal fields; e.g. the implementation in existing hos-pitals of phase-contrast imaging techniques, developed at synchrotrons to provide high-reso-lution images with micrometer resolution, could enable significant advancement in clinical diag-nostics (the X-ray energy of a tabletop XFEL can be as large as required for medical diagnostics, i.e. above 20 keV).

Ultrashort light pulses are powerful tools for time-resolved studies of molecular and atomic dynamics. They arise in the visible and infrared range from femtosecond lasers, and at shorter wavelengths, in the ultraviolet and X-ray range, from synchrotron sources and free-electron lasers. The ultrashort duration of these radia-tion beams will provide unprecedented time-resolved measurements down to the motion of electrons on atomic scales, and a zooming onto the two fundamental molecular building blocks, the electron and the atom. It will enable

exposure of atoms and molecules to relativistic intensities before their disintegration. Coherent diffraction on single molecules will then become accessible, opening an entire new field of re-search.

Time-resolved absorption spectroscopy and Thomson scattering of high-density plasmas require penetrating radiation such as X-rays and an ultrafast time resolution to reveal the properties of the warm dense matter produced in a laser–plasma experiment. Time-dependent measurements of plasma temperature and density will provide a valuable contribution to the understanding of degeneracy and coupling, as well as long and short-range interactions be-tween charged particles in dense plasmas.

Finally, the simultaneous use of particles and radiation as probe or pump beams offers unique opportunities. As an example with societal is-


sues, the study of the ultrafast kinetics following matter excitation by high-energy particles is a major subject in radiation physics and in nucle-ar technology, with implications on nuclear re-actor lifetime. At present, the physical effects of intense particle energy deposition can only be accessed through modelling. There is a crucial need to look at vacancy dynamics that occur in the few-hundred-femtosecond timescale, by ultrafast X-ray or visible probing. Novel laser-based sources will provide the necessary tools.

Laser-based accelerators imply a large reduc-tion in infrastructure, particularly for shielding, and thus in size and cost. A purely laser-driven light source would inherently produce ultra-short-duration pulses and be perfect temporally synchronized with the driving laser. Time-re-solved experiments require at least two pulses, one to initiate the fast process to be observed and another to probe after a well-defined delay. The pump and probe would automatically be

synchronized with the electron bunch and thus the undulator radiation. However, there are no restrictions to laser pulses, and these may be converted into other forms, for example, elec-tron or proton pulses. Thus, a laser-based ac-celerator facility would be very flexible.

The advanced X-ray sources at ELI Beamlines Facility will be unique and complementary to the existing sources, delivering significantly higher photon flux and narrower spectral bandwidth for the experiments, helping to draw the details of the whole picture in a given scientific investiga-tion. Moreover, the development of laser based radiation sources, such as attosecond pulses, electrons, protons, neutrons and further ac-celerated particles will greatly benefit from the development of X-ray sources at ELI while it will be possible to have an in situ X-ray diagnostic of the laser irradiated sample and its subsequent dynamics. The goal will be to understand the mechanisms of the radiation production, to op-

timize the existing sources and even to develop completely new concepts.


Chapter 2


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[14] Y.Wang et al., Phys. Rev. Lett. 97, 123901 (2006)

[15] Y. Wang et al., Nature Photonics 2, 94 (2008)

[16] P. Jaeglé, La Recherche 184, 16 (1987)

[17] S. Heinbuch et al., Opt. Express 13, 4050 (2005)

[18] P.W. Wachulak et al., Opt. Express 15, 3465 (2007)

[19] L.B. DaSilva et al., Phys. Rev. Lett. 74, 3991 (1995)

[20] F. Albert et al., Opt. Commun. 142, 184 (1997)

[21] J.J. Rocca et al., Opt. Lett. 24, 420 (1999)

[22] J.E. Trebes et al., Science 238, 517 (1987)

[23] S. Suckewer et al., Science 247, 1553 (1990)

[24] L.B. DaSilva et al., Opt. Lett. 17, 754 (1992)

[25] L.B. DaSilva et al., Science 258, 269 (1992)

[26] D.H. Kalantar et al., Phys. Rev. Lett. 76, 3574 (1996)

[27] H.A. Baldis et al., Rev. Sci. Instrum. 73, 4223 (2002)

[28] H.P. Schlenvoigt et al., Nature Phys. 4, 130 (2008) and Refs. therein

[29] H.S. Park et al., Phys. Plasmas 13, 056309 (2006)

[30] F. Brunel et al., Phys. Rev. Lett. 59, 52 (1987)

[31] J.Ph. Goddet et al., Opt. Lett. 32, 1498 (2007)

[32] J.Ph. Goddet et al., Opt. Lett. 34, in press (2009)

[33] M.A. Duguay et al., Appl. Phys. Lett. 10, 350 (1967)

[34] F. Gruner et al., Appl. Phys. B 86, 431 (2007) and Refs. Therein

Chapter 3Particle acceleration by lasers

Authors: RNDr. Josef Krása, CSc. / Daniele Margarone, Ph.D.

Particle acceleration by lasers

Introduction

The ELI Beamlines Facility is supposed to be built as central laser facility with different ex-perimental stations that can be used and ac-cessed independently by various communities to best develop and use new particle and photon beams for scientific progress, but also to trans-fer the corresponding knowledge to industrial and societal applications. ELI will probably be the first facility providing beam time for pump-probe experiments using moderately relativistic ultra-short ion beams and synchronized high-intensity lasers, electron beams, X-ray sources or attosecond sources. What makes ELI addi-tionally unique is the ability to vary all these dif-ferent secondary sources independently as al-lowed by having independent individual driving laser beams and adapting them optimal to the requirement of the specific experiment.

The envisioned laser driven electron and proton/ion sources with huge energies (up to the GeV level), in combination with other high-energy

particle and radiation sources, require a spe-cific environment close to that of conventional accelerators. In contrast to the latter, which are planned for a dedicated energy range, the la-ser driven source is, according to the laser pa-rameters that will be available on ELI and the projected parameters of the produced source, a “broadband” facility, which has to fulfil at least two aspects: first, to explore the optimum mech-anisms to create efficiently the highest energy and flux accompanied by shaping the energy spectra with the available laser parameters, and second, to use them for the envisioned scientific program and different other applications.

In recent years the dramatic rise in attainable laser intensity has generated an even more dramatic emergence and new evolution of the fields of research associated with non-linear laser-matter interaction. Production and accel-eration of electrons up to 1 GeV over acceler-ating distances around 1 mm (100 meters for

conventional accelerators) and hadron acceler-ation to 100 MeV, are the clearly visible results of this evolution.

The spectacular increase in brightness and de-crease in pulse duration of particle beams will revolutionize the way of investigating matter. Fundamental events in biology, chemistry and solid-state physics can be recorded with ang-strom space resolution to capture electronic, atomic or molecular transient dynamics. Source compactness, broad spectral range and perfect synchronization of particle and radiation bursts are unique properties that could extend the breadth of applications. The high peak current of laser–plasma electron beams could lead to compact XFEL facilities, on a size affordable by small-scale laboratories. High dissemination to-wards multidisciplinary users is then foreseen in fundamental science, but also in other fields. Finally, time-resolved experiments, would sig-nificantly extend the field of investigation in the

71www.eli-beams.euParticle acceleration by lasers

Chapter 3


dynamics of matter, compared with currently available techniques using a visible pump and X-ray or visible probes.

It seems possible, at the first stage, to pump in-termediate laser amplifiers (50 J level per beam at target) with laser diode technology instead of flash lamp based lasers. This change opens the possibility to strongly increase the laser quality, shot-to-shot stability and repetition rate, which follows the path to provide particle and radia-tion sources with sufficient reproducibility to be interesting to users and for further source de-velopment.

The last five years witness a very significant progress on laser-plasma accelerators. More-over, significant improvements are expected on the state-of-the-art of particle accelerators. The sources parameters we are proposing for users are not yet demonstrated but they seem rea-

sonable for a 6-year horizon. It is also expected that a significant improvement on the source parameters related to the quality of the beam would be improved in future with multi-stage and all-optical injection techniques.

The end stage of ELI envisions a powerful la-ser, delivering a few kJ in 15 fs (~200 PW) with low repetition rate (minute based). Since all this stages will be constructed sequentially and the laser technology will evolve quickly, it is likely that the laser in 2015 will be further upgraded. Thus, it will be possible to get the 100 GeV elec-tron laser acceleration, which is one of the goals ELI intends to achieve. The emergence of many PW-class systems, in the next few years, will be used for demonstrating the 10 GeV electron laser acceleration; thus ELI and the rest of the world ultra intense laser labs can nicely cooper-ate to sharpen their overall skills and knowhows in a collaborative network in this development. On the other hand, this energy level could trig-

ger significant interest from the physics com-munity and could be a vehicle for getting their involvement.

In summary, the ELI Beamlines Facility will offer a versatile electron and proton/ion source emitting in an unprecedented en-ergy range and, moreover, it is worth noting that the facility with its unique variability in the parameter range given by the optional different laser beam parameters would not be interesting exclusively for basic science studies. The particle “beamlines” with the concomitant environment (diagnostics, ra-diation protection, etc.) will also allow ac-complishing multidisciplinary societal ap-plications, which are already in the focus of present day activities.


Current Status in Particle Acceleration Techniques

Electrons

Current CPA lasers range from the Terawatt to the Petawatt power and can produce laser pulses with durations below 20 fs. In this way a focused laser beam can produce intensities above 1019 W/cm2 with short durations. In laser plasma-electron accelerators, a longitudinal ac-celerating electric field is generated by the pon-deromotive force of the ultrashort and intense laser. This force, proportional to the gradient of the laser intensity, pushes the plasma electrons out of the laser beam path, separating them from the ions. This creates a travelling longi-tudinal electric field, in the wake of the laser beam, with a phase velocity close to the speed of light, most suitable for accelerating particles to relativistic energies. This electric field can reach amplitudes of several hundred gigavolts per meter. Electrons need to be injected into the wakefield with a sufficient initial energy so that

they can be trapped and accelerated.

Experimentally, two injection mechanisms have recently demonstrated the generation of high-quality quasi-monoenergetic electron beams. In the first mechanism, a single laser pulse is used to drive the wakefield to large enough ampli-tude such that electrons are injected into the rear of the first wake oscillation through trans-verse breaking of the plasma wave. The elec-trons then surf the wake and after outrunning the wave they form a monoenergetic electron bunch. This is known as the ‘bubble’ regime [1]. With current laser technology, electron beams in the 100 MeV range have been produced over millimeter distances [2,3], with relative energy spreads of the order of 5–10% and charge of hundreds of picocoulombs. A 1 GeV electron beam has been reported in a recent experiment, where the laser pulse was guided and evolved over a few centimeters in a capillary plasma discharge [4].

The second mechanism is based on the use of several laser pulses. In its simplest form, the scheme uses two counter-propagating ultra-short pulses with the same wavelength and po-larization. The first laser pulse, the ‘pump’ pulse, creates a wakefield, whereas the second laser, the ‘injection’ pulse, is only used for injecting electrons into this wakefield. The laser pulses collide in the plasma and their interference cre-ates an electromagnetic beatwave pattern that pre-accelerates some electrons. A fraction of these have enough energy to be trapped in the wakefield driven by the pump pulse and further accelerated to relativistic energies. Although this scheme is more complicated experimen-tally, it also offers more flexibility: experiments have shown that the electron beam energy can be tuned continuously from 10 to 250 MeV [5]. The electron beam has a quasi-monoenergetic distribution with energy spread in the 5–10% range, charges in the 10–100 pC range and its parameters are stable within 5–10%. This ap-proach is promising for the control of the elec-


Chapter 3


tron beam parameters and might enable tun-ing of both the charge and the energy spread. For instance, increasing the beam energy to the gigaelectronvolt range should decrease the relative energy spread to the 1% level.

The current state-of-the-art in laser-plasma electron accelerators is the demonstration of mono-energetic electron bunches in the sub-300 MeV range [2,6,7], the demonstration of 1 GeV mono-energetic acceleration using ex-ternal guiding [4] and the first demonstration of all-optical injection resulting in an electron beam with low emittance and energy dispersion [5].

Using present-day common laser facilities in the 100 TW range, low-emittance (Cowan 2004) protons are accelerated up to 20-30 MeV energies from thin (~20 µm) solid metal-lic target. The present record in terms of kinetic energy has been set by the Nova 1 PW laser facility at LLNL which has produced up to 56 MeV protons (Snavely 2000) and the Trident la-ser facility which has produced up to 59 MeV (Filippo 2008). These beams can be refocused and energy selected (Schwoerer 2006, Toncian 2006, Schollmeier 2008).

In standard conditions, protons contained in a surface layer (~20 Å thick) of hydrocarbon con-taminants are preferentially accelerated due to their highest q/m ratio. Heating of the target pri-or to the experiment eliminates these hydrogen

contaminants and allows acceleration of heavi-er ions (Hegelich 2006). By simply changing the substrate, laser-acceleration can therefore pro-duce a versatile, easy to modify source of ions. Conversion efficiency up to a few % has been measured from laser energy to proton energy integrated in the spectrum from 10% to 100 % of the cut-off energy.

Several mechanisms have been demonstrated to induce proton and ion acceleration, name-ly target sheath normal acceleration (TNSA) (Hatchett 2000), front-surface acceleration (Nemoto 2001), and shock acceleration (Silva 2004), the TNSA mechanism being the one, up to the presently accessible highest laser inten-sities that produces highest energy ions. Accel-eration takes then place at the target-vacuum interfaces where laser-accelerated relativistic electrons form a dense electron plasma sheath (field ~TV/m) that ionizes surface atoms and ac-celerates ions along the target normal direction.

Ions


High laminarity of the ion beam requires accel-eration from an initially cold surface (i.e. that has a sharp vacuum interface). Usually, this is ensured only at the target non-irradiated rear surface. Using very high temporal contrast laser pulses (i.e. contrast >1010), laminar accelera-tion is also possible from the target front that is then also unperturbed by the low-level la-ser pedestal (Ceccotti 2007). The increase of the laser temporal contrast also allows to use extremely thin targets (in the nm range) which have been shown to yield higher energy ions with increased efficiency (Ceccotti 2007, Antici 2007, Neely 2006), due to a combination of in-creased laser absorption and electron recircula-tion. Refocusing of the naturally divergent beam (due to the curvature of the electron sheath) is possible by using curved targets (Patel 2003) or optics, either conventional (Schollmeier 2008, Ter-Avetisyan 2008) or plasma-based (Toncian 2006).

Monochromaticity is made possible either by accelerating only a monolayer of elements (all ions then experience the same electric field and are accelerated to the same energies) depos-ited on a higher-Z material foil (Hegelich 2006), by using the specific electron distribution on the surface of isolated microdroplets (Ter-Avetisy-an 2006) or by selecting differentially focused spectral components (Fourkal 2002, Toncian 2006).

Acceleration has been tested using low-density plasmas (Willingale 2006, Antici 2009) and has been shown to be also efficient. Although it pro-duces beams of lower quality than from solids, it offers the advantage of being compatible with a high-repetition rate for the laser source.


Chapter 3


Scaling Laws for ELI laser intensity regime

Electrons

In order to estimate the electron beam param-eters that will be available for users of the ELI Beamlines Facility it is mandatory to specify the laser beam characteristics. The main laser pa-rameters are:

Beam Number up to 4 (electron sources can use up to 3 beams)

Energy up to 50 J @ 10 Hz

Wavelength 800 nm

Pulse Duration 15 to 150 fs

Contrast ns contrast (prepulse) and ps contrast (pulse pedestal)

The maximum intensity on pre-pulses or pulse

pedestal cannot disturb the plasma density; therefore we can set two contrast levels: on ns time-scale we need to avoid the gas ionization (intensity bellow 1012 W/cm2) while on ps-time scale we need at least to avoid bulk charge separation in the plasma (intensity bellow 1016 W/cm2).

We present three pulse energy levels possible at ELI at a repetition rate compatible with a source for users: 100 mJ – 1kHz, 1.5 J – 100 Hz, and 45 J – 10 Hz. We have reported the laser and electron source main parameters for the three simple regimes (see Table 1): blowout self guid-ed, blowout external guided and bubble.

At this point, there is no straightforward method to calculate electron beam properties, besides maximum energy and charge. Other important electron beam quantities, such as bunch energy dispersion, emittance and bunch length are very

sensitive to experimental parameters and can only be anticipated by fully explicit three-dimen-sional heavy particle-in-cell simulations.

This source specification should be tuned be-fore the construction phase since the state-of-the-art is in fast progress and new particle in-jection techniques may be included to improve the resulting electron beam parameters. Addi-tionally, scaling laws for laser-plasma electron accelerators resulted from these analyses can be used to design future electron accelerators [8].


Laser energy on target

100 mJ 1kHz

1.5 J 100 Hz

45 J 10 Hz

Blowout Self guiding τ [fs] 9.8 24.2 65.8

W0 [µm] 4.4 10.9 29.6ne[1018 cm-3] 23.2 3.83 0.63L [mm] 0.2 3.3 54.6∆ E[GeV] 0.1 0.62 4.59Q [nC] 0.13 0.31 1.04

External guiding τ [fs] 15.6 38.4 119.2W0 [µm] 7.0 17.3 53.7ne[1018 cm-3] 4.62 0.76 0.08L [mm] 1.8 26.4 793.1∆ E[GeV] 0.26 1.56 15.06Q [nC] 0.1 0.25 0.78

Bubble τ [fs] 6.6 12.2 26.8W0 [µm] 2.0 3.7 8.1ne[1018 cm-3] 57.3 24.92 8.74L [mm] 0.03 0.2 1.8∆ E[GeV] 0.03 0.18 1.5Q [nC] 0.55 1.54 5.71

Table 1: Electron sources for users – experimental parameters


Chapter 3


The “initial source” parameters available for ion beam acceleration will obviously be the same as already defined for electron acceleration (50 J per beam laser pulses, with a minimum la-ser pulse duration of 15 fs and a repetition rate of ~10 Hz) with an important constraint for ion generation, which is the prepulse laser contrast (about 1010).

A review of the maximum energy and proton number obtained in various experiments is shown in Figure 1. It is clearly shown that the maximum proton energy increases with the la-ser pulse duration. It has been demonstrated that the efficiency of the acceleration process also increases with the laser pulse duration and the laser irradiance as well (Fuchs 2006).

Ions

Figure 1: Review of cut-off maximum proton beam energy, as a function of laser intensity, as reported from published data. Ex-perimentally measured data are small dots, boxes and crosses corresponding to three pulse duration ranges are shown. Simula-tions performed at higher laser intensities planned for ELI are reported as big purple dots. Note that, as experimentally proven up to 1020 W.cm-2, the maximum proton energy for the extreme short pulses is ~ I, whereas for the longer pulses ~ I1/2.


ELI, in its various stages, will allow exploring new, efficient, ion acceleration regimes that have been observed in numerical simulations. As a result, and as shown in Fig. 1, several hundred MeV up to a few GeV protons could be achievable with the planned on-target in-tensities at ELI in its final stage (i.e. up to 1025 W/cm²). Supposing that the proton number would be similar to that obtained currently on existing facilities, and considering a high repeti-tion rate (e.g. 10 Hz), then we could obtain cur-rents up to a few tens of nA, comparable with a conventional accelerator.

Based on the present knowledge and known acceleration mechanisms, the source specifica-tions would be:

• 200MeVforprotons

• 50MeV/nucleonforheavyions

• >1012 ions/bunch

• <psdurationatthesource

• %∆E/E

• µmvirtualsourcesize

• fewdegreesangulardistribution

• kAcurrentatthesource

According to simulations, the predominant ion acceleration regime should be, when increas-ing the laser intensity, first (i) shock accelerated at the target front surface or in its interior for I>1021 W/cm2 (Silva 2004), followed by (ii) ra-diation pressure acceleration, so-called RPA, (Esirkepov 2004, Macchi 2005), when I≥1023 W/cm2, i.e. when the electromagnetic wave is directly converted into ion energy via the space-charge force related to the displacement of all electrons in a thin (nm scale) foil, allowing to reach GeV-scale energies. Multiple stage ac-celeration using stacked foils would offer the

additional prospect of further increase of the ion maximum energy (Kim 2009). At a similar inten-sity regime, other simulations have also shown that ions could be accelerated to energies higher than 10 GeV in the “bubble” regime of wakefield acceleration using near-critical density plasmas and mixed ions (Shen 2007). Such underdense plasma targets would have the benefit of allow-ing high repetition rate operation.

ELI Beamlines Facility will thus offer the pros-pect of producing and study a versatile ions source, at high repetition rate, while enhanc-ing simultaneously the high-energy end of the spectrum, the beam monochromaticity and the laser-to-ions conversion efficiency, all of which being crucial points for the development of ap-plications in various areas. The possibility to vary the pulse duration of the ELI laser beamlines of-fers also a way to create proton/ion bunches of different durations, offering a way to adapt the source to the various envisioned applications.


Chapter 3


Directions of Implementation at ELI

Electrons

A precise work plan is required to develop a laser-facility of accelerated electron beams in order to: study the different acceleration tech-nologies and choose between them the most adapted to ELI goals and performances; set the full metrology of the electron beams; detail the building constraints especially if using a multi-stage accelerator; consider the evolution of the accelerated particle beam considering ELI laser full performances; compare the results from nu-merical modelling with experiments to validate the codes and use them for improving the high-energy electron sources; etc. The evolution of laser plasma accelerators until 2015 (possible date for ELI activity) will develop in very differ-ent directions in order to improve or extend the parameters of the electron bunches.

Practical plasma densities for laser plasma ac-celeration with current laser technology range from 1017 cm-3 to 1019 cm-3. These densities correspond to plasmas obtained from low pres-sure gases (10–500 mbar). The traditional way to obtain an adequate gas target consists in the use of a supersonic nozzle. Another pos-sible target for laser-plasma accelerators are preformed plasma channels [9,10]. In this case the plasma is completely ionized and presents a radial parabolic density profile that can act as a graded index optical fiber. This external guid-ing can keep the laser beam focused for long distances increasing the energy gain of the ac-celerator.

It will be necessary to develop an adequate plasma target for high repetition rate (>10 Hz), short length (<1mm), high density (>1019 cm-3) to take advantage of the short pulse capability of ELI. Such a plasma target would allow to explore the “bubble regime” with the laser fo-

cused to intensities > 1020 W/cm2 and produc-ing high charge electron bunches (> 1 nC) with energies of about 1 GeV.

The work plan will be also aimed to improve the existing plasma sources for external guid-ing. The more important improvements are the reproducibility and uniformity of the plasma, the possibility of guiding laser beams with spot sizes focused to less than 50 microns (this re-quires a strong radial density gradient in the plasma density), the length of the plasma chan-nel, which needs to be increased to the 20 cm range in order to match the dephasing lengths available for ELI parameters.

It is mandatory to develop a new class of plasma sources of very low axial plasma density, strong radial density gradient and at least 1-meter length. This plasma source could be extended to 2-meter length in order to accommodate a


demonstration of a 15 GeV laser plasma collid-er. The development of meter long low-density plasma sources will be a crucial step to break the few (~4) GeV barrier possible for current plasma sources.

It will be also possible to develop a multi-stage acceleration setup. The acceleration in multiple stages is the key to high-energy acceleration techniques. The use of multi-stage acceleration combined with 50 J laser (as planned for ELI) allows four meter-long 15 GeV stages (exclud-ing laser and electron beam optics).

It will be important to develop efficient beam loading techniques. The use of external beam loading is probably the most effective direction. Two approaches can be followed: i) to maximize the bunch charge, or ii) to reduce the bunch en-ergy spread. The theoretical and experimental work on beam loading is just starting. The beam

loading techniques tested so far with success used longitudinal density steps or the collision of laser beam to promote localized injection of charge and therefore to reduce the energy spread.

Significant technical developments are also re-quired for the source development. Real-time laser beam metrology will be required to as-sure electron beam reproducibility and reduce the activation of the accelerator setup. Some of the electron beam diagnostics can be adapted from the conventional accelerators, others need further development and will require the use of short pulse lasers (for time resolved measure-ments of the electron bunches with 10 fs dura-tion). A hypothetical source design is reported below.

• Laser Target

Laser targets for electron accelerators are low

atomic number gases (H2 or He) or pre-formed

highly ionized plasmas made of these gases. These laser targets imply the introduction of significant amount of gas in the vacuum sys-tem. At ELI, these electron sources will operate for the first time at high repetition rate (10 Hz to 100 Hz). The amount of gas introduced in the vacuum system will increase significantly re-quiring: i) reduction of the gas release per shot, ii) increase of the pumping speed and optimiza-tion of the geometry of the vacuum chambers including gas traps (taking advantage of the directionality of gas jets), iii) use of differential pumping between sections of the vacuum sys-tem. The gas is introduced in the interaction point by means of: i) supersonic gas jets, ii) differential pumped gas cells, iii) other plasma sources. Supersonic gas jets introduce a much higher amount of gas in the system than gas cells or discharge based plasma sources.


Chapter 3


• Laser beam dumping / electron charge collection

In a laser-plasma electron accelerator, the great part of the laser beam energy will be absorbed by the plasma and transferred to the electron beam. However, for a practical high repetition rate accelerator, special care needs to be taken with the laser light transmitted by the plasma, especially if sensitive applications are ahead in the beam axis. One possibility to mitigate this problem consists in the inclusion of a short sec-tion of a high-Z gas (example Ar) and uses it as a divergent lens for the laser light (due to ionization induced defocusing). It is important to reduce the amount of electromagnetic noise resulting from the ultra-fast rise-time pulsed currents in the accelerator. This noise strongly affects the electronic systems in the laboratory. This problem can be well mitigated if a collima-tor/beam dump consisting in a cylinder with a hole on the axis collects all the higher diver-gence (lower energy) electron ejected from the

plasma. This collimator/beam dump will also concentrate most of the radiation in the target chamber reducing the activation of other ele-ments. These two elements need to be carefully designed in order to maximize their efficiency in such a way that the electron beam properties would not be affected.

• Electron beam optics / beam deflection

Current experiments are producing a significant amount of hard radiation due to betatron oscil-lations of the electron beam in the ion channel. This radiation may affect the electron beam ex-perimental setup. Therefore the electron beam should have the possibility to be magnetically deflected from its original axis as well as to be imaged from the laser ion channel exit to the target. Since the electron beams will be consid-ered as a source for users, we have to introduce some of the beamline concepts of conventional accelerators as well as diagnostics. The mag-

netic field used to deflect the beams out of the original axis should have the freedom of being deflected to at least two different axes: the ex-periment target beamline and the diagnostic beamline. This property is very important since the beam can be optimized and then sent to the experiment by changing only the magnetic field.

• Electron beam diagnostics

Different electron beam diagnostics are neces-sary to measure and control the beam proper-ties. These include both beamlines (FCT/ICT current transformers, beam positioning, pep-per-pot emittance diagnostic, beam shape opti-cal transmission radiation, OTR, diagnostic) and diagnostic beam line (high-resolution magnetic electron spectrometer, temporal bunch charac-terization).


• Electron beam target station

The target stations for electron beams should be flexible in order to accommodate multipur-pose experiments. These can range from single electron beam experiments to complex pump-probe experiments involving the electron beam and multiple laser beams (synchronized with the electron beam).

• Electron beam dump

The electron beam must be absorbed in a ma-terial medium. Depending on the energy of the electron beam and on the atomic number of the material, the beam dump may require several meters to stop the electrons generated with ELI beams (up to 15 GeV / 1 nC / 10 Hz) on a single stage accelerator.

• Vacuum system

Typical vacuum systems for laser-plasma inter-

action are high-vacuum systems (~10-4 mbar), while conventional accelerators are ultra-high-vacuum systems (< 10-8 mbar). This is because the first ones release considerable quantities of gas and the latter ones require ultra-high-vac-uum to avoid damage by electrical disruption. At ELI we see no reason to improve the vacuum level. However, the particle sources may release considerably higher gas quantities for the vacu-um system due to the high repetition rate (10 Hz and higher). This requires a clever design of the vacuum system in order to keep the high-vacu-um necessary for high-intensity laser operation.

• Materials

High repetition rate laser-plasma electron ac-celerators will activate the materials in the in the experimental areas. We can anticipate, based on previous experience with smaller scale la-sers, that operation of the rooms will be strongly affected by the activation of the room compo-

nents if the materials in the room are not prop-erly chosen. The materials in contact with the primary as well as secondary radiation should have low atomic number and short decay life-times such as aluminum, plastics and carbon fiber. Materials as stainless steel, iron or copper should be avoided since they have intermediate decay lifetimes when activated, increasing the radiation levels in the room.

• Target station space organization

We can foresee two types of target stations: i) basic experiments without electron beamlines and ii) beamline experiments. A possible gener-ic setup for these two possible target stations is presented, as a simplified schematic, in the figures below (2a and 2b). The second type will require a larger area since the electron beam optics and diagnostics will require a consider-able length. In Figure 2 we can see a simple schematic of the second type of target station.

• Compact laser-focusing in plasma ac-


Chapter 3


celerators

In current laser-plasma accelerator experiments the laser beam is approximately Gaussian and the focal distances are in the range 0.2-3 me-ters, depending mostly of the available laser pulse energy. ELI will allow scaling the electron bunch energy by more than an order of magni-tude. As a consequence, the required focal dis-tance should scale also one order of magnitude. Such a long focal distance requires large vac-uum vessels and laboratory space, introduces instability in the laser pointing and considerably constrains the design of the electron beam op-tics between acceleration stages. Therefore, the source design would be strongly simplified if the focal distance could be reduced to the meter range. Two possible directions to achieve this reduction were proposed in the previous sec-tion: reduction of the beam diameter for non-fully compressed pulses, and the use of plasma lenses. Other possibilities are the use of a diver-gent mirror with high-damage threshold coating

Figure 2: Target station setup in experiments without electron beamlines (a) and with electron beamline (b) experiments.


(technique already used at Rutherford Appleton Laboratory, U.K.) or random phase plates. Ions

Based on the presently available projections for the production of proton and ions sources (see Figure 1), the maximum energy at the first phase of ELI project could reach ~200 MeV for protons and ~50 MeV/nucleon for carbon ions.

The source design follows from the fact that the dominant mechanism (TNSA) is currently well-known and optimal designs exist to exploit it. They however require a stringent temporal contrast for the laser pulse and the associated sophisticated targetry (ultra-thin target). In or-der to use the high repetition rate of the laser, one needs also to consider all problems related to such “high-repetition” targetry, i.e. a flexible way of replacing the target once it has been shot by the laser. One possible option could be to use a “film-target” such as a strip of metal

or a very huge metallic surface. However, these solutions might not be adapted for very thin tar-gets (nm scale) where a different type of targ-etry needs to be implemented (e.g. a series of independently mounted targets or gas-jets).

A very general set-up delivering an ion beam is shown in Figure 3. The probe beam can be used to diagnose the electron population at the front side (Antici 2008), the auxiliary beam at the right side can generate protons that can be used for proton radiography of the rear sur-face field (Romagnani 2005), whereas the film behind the target can be used as detector for protons. The spectrometer (spectro) and Thom-son parabola behind the target can be used as complementary diagnostics for detecting pro-ton/ion energies.


Chapter 3


The diagnostics should comprise:

• Spectrometer

• Emittance measurement

• Source size

• Divergence measurements

• Source duration

• Flux/charge

With the maximum energy planned for the initial ion source, most of these diagnostics, working adequately in the projected energy range, have already been designed, calibrated and used ex-tensively in several laboratories (nuclear activa-tion diagnostics, films, spectrometers for pro-tons and electrons, etc.). For energy resolved beam profiling, new detectors are required to deal with the higher particle energies and flux-es. New detectors based on nuclear activation are currently developed; they could detect the

much higher proton numbers, still providing ex-cellent spatial and spectral resolution and abso-lutely calibrated. Compound detectors also will measure electron propagation inside solid den-sity material using (γ,n) reactions with real and virtual photons. Those equipments will be used for the initial source as a pool and the expertise shared by several research teams. The diagnos-tics for higher repetition rate with high spatial resolution will need further improvements.

Figure 3: Schematic of an experimental setup for generating laser-accelerated ions. OAP stands for off-axis parabola, T.P. for Thomson parabola.


Based on the above source specifications, the following points will be required from ELI in or-der to be able to deliver the source:

• minimum two full power beams + lower power probe (non-harmonic preferably, possibility to be used chirped), all with their own compressors to vary the pulse duration

• feedback control of synchronization better than 10 fs

• temporally stacked pulses on the same beam to perform plasma conditioning

• polarization control (Linear and Circular)

• knowledge of pulse energy on target

• laser parameters stability (better than 1%)

• tunable temporal contrast with the pos-sibility of having high temporal contrast (i.e. a laser pedestal with laser intensity <1010 W/cm2)

• frequency-doubling or plasma mirrors set-up will be required if laser temporal con-trast does not meet requirements

• variable duration for the laser pulses and single shot control of the pulse shape

• tightest focusing (with pointing better than 0.1 mrad)

• focal spot shaping, wave front metrology and control

• repetition rate (single pulse may be needed for specific applications)

The interface with other sources represents a strong need to take advantage of unique syn-ergies. For example, coupling with high-energy electron beam production would allow using the ion beams to probe the fields in the electron acceleration and thus to obtain a better under-standing of the physical mechanisms. Coupling with X-ray sources would allow using them as

a probing tool for warm dense matter produced by ion beams, thus measuring the plasma pa-rameters and probing the atomic structure of the heated matter. This would allow for micron resolution radiography with highest temporal resolution of ELI experiments. Enough space needs also to be reserved to build an ion opti-cal system for proton-irradiation experiments, which will benefit also from linking one target area to the other.

Besides a source for ultimate high particle en-ergy (with reduced repetition rate) it should be possible to operate also a proton source with high repetition rate and a broad variability in the energy and flux, allowing also for more techni-cal relevant applications as well as for studies on the coupling with common accelerator set-ups (seeding of cavities, etc.).

In order to obtain a highly performing ion source,


Chapter 3


we will need to address and optimize the fol-lowing general issues: maximum charge, en-ergy spread, divergence, emittance, maximum energy. This will likely require some laser pulse shaping (uniform/flattop), the development of high repetition-rate capability (for the detectors, targets, etc.), new detectors (potentially very large for GeV ions detection), and some pos-sible coupling to conventional technology for post-acceleration/focusing. A crucial point to address will be the source reproducibility (laser stability).

Since we expect temporary activation during experiments at high repetition rate, the experi-mental stations should be isolated by thick walls in order to allow the access to other experimen-tal and laboratory common areas. In this way, it would be possible to use permanently the most expensive elements in the laboratory (pulse compressor, beam delivery).


As already explained above, we are proposing two experimental programs/rooms for laser-plasma based electron sources: one dedicated to supply sources for users (previous section) and the other dedicated to source development and advanced source use (this section). The ex-perimental program for electron source devel-opment should focus on the most important el-ements of the roadmap for future laser-plasma accelerators requiring one or more laser beams with the ELI size as well as a large laboratory.

When using external guiding (a plasma channel with a length equal to the dephasing length) the maximum energy gain of the accelerator can be optimized. Therefore, external injection can result in a great enhancement of the accelera-tor efficiency. Moreover, the external injection

Source Development

Electrons may also result in the improvement of the en-ergy spread and emittance of the final electron beam. This improvement is related with the fact that the injector stage can be designed for a higher plasma density than the second accel-eration stage and therefore the electron bunch will be accelerated by a more uniform electric field than in the injection stage.

At this point we can foresee three techniques for external injection: i) the use of two separated stages with laser and electron beam optics be-tween them, ii) the use of a higher plasma den-sity zone in the beginning of the accelerator, and iii) the use of an all-optical injection scheme. So far, only the last method was demonstrated experimentally with success in a configuration of colliding pulses. The methods i) and iii) re-quire at least an additional synchronized high power laser beam. The method ii) just requires that the plasma source acts as a lens over the laser pulse, changing its intensity and size ac-

cording to the plasma density. Such a experi-mental setup, with one or two ELI 50 J beams, would be able to produce electron bunches in excess of 1 nC and 10 GeV depending on the configuration.

Acceleration in multiple stages will allow to mul-tiply the final energy of the bunch by the num-ber of stages keeping a high level of compact-ness of the entire system. By means of staging it is possible to achieve higher energy electron bunches, by using a set of synchronized laser beams, than in a single stage accelerator, by using a laser beam with the same total energy. However, the final bunch charge in the staged accelerator is smaller. As an example, the use of four 50 J ELI beams in a four-stage accelerator would produce an electron beam with a charge of 0.78 nC and an energy beam higher than 60 GeV, according to the electron acceleration scaling laws. Using the same scaling laws in the external guiding blowout regime, a laser beam


Chapter 3


with the total energy of 200 J would produce an electron bunch of about 1.25 nC with an energy close to 40 GeV.

The long focal length interaction station of ELI will enable to extend scaling of particle accel-eration in nearly one-dimensional theoretical expectation that in fact has never been conclu-sively tested in latest lesser energy laser experi-ments. For the latter purpose, it is necessary to avoid self-trapping and coherent wakefield without break and with weak transverse focus to prevent transverse emittance blowup. Unlike the contemporary experiments at relatively high plasma density, this high energy capability of ELI allows experiments with lower density and thus higher energy gain with near one-dimensional laser focus. The acceleration of more than TeV electrons with ELI seems to be possible in one stage at lower density than contemporary ex-periments. This should pave a way to investi-gate the scaling of the future path in terms of

the energy frontier.

The benefits of staging need to be balanced with the cost of synchronization of different la-ser beams with the electron bunch being ac-celerated in addition to the electron beam optics required between each stage. However, staging will be a high demanding technological chal-lenge since a perfect synchronization of the la-ser and electron beams is required in a distance of several tens of meters.

Ions

This section is intended to outline the program that is envisioned to push the development of the ion sources within ELI above the limit that is set by the first stage of ELI (i.e. up to 50 J laser pulses), and that has been addressed in the previous section. Based on the projection of Figure shown above, several hundred MeV up to a few GeV protons could be obtained with the ultimate ELI parameters, i.e. using laser intensi-ties toward 1024 W/cm2. This would represent extremely significant improvements, compared to present state-of-the-art, of ion accelerators. The acceleration length can be as low as a few tens of microns, however with a downside asso-ciated with a non-guided process that is a high-er beam emittance and divergence. These pa-rameters have not been demonstrated because the required laser parameters are not available. However, based on simulations, they seem rea-


sonable. It needs also to be demonstrated, but is equally expected from simulations, that the beam quality can be further improved (mostly in terms of divergence and monochromaticity, as the present ion sources are already extremely bright). This would also be a major achievement and would serve users with great benefits. One should add that at these intensities protons should reach the ultra-relativistic regime and in that regime different acceleration schemes than what mentioned above might occur.

The ELI ultrarelativistic feature introduces the dynamics of protons to behave similarly to electrons in the intense laser fields. Be-cause of this feature, it is foreseeable that in-tense laser pulse from ELI beyond intensity 1024 W/cm2 can drive protons to high energies just like electrons are driven by laser wakefield beyond 1018 W/cm2. The energy gain scaling in this regime is proportional to the laser intensity and since accelerated protons move together

with the laser pulse (i.e just like a piston), the energy gain is adiabatic and thus the energy spectrum is monoenergetic.

We will have to take into consideration not only the maximum ion energy that will be reached through the various mechanisms under study, but also the following parameters, in order to decide for the most suited ones for downstream applications:

• laser to ions conversion efficiency

• ability for beam handling & selection (either through target engineering or conventional solutions, e.g. quadrupoles)

• ability for energy selection (with particular target/cylinder/capillary/others)

• ability for increased beam stability (energy distribution, particle numbers, emittance)

• increased efficiency, even at modest ener-gies

The design strategy for the source development will therefore require to study various paths which can be pursued for ion energy increase, including:

• simple laser intensity/energy increase

• use of ultrathin targets (requires very high contrast, circular polarization), production of protons and heavy ions

• use of small (mass-limited) targets (solids, µ droplets, spray)

• exploit/explore new mechanisms/regimes, other than TNSA, at higher intensities (e.g. shock acceleration, transparency regime, ponderomotive acceleration, etc). For this, the preplasma will have to be tuned at the target front, which will require an adjustable


Chapter 3


prepulse

• test the use of underdense targets (foam, gas, cluster, spray).

Different set-ups will be used to study these various aspects. Once a route is chosen, based on the results obtained throughout the study, a final set-up will be chosen and implemented. For all set-ups used through the course of the study, the issue of target development will be addressed with consideration for repetition rate, and the study of using either structured solid targets (layered, pre-ionized, embedded dots, isolated targets, µ droplets of different size) or of less than solid density where optimum pre-pulse will need to be adjusted.

We will need, along this source development program, to clarify the paths that are today envisioned from theoretical studies and that could lead to energy increase up to the range of

200 MeV - 1 GeV. Such energy levels are ex-tremely attractive to reach as they will open many new applications with high impact.

As mentioned above (in the state of the art sec-tion), a possible physical mechanism for ion acceleration that could be used to reach high ion energies would be the radiation pressure in solids. In this regime the plasma electrons are pushed steadily by this force, and ions are ac-celerated in the strong electrostatic field form-ing a shock-like structure. The use of circularly polarized laser light might further improve the efficiency of such ponderomotive ion accelera-tion, avoiding strong electron overheating. It has been shown in simulations that it allows one to obtain a quasi-monoenergetic ion bunch in a homogeneous medium by adjusting the laser pulse and plasma parameters. Compared to the target normal sheath ion acceleration, which is the dominant mechanism at the currently (2009) explored laser intensities, the pondero-

motive mechanism allows a better control of the ion energy distribution and acceleration of a much larger number of ions from a smaller target area. It will be also necessary to investi-gate in parallel the influence of radiation losses increasing with the intensity.

A series of PIC simulations, to complete the scaling of Figure and to check the parameters that could be expected for the ion source at the 100 PW range, have been performed (see Figure 4). This analysis, made for ELI PP-WP6, focuses on specifying the source term for the most extreme conditions provided – 133 PW – assuming a beam energy of 2 kJ in a 15 fs du-ration. The energy spectra from these simula-tions allow the conclusion that particles with up to a few GeV can be generated with this setup (Table 2 and 3).


In the studied configurations the particle beams are not collimated. The divergence can be actu-ally quite large, with denser side peaks of less energetic protons. It was also found that a sig-nificant fraction of energetic protons is travelling backwards. Thicker targets make a relatively uniform electron distribution in all the directions with some random spikes. Concerning the con-version efficiency, the 10 and 20 µm thick tar-gets reach lower peak energies, but the proton yield is higher for the protons down to half of the peak energy:

• 1 µm, 2-4 GeV range: ~5 x105 protons/(MeV.mrad. µm)

• 10 and 20 µm, 0.5-1 GeV range: ~5x106 protons/(MeV.mrad. µm)

General Type Simulation PIC 2D fully relativistic code

Target

Type Thin solid target

Material H

Density [g.cm-3] 0.088

Thickness [mm] 1 ; 10 ; 20

Laser

Energy [J] 2000

Pulse length [fs] 15

Wavelength [nm] 800

Intensity [W/cm2] 1.6 1023

a0

276

Layout

Focusing system (f/#) f/10

Beam waist [mm] 5.1 (1/e2 intensity radius)

Polarization Circular

Angle of incidence [˚] 0

Table 2: Summary of the input conditions for the Osiris simulations


Chapter 3


These numerical simulations foresee an order of magnitude increase of the peak energy rela-tive to the most energetic particles accelerated by today’s operating lasers. They also show that for radio safety concerns, it will be important to optimise the configuration, namely by laser and/or target shaping, in order to improve the beam collimation and, eventually, create or re-inforce its mono-energetic features. Further 3D simulations will be performed along the course of the ELI planning phase to accurately confirm the expected parameters of the future source.

Target thick-ness (mm)

Peak energy (GeV)

Proton FWHM

divergence (rad)

Proton Electron

1 4.6 2.9 0.510 1.3 0.9 1.620 1.3 0.5 1.3

Table 3: The maximum energy and beam divergence for the different target thicknesses

Figure 4: Maximum proton energy scaling with laser intensity. Purple dots are experimental values, blue are from simulations. The F/# rectangles show the intensity ranges achievable with the highest intensity ELI beam line using three different focal distances. The two darker blue squares at 1.0x1023 W.cm-2.mm2 show the results from the Osiris simulations on the 1 and 10 µm targets.


Another potentially promising way to accelerate ions to high energies is to use underdense or near-critical density targets (Esirkepov 1999, Yamagiwa 1999, Sentoku 2000). That approach has recently received much attention (Willingale 2006, Yogo 2008, Antici 2009). Compared to solid targets where laser absorption is limited to the target surface, the laser pulse inside low density plasmas heats electrons on a large vol-ume leading to higher laser absorption. This acceleration regime is also advantageous for applications since less debris are produced. It is more adapted for utilization with high repetition rate lasers. The laser contrast, which can be problematic with thin solid targets, is less detri-mental as the gas jets are transparent for laser intensities below 1013 W/cm2. The main accel-eration processes in this low density regime are still being debated. It appears from simulations that the acceleration processes depend strongly on the characteristics of the density gradient. We should also mention that, at extreme laser intensities reachable with the ultimate stage of

ELI, the use of underdense targets also offers the potential for bubble regime acceleration at extreme > GeV energies (proton acceleration using mixed ions).

The experimental setup designs will benefit from progress that will be accomplished a route to ELI, in partner laboratories, and also in the initial phase of ELI when the initial source will be delivered to users. The amount of work that will have been done at that time will allow to de-cide on the best experimental set-ups that will be most adequately suited to conduct the above mentioned design strategy for optimal source development.

The diagnostic requirements are the same as for the initial source but specific issues will arise from the strong energy increase of the particles produced at extreme laser intensities. There will be an additional requirement for: pion, muon

and neutrino beam detection; neutron beam de-tection and transport. As mentioned above, ELI, in its final phase, will likely be able to produce various kind of radiations (photons, particles) whose energy will range from MeV to GeV. Be-sides the number of particles per shot will be huge (>1012 protons in the 5-6 GeV range). The characterization in energy and beam spreading of these particles will be a challenge since no unique detector can be used. This constitutes the specification for the diagnostics to be de-veloped for the source development program.

Beyond 100 MeV and in the GeV range one must compose between the brightness of the radia-tion source and the variety of particles emitted together. Specific measures are required to plan for a beam stop of the bright proton/ion bunch-es in order to reduce radioactivations within a manageable space of ELI infrastructure. Nucle-ar high energy techniques could be considered, but these are huge equipments requiring very large experimental area.


Chapter 3


Laser-plasma accelerators produce short pulse electron beams (typical duration 10 fs) the bunch charge depends on laser energy and can exceed 1 nC with ELI. The emittance is naturally low due to the acceleration in the ion channel. The energy spread of the bunch can as low as a few percent. Except the energy spread, which can be improved with new beam loading tech-niques, these characteristics make these ac-celerators ideal for driving free-electron-lasers. A new generation of short period undulators, specially designed for free-electron-lasers with laser-plasma accelerators, was started to be developed.

Another application exploring the unique prop-erties of laser-plasma accelerators is the gen-eration of collimated beams of hard X-rays by

Directions of Implementation at ELI

Electrons betatron oscillation of the electron bunch in the ion channel [11]. Most of the applications of conventional accelerators are also possible with laser-plasma accelerators and sometimes at a reduced cost.

Up to now, X-rays with energies of a few mega-electronvolts represent the vast majority of ion-izing radiations used for cancer radiotherapy of several million patients throughout the world. Higher quality, more energetic electron beams, such as those produced by laser–plasma ac-celerators, could be used for radiotherapy and provide better clinical results. It was shown that such beams are well suited for delivering a high dose peaked on the propagation axis, a sharp and narrow transverse penumbra, combined with a deep penetration [12].

The real-time investigation of relativistic particle interactions with biomolecular targets opens ex-

citing opportunities for the sensitization of con-fined environments (aqueous groove of DNA, protein pockets) to ionizing radiation. However, compared with classical dose rate delivery in radiotherapy (1Gy/min) the very high dose rate delivered with laser–plasma accelerators (1013 Gy/s) may challenge our understanding of biomolecular repair, as ultrafast radiation per-turbations may be triggered on the timescale of molecular motions, angstrom or sub-angstrom displacements [12].

Electron beams produced in laser–plasma ac-celerators can be used to generate secondary radiation sources. The electron beam energy is efficiently converted into multi-megaelectron-volt Bremsstrahlung photons when it interacts with a solid target of high atomic number, pro-viding a submillimetre pulsed-ray source that is significantly smaller (450 µm) and of shorter duration (in the picosecond range) than other sources available today. Ultrashort-ray sources


are interesting for several applications, includ-ing imaging material compression to high den-sity. A train of short laser pulses may enable recording of movies of dense objects under fragmentation, or of the damage evolution of structures with a spatial resolution of 100 µm. Light and flexible devices for non-destructive material inspection would also be interesting, with potential applications in motor engineering, aircraft inspection and security. The ultrashort duration of these particle and radiation beams will provide unprecedented time-resolved mea-surements down to the motion of electrons on atomic scales, and a zooming onto the two fun-damental molecular building blocks, the elec-tron and the atom [12].

A hypothetical but credible scenario which could be achieved is the realization of a TeV collider that is based on laser acceleration and potentially cheaper than the conventional tech-nology. However, this deliberation is based on

future anticipated extension of technology of lasers and others. Another interesting physics objectives is the investigation of the space-time structure using high energy photons and, in particular the Landau-Pomeranchuk-Migdal ef-fect, which is the modification of the interaction of photons and electrons within high density matter. An additional application of laser ac-celeration is the electron-positron pair creation by laser-accelerated electrons colliding with an intense laser beam.

The most attractive and fundamental charac-teristics of ELI facility is that its suite of beams comes in perfect synchronism with its optical pulses themselves. Thus all the high energy beams (electrons, ions, gamma-rays) are syn-chronous with one another and with the optical beam, whose structure is typically constituted of ultrafast bunches. This provides a basis for ar-ranging a marriage between different beams to collide or influence the others. An example may

be a set of intense optical beams counterpropa-gating against a laser accelerated high energy electron bunch. Such a setup may be useful for exploring the highly nonlinear QED effect.


Chapter 3


Laser-accelerated ion beams are currently used mainly in two applicative areas: (i) warm dense plasma generation and (ii) probing of matter and/or electric and magnetic fields, but have potentially very close parameters to other sig-nificant applications.

At the boundary between condensed matter and plasma physics, the study of warm dense mat-ter (WDM), i.e. of dense matter (1-10 g/cm3) at high temperature (1-100 eV) (Ichimaru 1982), is essential since it is relevant to a wide range of disciplines. Indeed, the matter in this state is found not only in astrophysical objects but also in the transition from solid density matter to the plasma state. Using high currents of laser-ac-celerated ions presents a significant advantage over other techniques to produce WDM (Patel

Ions 2003). Indeed, ions have intrinsically in-depth energy deposition capability. Moreover, being produced in a very short time (few ps), the ions can also deposit energy potentially in a shorter time scale than the expansion time of matter heated to a few eV. This technique allows one to produce sufficiently large volumes of WDM to permit quantitative observation, thus opening the way for measurements of WDM parameters, such as stopping power, equation of state and others.

ELI Beamlines Facility by producing a high num-ber of ions at high energy in a compact manner and in a short time-scale, will offer a decisive advantage when producing such WDM by al-lowing to increase the volume of heated mat-ter and to push the achieved temperatures up to 10 keV range, as required e.g. for laboratory astrophysics.

Protons are also relevant as a diagnostic tool for fusion science, they are e.g. employed in experiments that study laser-plasma or hydro-dynamic instabilities or as a tool to diagnose target compression.

Short, low-emittance ion sources produced by laser-acceleration are already used to perform pump/probe ps-resolved and µm-resolved ex-periments to probe electric and magnetic fields in plasmas (Borghesi 2002). This technique has proved to be an essential tool, yielding the dis-covery of entirely new phenomena by diagnos-ing previously inaccessible observables (Rygg 2008). Besides the possibility to take 2D snap-shots of electric and magnetic field structures in plasmas the usually broader energy distribu-tion of protons in combinations with their time of flight effects allow also to track the whole dynamics of field evolution in plasmas (Romag-nani 2005, Sokollik 2008). Due to the relatively modest ion energies achieved up to now, this


technique is however limited to probing fields in low-density media. For the same reason, dense matter (e.g. shocked matter) probing is also possible using this technique in order to retrieve density (and not field) information, but is pres-ently limited to very thin samples.

At relatively modest energies (~100-200 MeV) ELI will allow extending the range of such field/matter probing application. It will then allow to study electron transport in dense matter (as re-quired for the optimization of secondary X-rays or of nuclear reactions within dense materi-als to produce positrons or γ-rays) or shocks in compressed material for laboratory astro-physics and geophysics. Higher ion energies (up to 800 MeV), will allow to radiograph very dense and thick objects (e.g. matter at 50 g/cm2). Here, compared to conventional accelerators (e.g. the LANSCE 800 MeV pro-ton accelerator found at LANL for dense mat-ter probing), the excellent emittance of ELI ion

sources, in combination with its short pulse du-ration, will be a significant advantage, allowing unprecedented spatial and temporal resolution.

The first potential area of application for ion sources produced by ELI lies within the area of bio-chemistry helped by ion beam irradia-tion for conditioning materials (using radicals, e.g. hydroxyl radical yields in the tracks of high energy C6+ and Ar18+ ions in liquid water) to steer reactions. For this, ELI will offer interesting complementary prospects with cyclotron facility like GANIL as, again, the ion beams will be pro-duced with the possibility of offering simultane-ously other sources of companion radiation (e.g. X-rays, electrons, etc).

High yield medical applications can also be in-vestigated by using ELI-based ion sources. For medicine, two main applications can be con-sidered: first, proton therapy assisted by lasers

(Malka 2003, Bulanov 2002). Here, the proton energies ought to be within the therapeutic window, i.e. between 70 and 250 MeV with a ~ nA current (1010 part/s) with spectral and spatial beam shaping. Not only protons, but also other type of ion beams, such as carbon or lithium beams, will also be produced in this prospect, as they appear nowadays to be of ma-jor interest for hadron therapy. Another potential advantage of such a source is that the energy and quantity of the ions could be adapted from shot to shot (i.e. during the treatment time), which allows a more modular form of treatment compared to conventional accelerators. By fos-tering development of such technique, ELI would help promoting proton source for hospitals to treat cancer tumors. ELI will allow estimation of the cost and optimization of the source through the guidance of radiotherapists. Moreover, elec-tron and proton beams with a pulse length in the ps or fs time scale will provide a unique op-portunity to study new and important biological radiation effects.


Chapter 3


ELI could also be used for time-resolved biol-ogy studies. There is a fundamental difference between the impact of a heavy ion and a pro-ton beam. The difference in the delta electron density caused by the impacting ion results in biological effects, which are not known below a microsecond timescale. It manifests itself in certain tumors which are resistant to proton therapy, but not to e.g. carbon therapy. Increas-ing the dose rate, but keeping the total dose constant could lead to higher biological ef-fectiveness and would allow proton therapy to new types of cancer not applicable today. Here basic research is required to investigate dose dependent effects, like water radiolysis close to cell DNA or multiple impacts within the chemi-cal recombination timescales. Short proton and ion bunches from ELI would increase the acces-sible range of particle energy and flux by orders of magnitude from the present status. Higher particle energies could also stretch the range for radiobiological research from solar cosmic radiation to the realm of interstellar radiation

background experiments.

Reaching extremely high ion energies, in the GeV range, would allow to use ELI in another extremely attractive area, namely as a source for spallation. This opens all the downstream study of neutron beams, or as a source for the transmutation of radio-nucleides. Currently there is only one very costly facility (SNS, re-cently completed in the USA) that is able to per-form transmutation of nuclear waste, moreover, conventional neutron sources are currently not very flexible and are of low brillance. Here ELI would again offer a complementary tool to con-ventional sources like the SNS to probe matter but in a time-resolved manner that is not pos-sible at the SNS. Indeed, the neutron, produced through D-D monochromatic reactions, should be bunched and keep the short duration of the initial ion source.

Finally, proton beams with energies > GeV could be used for laser-driven heavy-ion col-lider experiments (e.g. ion beams driven by opposing two ultrarelativistic laser pulses) with about 1 million events per laser shot (Esirkepov 2004). For this, beams containing about 1012

protons in the 5-6 GeV range, as predicted by (Pukhov 2003), could be suitable. Pair creation through laser-proton collisions (Keitel 2004, Di Piazza 2008) would also be a unique applica-tion allowed by ELI when using both the high-field laser beams and the extremely high energy proton beams, both produced by the facility.

The potential for drastic cost reduction of par-ticle accelerators for cancer treatment is poten-tially extremely important for medical business. The interest is that, by utilizing laser acceler-ated protons, the generating laser beams can be transported close to the Gantry-like set-up where the protons are produced close to the patients. Therefore no expensive measures for


radiation protection of the proton beam trans-port as in conventional facilities are required. This arrangement makes also easier to vary the proton dose in terms of energy spectra and pulse duration. If realized, this scheme could stimulate a large market of photo-medical in-dustry with huge investment leverage. In fact, current limitation of particle beam therapy with the conventional accelerators is its high cost. Thus, small laser accelerators could lead to a revolutionary change in the cancer therapy scenery and thus open a vast photo-medical industrial market.

Currently there are about 30 conventional parti-cle therapy facilities in operation worldwide: e.g. Loma Linda University in California, USA (http://www.protons.com), PSI (Paul Scherrer Institut) in Villigen, Switzerland (http://www.psi.ch), HI-MAC (Heavy Ion Medical Accelerator) in Chiba, Japan (http://www.nirs.go.jp), Institut Curie in Orsay, France (http://protontherapie.curie.info),

NPTC-MGH (Northeast Proton Therapy Center - Massachussets General Hospital) in Boston, USA (http://www.massgeneral.org). Thousands of patients per year are treated in the existing facilities. The average cost of a conventional proton therapy facility is 70-100 M USD con-sisting of 40 M for building and radiation shield-ing, 15-30 M for accelerator itself, and 15-30 M for gantries and other related costs. The cost for a carbon therapy facility is about 2-3 times higher. On the other hand, the best estimate for a laser-based proton facility is currently 4.5 - 7.5 M USD consisting of 0.5 M for building/shielding, 3-5 M for accelerator (laser + target), and 1-2 M for gantries/others. The main ad-vantage is evidently the drastic decrease in the cost of building and shielding (about 100 times), which could permit rapid spread of commercial proton facilities around the world.

The ELI facility can provide a major contribu-tion for the development of future high-quality

and low-cost proton sources for cancer thera-py. In particular, ELI can first optimize the ion source, and then in collaboration with industrial partners design and test the prototypes. In par-ticular, shielding and beam transport design will concern magnet systems for electron stopping and ion energy selection, beam stoppers, col-limators. The sources to be considered will be primary protons/electrons, secondary protons/electrons, X-rays and neutrons, implying differ-ent shielding materials (lead, tungsten, copper, steel, polyethylene, heavy concrete). Thus the radiation protection, extremely important for practical implementation in therapeutic facili-ties, can be verified.

Co-operation with companies working in the field of conventional accelerators, such as HI-TACHI (http://www.hitachi-hitec.com/global), IBA (http://www.iba-worldwide.com), VARIAN (http://www.varian.com/us/oncology), SIEMENS (http://www.medical.siemens.com) or ACC-


Chapter 3


SYS (http://www.accsys.com/) is anticipated. In fact these companies can also transfer their know-how and suggest the way to fit conven-tional techniques to the small laser accelerator machines. Thus, an efficient technology trans-fer towards commercial sector and back is ex-pected, e.g. in terms of accelerator and gantry coupling, beam transport, particle selection, collimation, monochromaticity, scanning beam systems, etc.

It is worth to mention the importance of the closely collaboration between ELI laser acceler-ator researchers and the conventional accelera-tor community. In fact, the level of sophistication and the long years of experiences that the ac-celerator physics community has pioneered and accumulated are among the most spectacular technologies that the last 100 years of human history have witnessed.

Besides the medial applications, protons are used in a variety of industrial applications, rang-ing from radiography of paintings and art objects to colouring of precious stones. It is therefore likely that a facility, offering high-precision, high luminescence protons beam, better and cheap-er than conventional proton facilities, will soon attract industries for their potential applica-tions. However, since the characteristics of the achieved proton beams are still to be refined, a precise definition of the technology transfer between industry and ELI is still premature.

References[1] A. Pukhov & J. Meyer-ter-Vehn, Appl. Phys. B 74 (2002) 355–

361.

[2] S. Mangles et al., Nature 431 (2004) 535–538.

[3] J. Faure et al., Nature 431 (2004) 541–544.

[4] W.P. Leemans et al. Nature Phys. 2 (2006) 696–699.

[5] J. Faure et al. Nature 444 (2006) 737–740.

[6] C.G.R. Geddes et al., Nature 431 (2004) 538

[7] J. Faure et al., Nature 431 (2004) 541

[8] W. Lu, et al., Phys. Rev. ST Accel. Beams 10 (2007) 061301

[9] A. Butler, et al., Phys. Rev. Lett. 89, 185003 (2002)

[10] R. Bendoyro., et al., IEEE Trans. Plasma Sci. 36, 1728 (2008)

[11] M. Fuchs et al., accepted by Nature Physics (2009)

[12] V. Malka et al., Nature Phys. 4 (2008) 447

Chapter 4Applications in molecular, biomedical, and material sciences

Author: Ing. Libor Juha, CSc.

Applications in molecular, biomedical, and material sciences

Temporal scales of particular processes are as follows: looking at electron dynamics requires resolution better than 1 fs; nuclear dynamics (molecular vibrations, phonon dynamics) have characteristic times in ~ 10 fs; intramolecular dynamics in large molecules take times > 1 ps and only molecular fragmentation, real chemi-cal change, radiative transitions in molecules (fluorescence) occur at longer time scales. Us-ing a single sub-picosecond pulse also avoids the influence of radiation damage to probed system on the measurement.

Introduction

Funding and building such a large-scale facil-ity like ELI Beamlines require providing a con-clusive evidence that the infrastructure is able to take up the great scientific challenges of the future. Great challenges in molecular, biomedi-cal, and material sciences (MBM sciences) of the 21st century can be formulated as follows:

1. Measuring the mechanisms of physical and chemical processes at the atomic scale. We need to develop „cameras“ for making mo-lecular movies and learn how to use them effectively to probe matter in quick motion.

2. Controlling electronic processes in matter. In addition to that, nuclear dynamics follow-ing the electronic events should represent a subject of control.

3. Understanding the complexity - efficient methods should be developed to control and investigate various processes in real, i.e., highly complex, systems in the state as

they are present in nature.

4. Nanometre scale imaging of arbitrary ob-jects in their native state, e.g., capturing a living cell at nanometre resolution. Nano-metrically resolved dynamics of their re-sponses to various stimuli.

Below, we are going to describe how ELI will provide numerous powerful tools allowing effi-cient research in the above-mentioned areas.

Why MBM sciences need the ultra-short pulses ?

105www.eli-beams.euApplications in molecular, biomedical, and material sciences

Chapter 4


a) because of probing and/or pumping very diluted systems – if we investigate a sys-tem containing the target species (e.g. ions, clusters, molecules) at very low concentra-tion, we need very high „concentration“ of photons. Typical examples of such a system represent a cloud of highly charged ions, species doping a certain solid material and/or atomic, molecular or cluster beams.

b) because of probing and/or pumping a par-ticular process in very complex system – if the system is composed of numerous com-pounds in several phases, the deposited radiation energy is distributed into many different channels. Thus only a minor por-

tion of the pulse energy is utilized to initiate and/or visualize the particular species and/or process investigated. We should take into account that most of systems whose investigation is motivated by an application are very complex, e.g., cells, tissues, or-ganisms, artificial nanostructures, geologi-cal matter, etc.

Why MBM sciences need energetic pho-tons and charged particles ? Since ELI provides a unique opportunity of per-

fect spatial overlap and temporal synchroniza-tion of an fast optical laser beam with beams of ionizing radiation at highest parameters achiev-able using the technology of the near future. Such a combination allows investigating very early stages of photochemical and/or radiation chemical processes.

Why MBM sciences need ELI ?


At the MBM station intended to be built and operated at ELI, following main layouts will be constructed, commissioned, and utilized in mo-lecular, biomedical and materials research:

a) the pulse radiolysis device with sub-ps res-olution should be based on a combination of ELI-driven particle (i.e. electron, proton, highly charged ions) and/or energetic pho-ton beam and properly delayed portion of the primary ELI beam for analysis of parti-cle-generated transients,

b) ELI-driven electron beam will serve in the time-resolved electron diffraction appara-tus; timing with a portion of the ELI optical beam allow to investigate structural chang-es in photo-transforming systems,

Directions of imple-mentation at ELI

c) coherent XUV/X-ray sources operated at ELI will be used for diffractive imaging of various objects, from single molecules to living cells; timing the short-wavelength beam with the long-wavelength one offers a possibility to investigate a fast response of the investigated system to high fluxes of low-energy photons, and

d) ELI-drive short-wavelength sources allow looking also at spatio-temporal momentum patterns of photo-electrons and secondary electrons; this layout may provide important information on fast electronic and structur-al dynamics of highly energized molecular and supra-molecular systems. Coupling the short- and long-wavelength laser fields al-lows to clock the system on the very fine time scale (optical streak camera with ultra-high temporal resolution will be developed).


Chapter 4


Measuring atomic-resolution images of materi-als with X-ray photons during chemical reac-tions or physical transformations resides at the technological forefront of X-ray science. New X-ray–based experimental capabilities have been closely linked with advances in X-ray sources, a trend that will continue with the impending arrival of X-ray–free electron lasers driven by electron accelerators.

X-ray diffraction has proven to be among the most powerful tools for determining the atomic structures widely used in scientific disciplines ranging from biology to solid state physics. Al-though all objects scatter X-rays, crystal forma-tion has been an essential step in the measure-

Key research topics

X-ray coherent imag-ing with atomic reso-lution

ment of atomic resolution structures for the majority of materials. Because of the regular arrangement of atoms in a crystal as opposed to an amorphous material, the X-ray scattering from the repeating structural unit adds coher-ently at periodically arranged Bragg peaks. This amplification greatly reduces the X-ray fluence required to measure a high-resolution diffrac-tion pattern, but away from the Bragg peaks the scattered intensity remains too weak to be measured, losing much of the information con-tained in the continuous molecular transform of the underlying structural unit. Even so, crystal-lography has remained the only method to mea-sure diffraction at the large scattering angles required for the determination of high-resolu-tion structures. The inability to crystallize impor-tant samples, however, has impeded progress in materials science and structural biology [1].

Measuring the continuous X-ray scattering pat-tern directly from a nonperiodic object provides

an alternative method for structure determina-tion [2]. A real-space image with atomic resolu-tion can be obtained from the measured X-ray scattering pattern computationally. The numeri-cal determination of the image requires that the phase of the diffracted light has to be deter-mined. However the scattering pattern contains the information on the intensity of the scattering radiation rather than the amplitude and thus no phase information can be directly measured. A variety of methods have been developed for alle-viating the information deficit in crystallography, such as examining the wavelength dependence of the diffraction pattern near an atomic absorp-tion edge or by knowing part of the structure or a similar structure. With coherent diffractive imaging, an alternative route to reconstructing the scattered X-rays into an image can be used. Sayre has noted that the continuous diffrac-tion pattern of a coherently illuminated unit cell contains twice the information obtained from the diffraction pattern of a crystalline arrange-ment of identical copies of that unit cell [2,3]. If


adequately sampled, this pattern provides the exact amount of information needed to solve the phase problem and deterministically invert the X-ray scattering pattern into an image of the object.

This imaging technique requires the illuminat-ing X-ray beam to maintain phase coherence across the entire width of the object. For co-herent imaging, as with crystallography, X-ray exposure determines the achievable resolution, and radiation damage sets the maximum dose. The necessity of limiting the dose without the benefit of Bragg amplification inhibits coher-ent imaging from achieving atomic resolution at synchrotrons designed to produce high av-erage, but low peak, flux, and has limited the technique to relatively large objects [4,5]. On the other hand, free electron lasers operating in the extreme ultraviolet have demonstrated that “instantaneous imaging” can be achieved [6] and X-ray–free electron lasers (LCLS, XFEL)

Figure 1: (A) Schematic representation of photogenerated softening of the interatomic potential in InSb. (B) Time-dependent distribution of atomic positions following bond softening. The initial Gaussian distribution broadens linearly with time and with a velocity determined by the root mean square atomic velocities before laser excitation. (C) Schematic representation of a pho-togenerated shift in the equilibrium bond length in a bismuth crystal. (D) Time-dependent distribution of atomic positions after displacive vibrational excitation. The frequency of the coherently excited vibration determines the period of the oscillation in average atomic position, whereas the magnitude of the shift in equilibrium position determines the amplitude of the oscillation. Reproduced from ref. [8]; for the original data see references cited therein.


Chapter 4


[7] have the promise of making atomic resolu-tion imaging possible [8]. Delivering the radia-tion dose to the sample in an extremely intense single X-ray pulse before the onset of radia-tion degradation provides a strategy for high-resolution X-ray imaging without crystallization [9,10]. Moreover, ELI would make possible to study non-equilibrium structural dynamics of transient phenomena initiated by ultrashort la-ser pulse in femtosecond and attosecond time scale.

Figure 2: Schematic depiction of single-particle coherent diffractive imaging with an XFEL pulse. The intensity pattern formed from the intense X-ray pulse (incident from left) scattering off the object is recorded on a pixellated detector. The pulse also photo-ionizes the sample. This leads to plasma formation and Coulomb explosion of the highly ionized particle, so only one diffraction pattern (a single two-dimensional (2D) view) can be recorded from the particle. Many individual diffraction patterns are recorded from single particles in a jet (travelling from top to bottom). The particles travel fast enough to clear the beam by the time the next pulse (and particle) arrives. The data must be read out from the detector just as quickly. Reproduced from ref. [8].


Another method for imaging small atomic clus-ters is X-ray holography with atomic resolution [11,12]. In contrast to X-ray coherent scatter-ing, X-ray holography records three-dimen-sional image on a two-dimensional recording medium and, in principle, all the information needed for the image reconstruction can be obtained in a short single shot. The holograms contain information not only on the intensity but on the phase of the scattered radiation as well. Namely, in the case of X-ray fluorescence holography, the fluorescence emitted by the atoms of a selected element (emitters) is used as a holographic reference wave. The portion scattered by neighbouring atoms constitutes an object waves. The interference of the reference wave and the object wave constitutes the ho-

X-ray holography with atomic resolu-tion

logram. Several concepts of X-ray holography with atomic resolution using X-ray fluorescence [11-14] or anomalous X-ray diffuse scattering has been developed [15-17].

Nowadays, a large number of emitter atoms have to be illuminated in order to get a detect-able signal even when using most powerful

synchrotron sources of x rays. Then a numerical reconstruction of measured holograms provides an average neighbourhood of all emitters illumi-nated by an incident beam. Consequently, use-ful structure information can be obtained only in the case where all the emitters have identi-cal environments. This fact strongly limits the samples suitable for holography experiments. Obviously, all single crystals fulfil this criterion.

Figure 3: (a)-(d) Holograms of CoO measured at Ephot 6925, 13 861, 17 444, and 18 915 eV, respectively. (e) Atomic images obtained from the combination of these measurements. Reproduced from ref. [14].


Chapter 4


However, whereas orientational order is nec-essary, translational periodicity is not required and, in principle, the holography can be applied to small clusters of atoms like protein mol-ecules, viruses, nanostructures etc. when using a much more powerful source of X-rays. Again, radiation damage by deposition of the energy of X-ray photons into the sample limits the structural studies on non-repetitive and/or non-reproducible structures. However, as shown by computer simulations [9], useful structural information can be obtained before radiation damage destroys the sample when using high X-ray dose rates and ultra-short exposures, i.e. femtosecond X-ray pulses generated at ELI.

Time-resolved X-ray diffraction

The extension of time-resolved X-ray diffrac-tion, widely used in nanosecond and picosec-ond time domains [18,19], to the femtosecond domain is an important challenge, as the nature of chemical reactions and phase transitions is determined by atomic motions on these tim-escales. An ultimate goal is to study the struc-ture of transient states with a time resolution shorter than the typical period of vibration along a reaction coordinate. Recent developments in time-resolved X-ray diffraction, using both syn-chrotron and laser-plasma based sources, have led to the capability of directly observing struc-tural dynamics on picosecond and femtosecond time scales. This has resulted in a number of novel experiments, including the investigation of structural changes induced by short-pulse-laser irradiation of organic materials [20,21] as

well as ultra-fast laser-induced phase transi-tions [22-24].

The use of standard X-ray diffraction methods (monochromatic and Laue single-crystal dif-fraction, powder diffraction) in a pump-probe mode using still shorter pulses (ELI is supposed to provide femtosecond or attosecond flashes) may be very beneficial to the study of ultrafast changes in the electron density in materials in-duced by high-intensity ultrashort laser pulses available at the ELI Beamlines Facility.


The pulse radiolysis method represents a gen-eral technique for the study of fast chemical re-actions and related processes. In particular, this technique contributed significantly to under-standing the chemical and biological action of ionizing radiation. The technique of pulse radi-olysis is based on the external perturbation of a chosen system by a nanosecond (recently pico-second) to microsecond pulse of ionizing radia-tion (typically 10-MeV electrons) of sufficiently high intensity to produce an instantaneous concentration of transient species which is high enough to be directly observable (mostly due to time-resolved optical absorption, luminescence, EPR, electrical conductivity, etc.). The pulse ra-diolysis celebrates fifty-year anniversary right now, because its development became feasible in 1959 upon the availability of pulsed electron

Sub-picosecond pulse radiolysis

accelerators exhibiting appropriate pulse dura-tions and energy contents. Actually, it is one of the earliest pump-probe techniques. More de-tails on earlier stages of the development of this technique can be found for example in [25,26].

The advent of technologies providing ultra-short laser pulses in the eighties triggered the new period in the pulse radiolysis development al-lowing the study of primary radiation chemical processes with a picosecond temporal resolu-tion. In the currently used techniques, the ultra-short laser pulse is split into two parts. One of them represents and/or drives a pulse beam while another one serves as a probe. There are two particular ways how to utilize these pulses in the pulse radiolysis.

a) The first portion of the light pulse triggers the photo-injector in a suitable electron accelerator providing an electron bunch

which can be well synchronized with the second part of the light pulse. The second light flash may be delayed using an adjust-able path length to be used as a probe of transient events initiated by the electron bunch. More details on such devices and their use can be found in [27-29].

b) The ultra-short laser pulses have sufficient intensity to ionize irradiated system due to multiple photon ionization. Therefore, the optical laser can provide a pump beam di-rectly. The second part of the beam helps to follow the event, usually to be convert-ed into synchronized white light flash. For more details see [30-33].

The temporal resolution achieved with the first class of ultra-fast pulse radiolysis devices re-main at a picosecond level [29]. Following the latter way, we may achieve a sub-picosecond resolution [30]. However, this arrangement is


Chapter 4


associated with several difficulties (e.g., only a minor part of absorbed energy is ionizing the matter; a significant portion of the deposited energy is dissipated resulting in a temperature jump [32] which makes the interpretation of re-sults difficult) and it should always be kept in mind that the pulsed laser beam ionizes irradi-ated matter in a different manner than the real, high-energy ionizing radiation.

The picosecond resolution limit should be bro-ken because products of radiation induced chemical reactions are determined by rapid pri-mary processes [34] such as energy transfer, thermalization and solvation. Some experiments carried out using the above mentioned methods demonstrate that these initial events occur in liquid water on time scales < 5 ps and involve a complicated interplay between electronic re-laxation and vibrational energy redistribution. These experiments also show that the chemi-cal processes originating from ionizing radia-

Figure 4: Fig. 4. Primary events initiated by high-energy ionizing radiation of liquid water [33]. The wide impact of ionizing radia-tions inducing electronic or vibrational excitations and ionizations concerns inelastic interactions of energetic charged particles and X- or γ-rays with water molecules. Most of these physical and chemical steps take place in less than one picosecond (< 10-12 s) and involve confined spaces commonly called spurs. The interaction of ionizing radiations (relativistic particles, γ- or X-rays) with water molecules induces energy deposits and secondary electrons in tracks. These physical events trigger the formation of nonequilibrium electronic configurations (quasi-free or dry electron eqf−), p-like prehydrated electron ep−, elec-tron–radical pairs and fully hydrated electron ground state es−. Simultaneously, the primary water molecular cation (H

2O+) reacts

with surrounding water molecules via an ultrafast ion–molecule reaction. The latter event occurs in less than 100 fs, yielding a strong oxidant (hydroxyl radical OH) and hydronium ion H

3O+ (hydrated proton). This reaction is likely one of the fastest that oc-

curs in polar molecular solvents and represents an ideal case to learn more about ultrafast proton transfer in water. A favorable structured environment induced by an aqueous hydronium ion (H

3O+)aq and hydroxyl radical (OH)aq can be created before an

excess electron gets to an equilibrium state.


tion are unique and cannot be reproduced by laser photons alone. Due to the lack of a suit-able femtosecond source of ionizing radiation, knowledge of the primary processes in radiation chemistry and radiobiology remains poor. The large variety of femtosecond sources of ionizing radiation, which could be in the pulse radioly-sis technique utilized as a pump, synchronized with the UV-Vis-NIR laser beam, which would be used as a probe, will be available at the ELI Beamlines Facility.

Initial pulse radiolysis experiments [33,35,36] have already been carried out using the ultra-intense, ultra-fast laser beams both generating and accelerating electrons to MeV energies. Their results clearly indicate that an application of more advanced sources of this kind could shed light on the course of elementary ioniz-ing events in nascent spurs formed in irradi-ated liquid water, schematically shown in Fig. 4 [33]. Experiments with femtosecond pulses of

ionizing radiation promise providing the direct information about the magnitude of primary ion-ization events, depending up to now just on in-direct approaches such as the inverse Laplace transformation of a concentration dependence of the solvated electron yield using scavengers or stochastic modelling of non-homogeneous radiolytic events with Monte Carlo methods.

As we mentioned in the Introduction, very high „concentration“ of photons and charged par-ticles provided by various ELI-driven secondary sources allows to investigate very diluted sys-tems, i.e., systems containing the target spe-cies (e.g. ions, clusters, molecules, nanocrys-tals) at very low concentration. Typical examples of such systems represent streams of weakly bound (van-der-Waals) clusters and/or cluster ions, which we describe below in details.

Photochemical processes of molecules in bulk environments are dictated by the interaction of molecules with the solvating species. By the way of example, the photoexcitation in aqueous systems can lead to the production of the H

3O

Influencing and probing diluted sys-tems


Chapter 4


molecular radical (or solvated electron) which in turn can interact with embedded species such as various biomolecules and transfer the excitation to these molecules leading to their dissociation. On the other hand, the solvent molecules can change the photochemistry of embedded molecules by closing some disso-ciation channels. Such processes are relevant e.g. to DNA radiation damage, etc. However, the understanding to these complex processes at the molecular level can hardly be provided by the condensed phase studies due to the com-plexity of the systems involved. The gas phase studies of the isolated molecules, on the other hand, neglect the important action of the sol-vent and therefore the insight is also very lim-ited. Here molecular clusters were established as a tool to reveal the details of photodissocia-tion and photochemistry of solvated molecules. The relevance of cluster studies can range from biological species (e.g. heteroaromatic ring molecules in water clusters) to chemistry of at-mosphere (hydrogen halides, HNO

3, NO

x, etc. in

water clusters and nanoparticles).

We propose to build, at a specialized MBM sta-tion operated at ELI Beamlines Facility, a molec-ular beam apparatus to produce free molecules and clusters in vacuum. Additionally, an elec-tron gun will be used to excite the molecules and clusters and eventually to produce negative clusters by an electron attachment or positive ones by electron impact ionization. The neutral or charged species will be subsequently ex-posed to ultra-intense flash of VUV/XUV/X-ray radiation. Several tools will be used to analyze the interaction outcome: Ion imaging technique will provide 3-D information about the velocity of a dissociation product, from which the de-tails of the photodissociation dynamics can be revealed. Quadrupole and time-of-flight mass spectrometry will analyze the cluster composi-tion and intracluster reactions.

Thus a versatile apparatus will be obtained for the study of a variety of important processes in molecules and clusters: e.g. photodissociation at low energies (< 10 eV), and photoionization at higher energies (>10 eV). These processes differ in the bare molecules and clusters point-ing to the important effects of solvation. The tunability of ELI-driven sources will provide a unique insight into these processes. For ex-ample by scanning the radiation energy the ex-periment can be walked through the landscape of the system potential energy hypersurfaces, where various reaction channels can be opened at various photon energies. Enormous number of photons in a single pulse provided by numer-ous ELI-powered sources would allow studying processes with extremely small photo-ioniza-tion and/or photo-dissociation cross-sections. Another interesting option is represented by studying not so rare processes but occurring in low-abundant species.


The simple layout proposed in Fig. 5, which is based on a single ELI-provided beam, would be in the second stage of the project turned into a pump-probe scheme combining two pulses in time and space to investigate ultra-fast dynam-

ics of various elementary events in clusters with a femtosecond temporal resolution.

Experiments described above for van-der-Waals clusters can be in the proposed scheme carried

out also with molecules, molecular ions, cova-lent clusters, nanocrystallites, nanodroplets, and so on. Such research has strong sources of mo-tivation in areas of plasma physics (especially in the controlled fusion research and development and plasma astrophysics), plasma chemistry, cosmochemistry and astrobiology, chemistry of lower and upper atmosphere, chemical ca-talysis, bioorganic chemistry, nanotechnology, tailored syntheses of molecules (e.g., pharma-ceuticals) and advanced materials, etc.

Surfaces and interfaces of condensed phases can be also considered as diluted systems be-cause of their restricted volume, i.e., the strongly limited number of their constituents interacting with the incident radiation. Some experiments (see for example [37]) have already been done with high-order harmonics [38] in this area, but these investigations suffer from relatively low number of photons in the probe. The ELI driven sources of XUV radiation can be used for real-

Figure 5: Proposed experimental arrangement for a VUV/XUV/X-ray beam-induced photoionization and photodissociation of van-der-Waals clusters, to be implemented at MBM station of ELI.


Chapter 4


time observation of electron transfer processes at interfaces [39]. Electron transfer processes of valence electrons constitute the fundamental step in photochemistry, electrochemistry, and electron transport across boundaries. Under-standing the underlying mechanisms has im-portant implications for a range of fields from materials to life sciences. The time scale of electron transfer is determined by the interac-tion strength of the donor and acceptor states. For weakly interacting states, e.g. image po-tential states at metal surfaces, the electron transfer dynamics has been studied in the last decade by femtosecond laser pulses. For stron-ger covalent interactions, electron transfer oc-curs on an attosecond time scale and has been inaccessible in the time domain so far. Frequen-cy-domain experiments using high-resolution resonant X-ray spectroscopy indicate transfer times well into the attosecond regime, e.g. for sulphur covalently bound on a transition metal substrate. However, this method does not pro-vide real-time resolution as in pump-probe ex-

periments and is based on assumptions about the relaxation dynamics.

A question highly relevant to fundamental phys-ics is the dynamics of electron correlations and screening of charge carrier excitations in solid-state materials with high electron density. Theo-retical estimations show that the screening is built up within the inverse plasma frequency, i.e., well below 1 fs. Various ELI-driven XUV sources with its attosecond XUV-pump/XUV-probe ca-pability offer a unique opportunity to address these problems experimentally. Fig. 6 [39] il-lustrates possible generic schemes for probing electron transfer and screening processes at in-terfaces in a wide range of systems in real time. In a more general context, these studies aim at probing the evolution of non-stationary electron wave functions in condensed matter. In scheme (a) a XUV ultra-short pulse hν

1 resonantly popu-

lates an occupied state in an adlayer. A subse-quent pulse hν

2 monitors the ultra-fast electron

transfer to the substrate, which provides a sink for electrons. The electron transfer between the solid (acceptor) and the adlayer (donor) is there-by analyzed. Various model interfaces such as Bi/Si, CO/Pt, and H

2O/metal allow addressing

key questions for material science, catalysis, and life science. On the other hand, scheme (b) shows that charge screening can be studied by probing the change in binding energy upon electron injection into an unoccupied state of the adlayer. Due to electron-electron interaction the binding energy of a shallow core level will be renormalized upon electron injection. This change in binding energy is monitored in the photoelectron kinetic energy by a time-delayed second XUV pulse.


Figure 6: Proposed schemes for attosecond real time probing electron transfer and screening processes at interfaces [37].

Numerous particular problems, both technical and scientific, could be of great interest for in-dustry:

A. Better knowledge on early stages of radia-tion processes occurring on solid surfaces of various materials can be used for an op-timization of industrial radiation processing.

B. Ultra-high emittance beams of charged particles driven by ELI and above men-tioned probe techniques will serve for both development and optimized use of compact particle sources for radiation therapy of cancer.

C. The ultra-short pulses carrying an enor-

Potential for applica-tions, business and technology transfer


Chapter 4


mous number of energetic particles make it possible to investigate behaviour of com-plex systems composed of many chemical constituents in several coexisting phases. Such a complex system, e.g., living tis-sue, polymer composite and/or industrially polluted water represent a typical subject of interest in numerous industrial and bio-medical applications.

D. Ultra-fast signal transmission, detection and timing techniques developed for MBM experiments could represent germs for in-novative activities, especially in the com-munications and aerospace industry.

E. Procedures and techniques developed for time-resolved imaging would very likely represent an impulse for further develop-ment and major upgrades of conventional techniques of biomedical imaging.

F. Last but not least, pharmaceutical industry should represent the most motivated com-

mercial sector absorbing and stimulating results obtained from layouts looking at elementary molecular processes with fem-tosecond and sub-nanometer temporal and spatial resolution, respectively.


References [1] Z. Dauter, Acta Crystallogr. D62, 1 (2006).

[2] D. Sayre, Struct. Chem. 13, 81 (2002).

[3] D. Sayre, Acta Crystallogr. 5, 843 (1952).

[4] J. W. Miao, H. N. Chapman, J. Kirz, D. Sayre, K. O. Hodgson, Annu. Rev. Biophys. Biomol. Struct. 33, 157 (2004).

[5] H. N. Chapman et al., J. Opt. Soc. Am. A 23, 1179 (2006).

[6] H. N. Chapman et al., Nature Phys. 2, 839 (2006).

[7] (a) XFEL – The European X-Ray Free-Electron Laser. Technical Design Report (Eds. M. Altarelli et al.). Hamburg DESY 2006-097. (b) J. D. Bozek, Eur. J. Phys. Special Topics 169, 129 (2009) and references cited therein

[8] K. J. Gaffney and H. N. Chapman, Science 316, 1444 (2007).

[9] R. Neutze, R. Wouts, D. van der Spoel, E. Weckert, J. Hajdu, Na-ture 406, 752 (2000).

[10] J. C. Solem, G. C. Baldwin, Science 218, 229 (1982).

[11] A. Szöke, in Short Wavelength Coherent Radiation: Generation and Applications (AIP Conf. Proc. No. 147), Eds. D. T. Attwood and J. Boker (New York: AIP), p. 361 (1986).

[12] M. Tegze, G. Faigel, Nature 380, 49 (1996).

[13] T. Gog et al., Phys. Rev. Lett. 76, 3132 (1996).

[14] M. Tegze et al., Phys. Rev. Lett. 82, 4847 (1999).

[15] M. Kopecký, J. Appl. Cryst. 37, 711 (2004).

[16] M. Kopecký et al., Appl. Phys. Lett. 87, 231914 (2005).

[17] M. Kopecký et al., Phys. Rev. Lett. 100, 195504 (2008).

[18] J. Wark, Contemp. Phys. 37, 205 (1996).

[19] Time-resolved Diffraction, Eds. J. R. Helliwell, P. M. Rentzepis (Ox-ford: Clarendon Press 1997).

[20] C. Rischel et al., Nature 390, 490 (1997).

[21] F. Schotte et al., Science 300, 1944 (2003).

[22] B. J. Siwick et al., Science 302, 1382 (2003).

[23] A. Rousse et al., Nature 410, 65 (2001).

[24] A. M. Lindenberg et al., Science 308, 392 (2005).

[25] M. S. Matheson, L. M. Dorfman: Pulse Radiolysis, (Cambridge(MA)-London: The MIT Press, 1969).

[26] The Study of Fast Processes and Transient Species by Electron Pulse Radiolysis, Eds. J. H. Baxendale and F. Busi (Reidel 1982).

[27] T. Kobayashi et al., J. Nucl. Sci. Technol. 39, 6 (2002).

[28] J. Belloni et al., Nucl. Instrum. Meth. Phys. Res. A 539, 527 (2005).

[29] Y. Muroya et al., Rad. Phys. Chem. 77, 1176 (2008).

[30] A. Migus et al., Phys. Rev. Lett. 58, 1559 (1987).

[31] Y. Gauduel, in Lasers in Chemistry, Vol. 2, Ed. M. Lacker (Wein-heim: Wiley-VCH), p. 861 (2008).

[32] R. A. Crowell et al., J. Phys. Chem. 108, 9105 (2004).

[33] B. Brozek-Pluska et al., Rad. Phys. Chem. 72, 149 (2005).

[34] R. A. Crowell et al., Nucl. Instrum. Meth. Phys. Res. B 241, 9 (2005).

[35] V. Malka et al., Nature Physics 4, 447 (2008).

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[38] D. Riedel et al., Rev. Sci. Instrum. 72, 1977 (2001).

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p. 18; http://www.extreme-light-infrastructure.eu/pictures/ELI-scientif-ic-case-id17.pdf


Chapter 4


Chapter 5Plasma and high energy density physic

prof. Ing. Jiřří Limpouch, Csc. / Ing. Jaroslav Nejdl


Introduction

The ELI Beamlines Facility will open chances to study previously inaccessible states of mat-ter and interaction regimes of laser, X-ray and charged particle beams with various targets. The focused intensity of ELI laser will be up to three orders of magnitude higher than on any existing laser, and consequently, the parameters of the secondary sources (X-rays, charged par-ticle beams) will be massively superior to the existing ones.

The intensity will be so high that electrons will oscillate ultra-relativistically with γ up to 1000 in the laser field, and ion oscillations will be-come relativistic. Plasma with relativistic bulk electron temperature T

e ≥ 500 keV will be

formed and thermal bremsstrahlung will gen-erate γ-radiation capable to produce electron-positron pairs massively. The laser-accelerated ion beams and X-ray laser beams will be able to heat macroscopic mount of solid density matter isochorically to keV temperatures. Thus, fasci-

nating, fundamentally new plasma physics will be investigated on ELI infrastructure.

The additional essential advantage of the ELI infrastructure will be the synergy of laser and secondary sources. Thus, one experiment can use synchronized laser and X-ray and/or par-ticle beams for plasma production and interac-tion and/or for diagnostics. For example, electric fields in laser-produced plasma will be detected by beam of accelerated protons.

First, the development of the secondary sources (X-ray and charged particle beams) of ELI will benefit from the research of laser-target inter-actions. There is still vast space for improve-ment of the existing schemes for X-ray emission and charged particle acceleration, and innova-tive schemes will be proposed based on laser-plasma interaction research. Second, plasma is a suitable medium for parametric amplifi-

cation and compression of laser pulse. It can sustain much higher intensities than any other medium and application of stimulated Raman (SRS) and Brillouin (SBS) scattering is currently being explored experimentally. Third, the envi-sioned research will have impact on many other physics and science field. For example, novel fusion schemes will be proposed and explored. High energy density systems will be formed with parameters near to or scalable to those impor-tant for astrophysics and thus the basic data for modelling certain astrophysical systems will be obtained. Laser-produced plasmas may be also utilized in the technology of the classical ac-celerators as ultraintense electric or magnetic lens focusing intense particle beams (e.g. Large Hadron Collider) to a significantly narrower focal spot than via classical technology.

ELI will be a unique user facility and the ex-perimental studies of plasma and interaction physics, and of high energy density physics will

125www.eli-beams.euPlasma and high energy density physics

Chapter 5


be proposed and guided by user teams. Conse-quently, the research topics proposed here can-not be complete by any means, and they are not intended for setting any limitations on the future research. They can only serve as guidelines set-ting an insight that is required for the design conceptions of the interaction and diagnostic complex. The infrastructure ELI must provide very flexible environment that will enable very broad range of experimental set-ups.

Interactions of high-intensity lasers with tar-gets have been studied since the second half of 1960s, immediately after the invention of laser. The driving idea behind the largest lasers was inertial confinement fusion (ICF). Nanosecond laser pulses were used and the interaction was non-relativistic, the maximum laser intensities were up to 1016 W/cm2. Warm dense matter was produced near to the ablation surface and also by intense shock waves inside solid targets. Solid matter was compressed several times and temperature behind the shock wave reached up to 10 eV. The interaction physics was summa-rized in Ref. [1], the physics of laser-produced warm dense matter is described in Ref. [2]. With the invention of chirped pulse amplifica-tion (CPA), the studies of interactions of much shorter, femtosecond laser pulses with targets started in 1989 and the physical description of

this new interaction regime was published soon together with an overview of its prospective ap-plications [3].

The relativistic intensities were firstly reached at Los Alamos CO

2 laser Helios in 1978 [4]. Due to

the long wavelength 10.6 µm of the CO2 laser,

intensity of 1016 W/cm2 was sufficient for elec-tron to oscillate relativistically. However, the pro-gramme of fusion CO

2 lasers was abandoned

soon, because laser energy was predominantly transformed into fast electrons that preheat the fuel making fuel compression unfeasible. Rel-ativistic intensities were reached again in the end of 1990s by femtosecond solid state lasers, relativistic γ ≈ 25 was achieved in 1999 in LLNL where one of the NOVA laser beams was con-verted to CPA femtosecond laser [5]. High-pow-er femtosecond lasers are either high-energy long-pulse (~500 fs) low-repetition (10-3–10-4 Hz) Nd-glass-based lasers (Vulcan-PW, RAL, UK, FIREX-I, Osaka, Japan and Omega-EP, Roch-ester, USA), or lower-energy, short-pulse (~30

Current status


fs), higher-repetition Ti:Sapphire lasers (Astra Gemini, UK, Texas Petawatt, USA). Extremely sharp focusing lead to the record laser intensity of 2×1022 W/cm2 at 300 TW-Ti:Sapphire laser Hercules at Univ. Michigan, USA. ELI will fully exploit the existing laser technologies and at the same time explore the emerging novel ap-proaches such as DPSSL to generate extreme laser power and intensity.

Research of relativistic laser interactions with various targets is driven mainly by prospective applications. Laser propagation in long under-dense plasmas is examined in connection with laser wakefield acceleration of electrons [6]. Laser interaction with thin foils is investigated in order to improve ion acceleration process [7]. Mass-limited targets are used to diminish un-desirable lateral energy spread out of the focal spot [8], and thus energy density is enhanced. Cluster targets are used for initialization of vari-ous nuclear reactions [9] as the reaction rate is

increased due to ion energy enhancement via Coulomb explosion. Strong shock and radiative blast wave in cluster media can also be scaled to astrophysical conditions [10]. Isochoric heat-ing of thin foils by laser-accelerated protons was studied experimentally [11]. We have men-tioned only a few important examples, for com-pilation and presentation of a complete list is beyond the scope of this text.

We would like to stress that all pre-requisites for future successful plasma and high energy density physics already exist. There is a strong European community of researchers with vast experience in experimental, theoretical and nu-merical studies of relativistic laser interactions with various targets. This community is inter-connected as interpretation of experiments is not realistic without numerical simulations as full characterization of the system is not fea-sible with the required temporal and spatial resolutions. Many experiments are proposed

by theorists and guided by numerical simula-tions. There is long-time experience with run-ning large user facilities, including lasers, tar-gets chambers and diagnostics. Broad range of methods for diagnostics of laser-target interac-tion and of laser-produced plasmas, including active and passive optical, X-ray and particle methods, have been already developed.


Chapter 5


Directions of plasma, HEDP and interaction studies at ELI

In the following, we shall discuss several im-portant directions of plasma and high energy physics research at the ELI Beamlines Facility. This list is in no aspect complete and we can-not guarantee that the listed topics will be the most important. Firstly, no one can fully foresee the evolution of this booming field in the next five years; secondly the research topics will be selected from the user proposals. This list is intended as a guideline for the design of flex-ible target area facilities devoted to plasma and HEDP studies, and for the choice of basic on-site diagnostic equipment. The research topics are organized into the following particular top-ics.

The propagation of high-intensity, coherent, electromagnetic radiation in plasmas, and the resulting modifications of the plasma state, is the subject of the nonlinear optics of plasmas. Coherent radiation can scatter from and decay

into collective plasma modes and can create radiation at new frequencies. The plasma oscil-lations may become self-organized, nonlinear, kinetic states. Simple modes that have linear and fluid limits can be strongly influenced by nonlinear kinetic modes, such KEEN and KEIN waves. This subject is very exciting from theo-retical, computational, and experimental points of view because of the myriad states made pos-sible by the interaction of multimode fields, to-gether with wave-particle resonances, the non-locality of the self-consistent, plasma-particle dynamics [12].

Nonlinear optics of plasmas and laser interactions with un-derdense plasmas

Figure 1: Proton projection image of wake taken 20 ps after interaction of 35 fs Ti:Sapphire laser pulse of intensity 5×1019 W/cm2 with tenuous plasma formed in front of 0.9-µm-thick mylar foil by laser prepulse [13]. Small scale structures in the wake are clearly depicted.


This topic includes wake formation behind the laser pulse and studies of the wake properties (Fig.1) that are important for electron accelera-tion. Presently, the interest is concentrated on the bubble regime [14] that leads to formation of monoenergetic beam of electrons acceler-ated up to GeV energies. However, the limits of electron, positron and ion acceleration in the wakefields are not clear. Wakefields are multidi-mensional and the electron density in the wake may reach values orders of magnitude higher than the background density. The intensity of ELI laser will be so high that ions could be trapped and accelerated in the wake. The optimum con-figuration of laser beams for stable propagation over long distances in tenuous plasma is not known to date. The optimum regime for electron acceleration at ultra-relativistic intensities may differ substantially from the present situation.

Parametric scattering like SRS and SBS can influence propagation of laser beams and at

Figure 2: Electromagnetic soliton detected in numerical simulations of SBS [16]; blue – density, red – transverse electric field squared, green – magnetic field squared.

Figure 3: The phase space of relativistic KEEN wave plotted in Vlasov simulations of SRS [17].


Chapter 5


high intensities they could create the condi-tions for trapping laser light and formation of relatively long-lived electromagnetic solitons [15]. These solitons have been observed in three-dimensional PIC simulations and more recently in the nonlinear stage of SBS evolution [16] (Fig. 2). They can stay in the same place for a sufficiently long time (of about tens of ps) or slowly drift if the ambient plasma is inhomo-geneous. Fig. 3 shows SRS-induced non-linear KEEN wave. ELI will allow such structures to be manipulated, and used for studies of long-term effects of extremely high electric and magnetic fields on isolated nuclei. Backward Raman and Brillouin scattering can also lead to laser pulse amplification and compression [18] that could be used in interaction experiments or in laser technology. ELI will allow studying this process at much higher intensities.

ELI will create plasmas with temperatures exceeding 1 MeV. In these plasmas brems-strahlung photons will have energy sufficient for creation of electron-positron pairs (Fig. 4). This already observed process [19] is particu-larly efficient with high-Z target materials like

gold (Fig. 5). Creation of plasma with positron density comparable to ion density will be pos-sible only with the next generation lasers like ELI. Understanding how these plasmas behave when either their directed flow or their average energy is relativistic is a fascinating area for research. This includes understanding how to create useful electron-positron plasmas in the

Relativistic HED plasma

Figure 4: Diagram of pair creation. Electron colliding with high-Z ion emits bremsstrahlung (γ photon) that collides with another high Z ion and electron-positron pair is cre-ated.

Figure 5: Electron and positron spectrum, when 1 ps pulse of intensity 1020 W/cm2 was incident on thick gold target [19].


When an intense laser like ELI is incident on a solid-density target, a complicated suite of phe-nomena occurs. The electric field in such lasers is orders of magnitude larger than the fields that bind the electrons to the atoms, single free electrons can oscillate with energies in excess of 1 GeV, and the radiation pressure can exceed 10 Tbar. The absorption of an intense laser at a dense plasma boundary is not completely understood because of the complexity of the electron-distribution function and fields (both electric and magnetic) at the interface. Inves-tigation of radiation pressure effects on density steepening and consequently on electron spec-trum and temperature is important. Research of laser interaction with solid foils is needed for

laboratory. This topic is important in high energy astrophysics, in the development of compact photon and energetic-particle sources, and in extending the boundaries within which high en-ergy density systems have been explored.

Laser interaction with solid targets and dense matter

optimization of proton and ion acceleration for various applications, i.e. for optimization of fast ion source at ELI. As probe beams one can use either portions of the few-cycle laser pulse it-self or secondary electron, ion, or X-ray pulses of similar time duration or even much shorter, attosecond XUV pulses that can be generated by means of laser harmonics. Attosecond time scales govern electron motion not only in atoms and molecules, but also in metals and solid-density plasmas.

When ELI laser will interact with the corona of solid target, laser will penetrate to dense plasma via relativistic transparency and chan-nel boring by ponderomotive force. Recent theoretical paper [20] demonstrates that puls-es with intensities exceeding 1022 W/cm2 may penetrate deeply into the plasma as a result of efficient ponderomotive acceleration of ions in the forward direction. The penetration depth as large as hundreds of microns depends on the


Chapter 5


laser fluence, which has to exceed a few tens of GJ/cm2. The fast ions, accelerated at the bot-tom of the channel with an efficiency of more than 20%, show a high directionality and may heat the precompressed target core to fusion conditions.

Figure 6: Simultaneous subpicosecond time framed interferogram (a) and Schlieren image (b) of a radiative blast wave 24 ns after being launched into a medium of 6 nm Ar clusters by a 700 mJ, 750 fs laser pulse. As material is heated and ionized, the result-ing free electron refractive index change causes fringes to bend in the image (a). Image (b) highlights refractive index gradients. Here, a thin shelled blast wave has formed, with a strong radiative precursor leading the compression wave [10].

Atomic clusters have been shown to be very efficient absorbers of intense laser radiation. During Coulomb explosion of laser-heated clusters, high energy ions and hard X-rays are produced. The possibility of applying clusters for various applications, such as tabletop nu-clear fusion from exploding deuterium clusters and synchrotron radiation, is very challenging. Clusters were recently used to create high en-ergy density plasmas that drive strong shocks (Mach > 50) and radiative blast waves [10] (Fig. 6). The plasma states generated can be described by magneto-hydro-dynamic equa-tions that have a number of similarities. Careful application of these equations and similarities allow experiments to be scaled to astrophysi-

Laser interactions clusters and mass-limited targets


cal phenomena that have spatial and temporal scales that are greater by as much as 15–20 orders of magnitude. In this way, the radiative blast waves in the laboratory have been scaled those experienced in supernova remnants and the physics governing their dynamics investi-gated under controlled conditions.

Mass-limited targets are used in order to limit undesirable energy spread out of the laser fo-cal spot. Thus, high energy density deposition is achieved. For example, deposition of 1 J in a droplet 10 µm in diameter would correspond to deposition of about 10 keV for each particle. Since the expansion time of such a droplet is greater than 1 ps, during this time period one can study matter in a quite unusual state of very high density and temperature in the same time. The limited-mass targets made from transpar-ent materials allow one to avoid negative effects associated with pre-pulses and they could be used in many applications, in particular for ef-

ficient ion acceleration to high energies [8].

Figure 7: (Left) XUV image of proton heating of a 60 µm Al foil at D/ r=1.5, converted to temperature. (Right) X-ray pinhole camera image of a laser irradiated Al hemisphere (where protons are accelerated) with a 100 µm thick Al foil at D/ r=1.8 [22].


Chapter 5


At ELI it will be possible to heat macroscopic amount of solid density matter up to keV tem-peratures either directly by laser or by the secondary sources. While direct heating by laser uses heating by hot electrons confined by self-generated magnetic fields and shock waves launched at material interfaces [21], using ion or proton beam is very straightfor-ward. Isochoric heating of 15 µm-thick Al foils by laser-accelerated protons up to 80 eV has already been demonstrated at Gekko petawatt laser [22] (Fig. 7). Equation of state, opacities and transport properties of warm dense matter are key data for astrophysics that will be well measurable with ELI.

The proton beam generated by ELI will unite

properties unreached so far by any laser gener-ated proton beam: high energy combined with a high intensity and focusability. This makes it an ideal instrument for a plasma generation. Since the ELI performance covers a broad interval of energies and intensities of the generated proton beam, it will be possible to generate plasmas ranging from a deeply non-ideal state (warm dense matter) to plasmas with high energy con-tent, close to that formed in the tempers meant for a proton-driven inertial fusion. The warm dense matter is an object of physical interest in its own right, not to speak about the astrophysi-cal relevance. Any relevant quantity measured on this exotic state of matter will be of inter-est. For the astrophysical purposes especially the optical properties like the opacity coefficient in the optical and X-ray region, emission spec-tra, etc. On the high energy end we are likely to obtain proton-beam-generated plasma in a high power density regime. If such a regime is reached with the laser-generated proton beam of sufficiently high power density, the hot dense

matter beam plasma would become available to the physical research and, moreover, a very interesting transition from a highly non-ideal to the ideal state could be traced by simply varying the properties of ELI generated proton beam.

With ELI, it will be possible to create high energy density plasmas and probe it with the second-ary hard X-ray source. Thus, extremely impor-tant plasma opacities will be measured in this regime.

Warm dense matter studies


Plasma heating by proton beams is of interest for both the fundamental research as well as for the particle beam driven fusion experiments. Experiments of this kind have been going on for a long time using combined equipment of lasers to provide the stopping plasma and of accelerators to supply the proton beam. Unfor-tunately, just a very few laboratories can host experiments of this kind and they are mostly located on the accelerator sites. ELI will be the first purely laser laboratory, where the neces-sary proton beam could be generated by one of the ELI main laser beams. The stopping of charged particles in plasmas is a complex phys-ical process [23], which can hardly be reduced to binary Coulomb collisions governed simply by the Coulomb logarithm. Even then the Coulomb

logarithm is somewhat vaguely defined quan-tity and its determination is often provided by a hand-waving argument. Its measurement would thus shed more light on hitherto insufficiently clarified questions of plasma kinetics, especially in non-ideal plasmas. But even in rarefied plas-mas the stopping power is determined by other processes then binary collisions. The onset of collective phenomena is to be expected under the effect of the impinging proton beam, whose spontaneously created electric fields will add to the plasma stopping power. This phenomenon might show a resonant behaviour, anomalous dependences on the beam energy and plasma parameters. The expected flexibility of the ELI performance might help map out broad re-gions of both the stopping plasma and the pro-ton beam parameters not yet explored in the classical way. For the purpose of pre-forming the stopping plasma a part of the ELI pump-ing beams will have to be diverted on the ns or even ps level.

Stopping of proton beam in a pre-gen-erated plasma

ELI Beamlines Facility is not designed for com-plex inertial confinement fusion (ICF) experi-ments. However, many physical issues of ad-vanced ICF schemes, such as fast ignition or shock ignition can be addressed at ELI.

First, transport of high-current electron beams in dense plasmas is crucial for fast ignition via fuel heating by electrons (both for schemes us-ing or avoiding a cone). The ELI laser pulses will drive relativistic electron currents of the order of gigaamperes in overdense plasma targets [24]. In vacuum, electron current transport is limited by I

A = 17 βY kA due to magnetic self-interac-

tion. But, return current in a plasma tends to suppress B-fields. The system of counter prop-agating beam and plasma currents is, however, unstable and leads to beam filamentation and a host of secondary filament dynamics and merg-ing events (Fig. 8). At present, it is unclear to what extent the beam energy can be transport-ed to the compressed cores of fusion targets.


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The filamentation instability may be reduced by transverse beam heating and other effects. Again, the present predictions are based only on 3-D simulations [26] and require experi-mental verification. The filamentary structures evolve and change on the microscopic plasma time scale, which is in the attosecond regime at these densities.

Second, it will be possible to study shock-igni-tion-relevant strong shock propagation in dense plasmas and shock collision with a counter propagating shock wave at ELI. The temporal visualization of this process is obviously impor-tant for shock ignition feasibility and optimiza-tion. The vital temporal visualization of the col-liding shocks inside the compressed fuel can be done with a hard X-ray or even γ-ray imaging coming from the ELI main beam.

Figure 8: 3-D-hybrid-PIC simulation showing a continu-ous 1 GA beam of 1 MeV electrons with 120 keV trans-verse temperature injected into plasma with density rising exponentially from 2 to 100 g/cm3 after 1.2 ps [25].



ELI must provide versatile, user-friendly envi-ronment for a broad range of plasma and high energy density physics experiments. Experi-ments will profit from two basic distinctions:

a) superior focused laser intensity, and

b) synergy of laser and secondary sources.

It is not possible to foresee all important ex-periment proposals, but the brief list above in-dicates that the possibility to exploit at least two energetic femtosecond laser beams together with at least one beam with longer pulse (non-compressed nanosecond pulse or pulse com-pressed to ps range) would be vital for many experiments. The experiments may generate X-rays and charged particles for the interaction and/or for plasma diagnostics directly inside the target chamber. However, it may be of advantage for many experiments to exploit the advanced secondary sources available at ELI, i.e. coherent

or incoherent X-rays, electron and ion beams generated in an extra target chamber. Conse-quently, special ports have to be available for the secondary sources and special equipment for pointing and focusing of secondary sources into experimental target chamber. Very precise synchronization of the secondary sources with the interaction laser beam has to be provided.

The interaction chamber has to be prepared for various types of targets, such as gas jets of vari-ous forms, cryogenic gas jets producing clus-ters, micro-droplets produced by pulse nozzles, capillary targets for laser guiding, thin foils etc. High-precision automatic target manipulation and pointing must be provided.

The interaction chamber must be equipped with the basic optical, X-ray and particle diag-nostics. Optical and X-ray streak camera with the best available time resolution have to be

fielded. Fast X-ray, γ-ray and neutron detectors will be provided. Streaked and time integrated X-ray spectra are important part of diagnos-tics. Charge collectors, Thomson parabolas and electron and ion spectrometer have to be pro-vided. Active optical and X-ray diagnostics using shadowgraphy, interferometry and Thomson scattering must be possible.

The interaction chamber has to be shielded properly. The shielding requirements against radiation (γ-rays, neutrons and activation) have to rely on scaling of experimental electron dis-tribution and on scaling of radiation data from present petawatt experiments. Scaling will rely also on numerical simulations of at experiments at laser intensities expected at ELI. For the first stage the shielding should be comparable with shielding of Vulcan petawatt target chamber [27]. For medium and high-Z targets, an inter-action chamber shield blanket of 15 cm lead and 10 cm of high density polyethylene, and a


Chapter 5


shielded fire escape door were required. This shielding is combined with the 60 cm thick concrete walls surrounding the interaction area gives an attenuation of γ-rays with the direct attenuation of 50,000 at 6 MeV. Low activation

materials will be used for the interaction cham-ber construction.

The complex for plasma and high energy density physics should include at least two target areas (not counting the target area(s) for the second-ary sources like X-rays and laser-accelerated particles), so that two experiments may be car-ried out and/or prepared in parallel. The target areas may be developed gradually, so that the first target chamber will be ready for the first phase of ELI. This target area will stay in opera-tion in the ELI second and third stage for the experiments that do not demand full ELI power. The main target area has to comply with the requirements for experiments at laser intensi-ties up to 1024 W/cm2. The design can be final-ized using experience from novel high-intensity facilities including first stage of ELI.

Figure 9: Installation of shielding at Vulcan petawatt tar-get chamber at RAL, U.K.

As an international large-scale user facility, ELI Beamlines Facility must offer interaction areas with excellent parameters and complementary characteristics allowing experimental setups which are not feasible in any other existing laser laboratory. In practice, it means enough flexibil-ity in manipulation and timing of several laser pulses and/or pulses of secondary sources (X-rays and charged particle beams). Taking the advantage of inherent synchronization of all ELI lasers which originate from one master oscilla-tor, it will be possible to perform various interac-tion experiments with preformed plasmas and pump-probe experiments. The interaction areas must offer possibility of easy and accurate po-

Complex develop-ment for plasma and HEDP studies and its use


sitioning of various types of targets. ELI has to offer a broad range of standard on-site optical, X-ray and particle diagnostics. Active diagnostic methods must be developed, tested and ready for fielding in selected experiments.

The fundamental part of the complex for plasma and high energy density studies will be the inter-action areas. In the first stage of ELI, the project of interaction area(s) can start from the existing interaction areas at petawatt lasers like Vulcan petawatt interaction area (Fig. 9) mentioned above. The entry ports for incident laser beams should be based on the parameters and geome-try of ELI laser beams. Entry ports for secondary sources must be designed in accordance with their projects. The radiation shielding (γ-rays, neutrons and activation) must be designed ac-cording to the data from present experiments extrapolated to ELI parameters. Materials used for the construction of the interaction chamber should have low atomic numbers and short de-

cay lifetimes such as aluminum, plastics and carbon fiber. Materials as stainless steel, iron or copper should be avoided.

When the volume of interaction chamber is specified, the vacuum pumping system will be designed. High-vacuum systems (~10-4 mbar) are typical for laser-plasma interaction, ultra-high vacuum is not needed. The basic require-ment is a high pump power enabling pumping of large interaction chamber in a reasonably short time. The stands for positioning of various types of primary and secondary targets should be specified and designed.

The basic set of on-site diagnostics should be specified based on a preliminary set of typical experiments like those specified above. The di-agnostic ports should be specified. The typical setups for active diagnostic methods should be developed and tested at existing installations.

The plasma and high energy physics complex will be exploited as a user facility with experi-mental time granted to the most important and challenging project proposals by an interna-tional body. Only projects that cannot be carried out on other smaller facilities will be accepted. Often, the investigated plasma object will be prepared by a long or short laser prepulse, or by X-ray or particle source and then the interaction of the main laser beam with the object will be examined. The typical interaction temporal and spatial scales do not allow obtaining sufficiently detailed information using simple diagnostic methods. Thus, auxiliary laser beams, X-rays and/or particle beams will be used for active diagnostics. For example, underdense plasmas will be diagnosed by Thomson scattering of an auxiliary laser beam and dense plasmas will be diagnosed by Thomson scattering of beam of secondary X-ray laser. The electric fields in the target will be imaged by beam of accelerated protons.


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Consequently, the typical experimental set-ups will be complex and thus experiments should be prepared for a relatively long time. The key ele-ments of the setup and of diagnostic methods should be tested at smaller installations, if pos-sible. Strong technical support for the experi-mental groups must be provided by ELI infra-structure. Experimental time must be awarded only to fully prepared experiments in order to avoid inefficient use of the infrastructure.

Experimental research in plasma and high en-ergy density physics at ELI infrastructure will be mainly oriented to fundamental science. This will accumulate scientific knowledge of new regimes of laser and secondary source interac-tions with target and investigate unique states of matter that can be prepared only with ELI infrastructure.

However, there is also a large application poten-tial of the plasma and interaction physics inves-tigated at the ELI Beamlines Facility. First, the basic physics of laser-target interaction is es-sential for optimization of the secondary sourc-es. Generation of quasi-monoenergetic electron and ion beams is particularly novel and funda-

mental breakthrough was achieved in 2004 for electrons and in 2006 for ions. New accelera-tion mechanisms are still being proposed and the acceleration process is not yet optimized even for present day intensities. Numerical sim-ulations and theory predict possibility of signifi-cant improvements of the acceleration process at intensities achievable at ELI but an extensive search for suitable experimental set-ups will be inevitable. The possible improvements in the parameters of the secondary sources would significantly enhance their application potential that is discussed in detail in the descriptions of the particular activities.

Interaction experiments will be directly used in material science, for example for understand-ing the aging process in construction materials of nuclear power plants. One can think about experiments when ultrafast processes induced in material by laser-accelerated ion beam will be detected by synchronized ultrafast laser or

Potential for Applica-tions and Technology Transfer


X-ray probe. Indeed, the mechanisms leading to defect creation or phase transformation in ma-terials subjected to low or high-energy ions in-volve as initial steps ultrafast processes whose elucidation is still a challenge: in the case of elastic scattering, collision cascades – at pres-ent observed only through atomistic simulations – or intense and short-lived electronic excitation in the case of high-energy ions – for which es-sentially two models were proposed long ago; and there is no clear answer yet concerning their respective validity. It is essential in both cases to obtain observations of the state of the target in the first few picoseconds after passage of the particles, and only laser-based sources can make such observations possible. Addition-ally, ELI laser-target interactions will be intense ultrashort source of neutrons and γ-rays, and ultrafast response of materials on radiation damage could be probed by synchronized laser and X-ray pulses. Such experiments could bring essential progress to radiation physics that is vital for many major technologies.

Intense and short positron pulses produced at ELI may be also interesting from application point of view. Positrons are used in several techniques of material analysis: The lifetime of the positrons in solids, this being in the range of picoseconds, is inversely proportional to the electron density. Positrons are easily trapped at vacancy-type defects, where the positron lifetime is prolonged. The lifetime spectrum, therefore, tells about the type and density of the defect distribution in solids. On this basis, it has been possible, for instance, to build a positron microscope able to observe inside a material the appearance of clusters of vacancies located at the head of a scratch. In addition, the chemi-cal surroundings can be analyzed from the Doppler shift of the annihilation radiation. The Doppler effect can also be used to analyze the momentum distributions of the electrons in the solid in great detail.

By focusing ELI laser on high-Z material we shall

be able to produce intense pulse of γ-rays that can be used for nuclear transmutation of long-lived radioactive isotopes into less radioactive or short-lived products. This concept is being developed in the world for nuclear waste man-agement. The primary risk isotope is long-lived iodine-129 with high radiotoxicity and mobility, and this may be transformed to iodine-128 that decays with a half-life of 25 minutes to stable inert xenon-128. The experiments may demon-strate the feasibility of laser-induced transmu-tation process.

Laser-produced plasmas can be used in the classical accelerator technology. Laser-ion ac-celerator can serve as an intense ion source for classical accelerators. Additionally, laser-produced plasmas may serve as ultrastrong lenses for focusing of classical accelerator beam as they can sustain quasistatic electric and magnetic fields higher than any other sys-tem. Recently, a micro-lens for focusing and en-


Chapter 5


Figure 10: Schematic of the micro-lens device. A proton beam accelerated from a planar foil by the CPA1 laser pulse propagates

through a hollow cylinder side-irradiated by the CPA2 pulse.

Figure 11: RCF layers show that 7.5-MeV protons were selected and focused [28].

ergy selection of laser-accelerated MeV protons has been demonstrated [28] (Fig. 10, 11). It is anticipated that with laser of ELI scale it could be possible to develop ultrastrong lenses that could narrow the focus of large accelerator like LHC (Large Hadron Collider) significantly, thus beam intensity and collision probabilities will be enhanced.

The above discussed technologies could be potentially transferred to industry in the future and could bring an important (incl. commercial) profit for the whole society.


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[21] Y. Sentoku et al., Isochoric heating in heterogeneous solid targets with ultrashort laser pulses, Phys. Plasmas 14 (2007), 122701

[22] R.A. Snavely et al., Laser generated proton beam focusing and high temperature isochoric heating of solid matter, Phys. Plasmas 14 (2007), 092703

[23] R. Singleton, Charged particle stopping power effects on igni-tion: Some results from an exact calculation, Phys. Plasmas 15 (2008), 056302

[24] G. Mourou et al., Proposal for a European Extreme Light Infra-structure (ELI), Paris 2008

[25] J. Meyer-ter-Vehn et al., On electron transport in fast ignition re-search and the use of few-cycle PW-range laser pulses, Plasma Phys. Control. Fusion 47 (2005), B807–B813

[26] J.J. Honrubia, J. Meyer-ter-Vehn, Fast ignition of fusion targets by laser-driven electrons, Plasma Phys. Control. Fusion 51 (2009), 014008

[27] C.N. Danson et al., Vulcan petawatt: Design, operation and inter-actions at 5x1020 W/cm2, Laser Part. Beams 23 (2005), 87–93

[28] O. Willi et al., Laser triggered micro-lens for focusing and energy selection of MeV protons, Laser Part. Beams 25 (2007), 71–77.


Chapter 5


Chapter 6Exotic physics and theory

Authors: RNDr. Karel Rohlena, CSc. / Bc. Jan Prokůřřpek


Introduction

Ultra-high fields of high-power short-pulse lasers pose important possibilities for funda-mental physics. Albert Einstein's equivalence of matter and energy, elegantly expressed as E=mc2, has been confirmed countless times, most dramatically whenever a nuclear weapon detonates. But the process also occurs natu-rally in stars shining because atoms in its core fuse, transforming a sliver of matter into light. However, any equation works in both directions, at least theoretically, consequently it should be possible to convert energy into matter.

The recent progress in laser technology calls for a reassessment of intensity effects in Quantum Electrodynamics (QED) and the new prospects of measuring them. QED describes electromagnetic fields, such as those of light, and their interactions with matter. Physicists are eager to study it in order to convert photon en-ergy directly into creation of electrons and posi-trons. There is a plethora of strong-field QED

processes, which may be roughly categorized into two classes: “loop” (strong-field vacuum polarization, spontaneous pair production) and “tree-level” (perturbative pair production, pair annihilation, Compton scattering) processes. One-loop processes are of order h and thus of a genuine quantum nature, while tree-level pro-cesses generically do have a classical limit. As a result, one can introduce two distinct param-eters which characterize the different physics involved. The first parameter is the QED electri-cal field (see below), E

C, which depends on the

Planck’s constant and the speed of light, show-ing that it originates from a relativistic quantum-field theory. In an electric field of strength E

c an

electron acquires an electromagnetic energy equal to its rest mass mc2 upon traversing a distance of a Compton wavelength:

Hence, Ec may be viewed as the critical field

strength above which vacuum pair production becomes abundant. This is also borne out by Schwinger pair-creation probability given by the tunneling factor p~exp(-πE

C/E), where E de-

notes the “ambient” electric field one succeeds in achieving. Currently, this is E ≅ 1014 V/m im-plies a huge exponential suppression.

147www.eli-beams.euExotic physics and theory

Chapter 6


As already mentioned, turning matter into light, heat, and other forms of energy is nothing new. In 1997 a team of physicists at the Stanford Lin-ear Accelerator Center (SLAC) in the framework of the E-144 experiment have demonstrated the inverse process: "the first creation of matter out of light". They collided large crowds of pho-tons together so violently that the interactions spawned electron-positron pairs [1].

In order to generate a field as close as pos-sible to E

c, they used a 0.5 TW short-pulse

glass laser (λ=527 nm, EL=500 mJ) into a beam measuring 6 micrometers across at its narrowest point. Moreover, in order to increase the energy of the photons, the team collided the pulses with SLAC's 30-micrometer-wide pulsed beam of high-energy electrons (46.6 GeV).

When laser photons collided head-on with the electrons, they got a huge energy boost, much like ping-pong balls hitting a speeding Mack truck, changing them from visible light to very high energy gamma rays. Because of the laser's intensity, these backscattered gamma photons sometimes encountered several incoming laser photons simultaneously; a collision with four of them concentrated enough energy in one place to produce electron-positron pairs, as depicted in Figure 1. The highly energetic electrons, un-dergoing nonlinear Compton scattering with n laser photons, produce backscattered photons of 27–30 GeV which then collide with the laser photons producing electron-positron pairs via the multiphoton Breit–Wheeler mechanism. The number of positrons measured in 21.962 laser pulses was 175±13 [2].

The experiment was a proof of principle for a technology, based on intense laser beams boosted to enormous energies, for exploring the

quantum electrodynamics theory. The result can be viewed as the first direct demonstration of the “vacuum sparking" phenomenon, in which the energy of a very strong electromagnetic field promotes some of the fleeting "virtual" particles that inhabit the vacuum, according to QED, to become pairs of real particles.

Current status

Figure 1: An electron beam intersects a laser pulse, boosting photons to gamma energies and triggering an interaction that spawns particles [2].


Electron-positron pairs are often spawned in accelerator experiments that collide other par-ticles at high energies, and photons produced in the collision are what actually generate the pairs. But at least one of the photons involved is virtual, i.e. produced only for a brief moment in the strong electric field near a charged particle. The SLAC experiment marked the first time that matter has been created entirely from ordinary photons.

In 2000 at ATLAS laser facility of Max-Planck-Institut, a team of scientists succeeded in con-verting low-energy photons into positrons via electron acceleration in a plasma channel, by using a femtosecond table-top laser system [3]. The average intensity of this source of positrons was estimated to be equivalent to 23.108 Bq and it exhibits a very favourable scaling for high-er laser intensities. In the paper are described measurements of positrons produced by MeV electrons from a relativistically self-focused la-

ser channel in underdense plasma. The scheme employed is analogous to that used for the gen-eration of positrons from high-Z moderators in linear electron accelerators. It consists of two steps: first, using laser pulses from the 790 nm, 220 mJ, 130 fs, 10 Hz ATLAS laser system, a beam of electrons was generated in a gas-jet target; second, the electrons were converted in

a 2-mm-thick Pb slab to positrons. They were able to produce 106 positrons per laser pulse with a mean energy of about 2 MeV. Although for radiation safety reasons the measurements were performed using single laser pulses, there are no technical constraints hindering the op-eration at the laser repetition rate, i.e., 10 Hz. Thus, in repetition mode configuration, this ex-

Figure 2: Experimental setup on positron production using laser at MPQ [3].


Chapter 6


periment represents a positron source with an activity of 107 Bq.

Very interesting results have been also ob-tained using the Petawatt Laser at Lawrence Livermore National Laboratory [4]. This system uses one arm of the NOVA laser to amplify a frequency-chirped pulse to 1 MJ energy be-fore temporal compression to 450 fs. The peak power is well in excess of 1 PW, and the beam can be focused to a 10 µm spot, with a typical intensity of 3x1020 W/cm2. At these intensities the electric field at the laser focus is higher than 1013 V/m. On the face of the solid targets used in these experiments a plasma was generated by a pre-pulse at 2 or 10 ns before the main pulse. The pre-plasma served as a medium to self-focus the laser, thereby increasing the pondero-motive energy, and to produce electrons of even higher energy by a variety of laser acceleration mechanisms. The interaction of the electrons with the solid target produces positron-electron

pairs. The yield of positron-electron pairs in this experiments was of order 104 of the electron yield in the energy range 5-10 MeV, as reported in Figure 3 showing the positron distribution for a thin, 125 µm Au target. The number of re-corded positrons clearly exceeds the calculated yield expected for external pair conversion of Bremsstrahlung photons generated in the tar-get. The additional mechanism of “trident pro-duction”, i.e., direct positron-electron pair pro-duction by electron-ion collisions, is expected to have a similar total yield as the Bremsstrahlung component for these experimental parameters.

These data suggest that in future generations of ultra-high-intensity laser experiments, it might be possible to observe pair creation di-rectly in the collisions of the quivering electrons with the ions of the target plasma, and perhaps even progress towards a terrestrial relativistic pair plasma, which is relevant for astrophysical models of gamma-ray bursts or accretion disks.

The above reported experiments have led to a recent interest of the subject especially as to whether modern laser technology can produce the strong electric field required for experimen-tal verification. In the recent paper [5], the au-

Figure 3: Spectra of electrons and positrons measured in 600 J/0.5 ps shot on 125 µm Au target. Histograms show PIC simulation of electrons, and corresponding pre-diction for positron creation by Bremsstrahlung in the Au target [4].


thors treated the problem of electron-positron production in a standing wave of oppositely di-rected laser beams of plane transverse linearly polarized electromagnetic waves of fixed fre-quency, using a two level multiphoton resonant approximation. This approach, if experimentally implemented, will result in much higher elec-tron-positron production rate for the case of conventional femtosecond laser systems.


Chapter 6



The technological breakthrough of laser chirped-pulse amplification has led to unprec-edented laser powers and intensities, the cur-rent records being about 1 PW and 1022 W/cm2, respectively. Up to 3 orders of magnitude may be gained at the planned ELI facility. At a laser intensity of 1026 W/cm2, an electron will un-dergo an acceleration of 1027 g, comparable to the gravitational acceleration at the event hori-zon of a black hole. This high acceleration could be used to study Unruh radiation generation. At sufficiently high intensities, even vacuum can be broken down. The field necessary to achieve pair creation (“boil the vacuum”) is the above mentioned Schwinger field, E

C.

Although such fields are beyond the horizon, other nonlinear quantum electrodynamics ef-fects could be accessed at more modest fields. For petawatt, kilojoule-class lasers, a nontrivial electron-positron pair can be created. Nowa-days positron emission from direct interaction of

petawatt laser pulses with solid targets has the main disadvantage of no operation at high rep-etition rate, limiting the yield of positron emis-sion. However this limit will be easily overcome at the future ELI facility. In this point of view ELI will explore new regimes and try to map out the basic phenomena concerning the QED theory. Finally, it is noted that an alternate path to the Schwinger field could be an X-ray free-elec-tron laser. In the future proposed XFELs field strengths of 1017 V/m are expected and funda-mental physics with these fields constitutes an important research area. Various QED phenom-ena that can be study at ELI facility are reported and discussed below.

Electron-positron plasma is a plasmatic state of matter along with antimatter where heavy ions are replaced by the electron antimatter equivalent and where relativistic effects play an important role. The early pre-stellar period of the universe evolution is characterized by suffi-ciently high temperatures at which the creation of particles-photons, electron-positron pairs is a natural process. Other sources of quite dense electron-positron plasma are pulsars. Most the-ories state that plasmas near a black hole are so hot that electrons and positrons exist in equi-librium with each other. Such matter-antimatter is created due to the Hawking radiation near the event horizon of massive gravitational body.

On Earth, there is no such extreme gravitation

Electron-positron plasmas


field, but according to the theory of quantum electrodynamics the photon can be split up to electron-positron pair in presence of an elec-tric field with a threshold value of 1.3x1018 V/m, also known as the Schwinger limit (analyti-cally defined above). The Schwinger field cor-responds to a laser intensity of 5x1029 W/cm2. With weaker fields the sub-threshold production can be studied, where in the oscillating fields enhanced production of electron-positron pairs and their annihilation radiation are predicted. For a strongly focused laser pulse pair pro-duction is predicted already at 1027 W/cm2 [6], while for two counter-propagating pulses the threshold should occur at 1026 W/cm2 [7]. In this sub-threshold field strength region theoretical predictions differ by many orders of magnitude [8]. Experimental clarification of these conflict-ing predictions is required and from the above mentioned facts the investigation of propagation of strong e.m. waves (pulses) in relativistic hot electron-positron plasma becomes very topical.

These matter-antimatter plasmas can be cre-ated in laboratory using an ultra-high intensity lasers (PW class and more). Laser intensities exceeding 1018 W/cm2 couple most of their en-ergy to superthermal electrons with tempera-ture exceeding the rest energy of the electron (relativistic plasma). Positrons are created when the relativistic electrons interact with high-Z target ions. High magnetic fields generated by relativistic electrons help to confine them. PW class lasers with sufficient pulse length can generate positron densities (approximately 1022 cm-3 for solid gold targets) which exceed any laboratory source of positrons [9].

ELI laser facility might demonstrate the accel-eration of electron beams in the 100 GeV en-ergy range, which can result in an increase of the electric field strength up to 1018 V/cm in the electron beam frame. Using the imaginary time method this field predicts a production of up to 105 electron-positron pairs per shot in a small

volume of the time-space. The research pro-gramme at ELI can start with electron-positron generation from thin high-Z targets and then proceed to the lasers generating high electron beams which would collide and produce elec-tron-positron pairs in vacuum without a target. This could be carried out at laser intensities of the order of 1023 W/cm2.

Figure 4: Double illumination of a target with two petawatt laser beams could lead to the production of the electron-positron pair plasma [9].


Chapter 6


Vacuum four-wave mixing

Direct observation of elastic photon-photon scattering among real photons would be an important benchmark test of laser-based Quan-tum Electrodynamic (QED) experiments. Devia-tions from the expected scattering rate would indicate new physics in the low-energy regime. Throughout the last decades, several sugges-tions on how to detect elastic photon-photon scattering, using laser assisted schemes, have been put forward. Crossing electromagnetic waves can interact and yield new modes of dif-ferent frequencies. One of the most prominent modes in such a mechanism is given by the four-wave interaction mediated mode satisfying the resonance condition between the frequen-cies and wavevectors (i.e. photon energy and momentum conservation). It is therefore not a

surprise, given the evolution of laser powers and frequencies, that the search for photon-photon scattering using resonant four-wave in-teractions has caught the attention of research-ers in this area. This approach has progressed furthest in the experimental attempts to detect elastic scattering among photons.

Using four-wave mixing to stimulate photon-photon scattering has the advantage of not be-ing limited by the low scattering cross section, in the optical range. The resonance conditions should be ω

4=ω

1+ω

2-ω

3 and k

4=k

1+k

2-k

3,

between the vacuum generated photons and the laser pump sources, respectively (see Fig-ure 5). When the scattering of a laser beam by another laser beam is stimulated by a third laser beam, the scattering probability becomes to be measurable [10]. Since a perfect vacuum in the interaction chamber is in practice impos-sible to achieve, there will be competing scat-tering processes present. The effect of these

processes will rather effectively be suppressed due to plasma cavitations caused by the strong laser pulses. Progress of low-energy QED ex-periments could also prove to be useful for dark matter searches.

Four-wave mixing technique can be also used

Vacuum processes

Figure 5: Configuration of the incoming laser beams (rep-resented by the wave vectors k

1, k

2 and k

3) and the direc-

tion of the scattered wave (with wave vector k4).


for an ultrashort pulse source in the UV and VUV spectral range pulse generation in hollow wave-guides with unprecedented short-pulse dura-tions. The generation of a 2.5 fs pulse at 160 nm with energy of about 1 nJ has been pre-dicted by pumping all the energy into the fun-damental mode, well above its phase-matching pressure [11]. The energy can be increased by a factor of 20 using higher-order transverse modes. By changing the input idler frequency a detuning of the VUV signal can be realized.

Vacuum polarisation

In quantum field theory, and specifically quan-tum electrodynamics, vacuum polarization describes a process in which a background electromagnetic field produces virtual elec-tron-positron pairs changing the distribution of charges and currents which have generated the original electromagnetic field. It is also some-times referred to as the self energy of the gauge

boson (photon). According to quantum field the-ory, the ground state of a model with interacting particles is not simply empty space. Rather, it contains short-lived virtual particle-antiparticle pairs which are created out of the vacuum and then annihilate each other. Some of these particle-antiparticle pairs are charged, e.g., vir-tual electron-positron pairs. Such charged pairs act as an electric dipole. In the presence of an electric field, e.g. the electromagnetic field around an electron, these particle-antiparticle pairs reposition themselves, thus partially coun-teracting the field (partial screening effect, i.e. dielectric effect). The field therefore will be weaker than it would be expected if the vacuum were completely empty. This reorientation of the short-lived particle-antiparticle pairs is known as vacuum polarization.

Electromagnetic waves interact in vacuum ow-ing to the presence of virtual electron-positron pairs. In a typical experimental setup one strong

electromagnetic field polarizes the vacuum and another one, usually a weak field, probes the induced vacuum polarization. The polarized vacuum behaves like a dielectric medium and the light spot generated by a strong focused laser field acts as a microscopic piece of mat-ter that diffuses the probe field. This diffusion or diffraction alters the polarization state of the probe and if the probe initially is linearly polar-ized, then after the interaction it results ellipti-cally polarized, with the main axis of the ellipse rotated with respect to the initial polarization direction.

The observation of vacuum polarization effects is feasible in laser fields significantly weaker than the Schwinger field, thus it can be easily investigated in the ELI intensity regime. More-over the detection of vacuum birefringence is already possible by the available laser tech-nique with 1022 W/cm2 intensities if X-rays are employed as a probe.


Chapter 6


Vacuum birefringence

High quality vacuum according with a strong magnetic field (more than 5 T) serves as a bire-fringent medium. Due to the vacuum birefrin-gence linear polarization of the laser light turns into elliptical polarization. The basic experimen-tal setup should consist of a polarizer to reach as highest polarization as possible of the incom-ing laser beam, a strong dipole magnet (or a su-perconductive magnet) with the magnetic field B0 exceeding 5 T, and a sensitive ellipsometer as depicted in Figure 6. It is necessary to have large interaction region in order to make stron-ger elliptical polarization for its better measur-ing. For the plane wave in the magnetic field the electric intensity and magnetic induction will be E(r,t)=E

wei(k⋅r-ωt) and B(r,t)=B

wei(k⋅r-ωt) + B0, re-

spectively.

It should be also possible to make vacuum bire-fringence experiment without permanent mag-

netic field because the polarized vacuum acts as a medium with preferred directions dictated by the external fields which we assume to be generated by a high-power laser of frequency ω

0. Accordingly, there are two different refrac-

tive indices for electromagnetic probe beams of different polarization states. Distorting the vac-uum with lasers has been suggested long time ago [13] but was not considered experimen-tally for lack of sufficient laser power. ELI laser intensity regime and recent progress in X-ray detection will open the way towards novel ex-perimental capabilities. It is therefore due time to specifically address the feasibility of a strong-

Figure 6: The experimental scheme for the vacuum birefringence observation [12].

Figure 7: The vacuum birefringence turns the linear po-larization of the laser beam into the elliptical one in pres-ence of the strong magnetic field [12].


field laser experiment to measure vacuum bire-fringence. Such experiments are also necessary in order to test whether strong electromagnetic fields provide windows into new physics.

In the experimental setup proposed in [13] a high-intensity (petawatt class) laser pulse is fo-cused by an f/2.5 off-axis parabolic mirror. A hole is drilled into the parabolic mirror in align-ment with the z-axis in such a way that an X-ray pulse can propagate along the z-axis through the focal region of the high-intensity laser pulse. Using a polarizer-analyzer pair the ellipticity of the X-ray pulse may be detected. In Figure 8: a linearly polarized laser-generated ultra-short X-ray pulse is aligned collinearly with the fo-cused optical laser pulse; after passing through the focus the laser induced vacuum birefrin-gence should lead to a small ellipticity of the X-ray pulse which can be detected by a high contrast X-ray polarimeter [14]; extension of the setup for the generation of counter propagating

laser pulses and a high-intensity standing wave which may be used for pair creation (in gray). The whole setup should be located in an ultra-high vacuum chamber and entirely computer controlled.

Accurate control over spatial and temporal over-lap was convincingly demonstrated carrying out an autocorrelation of the laser pulses at full in-

tensity [15] and generating Thomson backscat-tered X-rays from laser-accelerated electrons [16]. This counter propagating scheme, a table-top “photon collider”, may also be employed for pair creation from the vacuum. For the X-ray probe pulse it was chosen an X-ray source of photon energy ω ≅ 1 keV, since the birefrin-gence signal is proportional to ω2.

X-ray polarimetry is currently sensitive to el-

Figure 8: Proposed experimental setup for the demonstration of vacuum birefringence [14].


Chapter 6


Unruh radiation is a quantum correction to the classical radiation rate that grows large only in situations where quantum fluctuations in the radiation rate become very significant. An ob-server or detector undergoing a uniform accel-eration a experiences the vacuum as a thermal bath with the Unruh temperature T=ha/2πk

Bc, c

is the speed of light, h is the reduced Planck's constant and k

B is the Boltzmann's constant. The

Unruh effect is close to the Hawking radiation at the horizon of a black hole where the observer staying near the horizon sees the thermal bath of the virtual particles whereas observer falling to the black hole does not see any. The linearly accelerated electron will execute a Larmor radi-ation and, as a response, the electron reacts to the vacuum fluctuations with a quivering motion in its proper frame and in turn triggers an addi-tional radiation. At the classical level, the same

lipticities of just about 10−4. Thus it should require intensities in the upper range of ELI specifications (1026 W/cm2). However, the situa-tion changes if polarized photon beams of MeV energies could be produced. Then the signal should increase significantly. Photon sources of this type can be obtained via Thomson or Compton backscattering off an electron beam generated, i.e., via laser wake field acceleration. Backscattering a probe laser (with a

0 < 1) off 3

GeV electrons, for instance, would yield 50 MeV γ -rays.

Unruh radiation linear acceleration induces a Larmor radiation. The Unruh radiation is induced by the reaction to the Larmor radiation and is a minute pertur-bation of it. The space where we are interested in detecting the Unruh radiation is along the ac-celeration direction where the Larmor radiation is the weakest. Therefore the two radiations can be treated as independent processes without interference.

The Unruh radiation can be discriminated from classical Larmor radiation via the different an-gular and spectral distributions and the distinct photon statistics. In the quantum case, the pho-tons are always emitted in pairs with maximally entangled polarizations whereas the classical radiation corresponds to a coherent state with Poisson statistics. The Larmor radiation has a blind spot in forward (and backward) direction where the Unruh radiation is maximal leading to a cone of “quantum domination” with a small angle. The Larmor and the Unruh radiation are


comparable when acceleration exceeds 1030 g.

The signatures of the Unruh effect might be detectable in future laser facilities generating electric fields not too far from the Schwinger limit (corresponding to an intensity of order 1029 W/cm2). However, when a 50 GeV elec-tron collides head-on with a laser beam of field strength E then the field strength in the rest frame of the electron is E’=2γE. Hence if the laboratory field strength of the focused laser beam is E=6.5x1010 V/cm, the field appears to be critical from the point of view of the electron. The corresponding laser intensity is I=1.1x1019 W/cm2 [18].

An experiment was proposed for the indirect signature in the Unruh effect [19]. The uniformly

accelerated detector acts as if it is immersed in a thermal bath, there is a finite probability that it absorbs a virtual particle from this bath and passes to an excited state. This process cor-responds to the emission of a real particle for a non-accelerating observer [20]. The opposite process, when the detector re-emits the virtual particle back into the bath in the accelerated frame and goes back to its ground state, also corresponds to the emission of a real particle for the non-accelerating observer. In the limit case in which the time between absorption and re-emission becomes arbitrarily small, the detector transforms into a scatterer which scatters vir-tual particles from one mode into another mode of the thermal bath in the accelerated frame. This process corresponds to the emission of two real particles by the accelerated scatterer for the non-accelerating observer. This effect is analogous to moving-mirror radiation [21] and can be interpreted as a signature of the Unruh effect.

Figure 9: Two-photon amplitudes for Unruh radiation on the left side and Larmor radiation on the right side [17].

Electromagnetic cascade initiated by very high energy electrons and photons is very important in a number of astrophysical situations. In the early Universe the high energy photons might be produced by some exotic sources such as primordial black holes and unstable heavy par-ticles.

In so-called Inverse Compton Scattering (ICS) a laser pulse collides with an electron bunch (see Figure 10) and forces the electrons to wiggle. This wiggle motion makes the electrons emit highly collimated synchrotron radiation in the forward direction, similarly to an electromag-netic undulator in a conventional synchrotron. The Doppler energy upshift allows one to reach

QED cascades: In-verse Compton Scat-tering


Chapter 6


high photon energies, e.g. 100 MeV γ -rays with a 10-GeV electron beam.

While in linear Compton scattering only the fun-damental wavelength is generated, non-linear ICS allows one to produce higher harmonics where the critical harmonic number scales with the third power of the dimensionless laser am-plitude a

0: nc~a

03. At high intensities one ex-

pects to see a substantial red shift of the usual kinematic Compton edge of the photon spec-trum caused by the large, intensity-dependent effective mass of the electrons within the laser beam. In addition, it was demonstrated that the motion of the center-of-mass frame for a given harmonic becomes intensity dependent. Tuning the intensity then effectively amounts to chang-ing the frame of reference, going continuously from inverse to ordinary Compton scattering with the center-of-mass kinematics defining the transition point between the two [22].

As above reported an important parameter comes into play, the “dimensionless laser am-plitude”, given as the ratio of the electromag-netic energy gained by an electron across a laser wavelength to its rest mass:

This is a purely classical ratio which exceeds unity once the electron’s quiver motion in the laser beam has become relativistic. It is suffi-cient to adopt a useful rule-of-thumb formula expressing a

0 in terms of laser power:

So that a0 is of order 102 for a laser in the pet-

awatt class. If the electrical field amplitude as-sociated with the laser beam stays far below the Schwinger pair-creation threshold, the natural process that comes to mind is the strong-field Compton scattering where a high-intensity beam of laser photons γL collides with an elec-tron beam emitting a photon γ. In this case one has to sum over all n-photon processes of the type:

Figure 10: Inverse Compton scattering scheme.


Then a pair cascades can be initiated by the Compton scattered photons as depicted in Fig-ure 11. The γ photon, in fact, can absorb a fur-ther energy from the laser beam and generate an electron-positron pair. Finally, each of them can in turn absorb energy and generate a γ photon, giving rise to a cascade process.

The physical scenario assumed is the collision of a high-intensity laser beam with an elec-

tron beam of sufficiently high energy γ ≥ 102 produced in a conventional accelerator or by a suitable laser plasma acceleration mechanism. The cross sections or photon emission rates are enhanced with increasing dimensionless laser amplitude. The possible effects are mostly of classical nature, being fundamentally due to the mass shift:

caused by the relativistic quiver motion of an electron in a laser field.

Ranked in order of their relevance the main in-tensity effects are:

1. red shift of the kinematic Compton edge for the fundamental harmonic

2. appearance of higher harmonic peaks n > 1 in the photon spectra

3. possible transition from inverse Compton scattering (ωI > ω) to Compton scattering (ωI < ω) upon tuning a

0.

The redshift may be explained in terms of the larger effective electron mass, m* > m, the generation of which costs energy that is miss-ing when it comes for “boosting” the photons to higher frequencies. To avoid significant energy losses, reducing the photon frequency, the am-plitude a

0 should not exceed a limit value. How-

ever, the generation of higher harmonics might partially overcome this problem. The transition from inverse to ordinary Compton scattering, once a

0 increases beyond 2γ illustrates the

energy “loss” just mentioned. When a0 = 2 the

laboratory frame can be interpreted as an inten-sity-dependent center-of-mass frame for which ωI

n = nω

n, at least for low harmonics. Thus,

Figure 11: Pair cascade induced by inverse Compton scattering.


Chapter 6


there is no longer energy gain of the emitted photons: the laser beam becomes so “stiff” that, in this frame, electrons begin to bounce back from it gaining energy rather than vice versa.

Nonlinear Compton scattering provides a unique testing ground for strong-field QED. ELI pulse power and duration will allow to perform the experiments required for measuring the effects listed above. In fact, ICS radiation produced with the 10-fs ELI laser system will have a duration comparable to the electron bunch duration, which in turn is expected to be on the same time scale as the laser pulse one. The laser am-plitude provided by ELI can be as large as 280 (in the case of 150 PW) and can provide intense beams of ultrafast radiation in the MeV energy region.

A quark-gluon plasma (QGP) is a state of mat-ter in which quarks and gluons are no longer confined to volumes of hadronic sizes. The fundamental theory for the strong interaction, Quantum Chromodynamics, estimates for the critical temperature at which the transition be-

tween ordinary nuclear matter and quark gluon plasma appears. Calculations indicate that the critical temperature should be 175 MeV in the

case of a vanishing baryon chemical potential.

The study of the QGP is also a testing ground for finite temperature field theory, a branch of theoretical physics which seeks to understand particle physics under conditions of high tem-perature. Such studies are important to under-stand the early evolution of our Universe: if the quark-gluon plasma is possible, the conditions of temperature and energy density in the very early stages (the first 25 µs) of the Big Bang would have certainly resulted in such a state existing before the temperature fell as the ex-pansion proceeded and the quark-gluon “soup” froze out into hadrons. This should be crucial to the physics goals of a new generation of uni-verse observations.

In laboratory, in order to produce a quark-gluon plasma, a high density state has to be created by means of symmetric compression by ultra-

Quark-gluon plasmas

Figure 12: At a certain density (temperature) hadrons lose their identity and a system of free quarks and gluons is created (quark gluon plasma) [23]


high intensity lasers or nuclear collisions based on laser accelerating techniques.

The evidence for bulk properties consistent with quark gluon plasma formation are accompanied by large energy density, entropy growth, plateau behaviour of the thermodynamic variables, un-usual expansion and lifetime properties of the system, presence of thermodynamic equilibra-tion, fluctuations of particle number or charge balance. The evidence for modifications of spe-cific properties of particles thought to arise from their interactions with quark gluon plasma can be the modification of widths and masses of resonances, modification of particle production probabilities due to colour screening and modi-fication of parton properties due to interaction with other partons in a dense medium.

In the extremely-high intensity limit the laser pulse-plasma interaction acquires new proper-ties, ranging from the effects of the radiation

friction force and Quantum Electrodynamics effects to nonlinear vacuum polarization and electron positron pair creation [24]. In this limit novel mechanisms of the ion acceleration come into play. Since the energy of the resulting ion bunch can be over 100 GeV per nucleon, this ion acceleration regime is suitable for quark-gluon plasma studies [25] and in connection with an application to the investigation of neu-trino oscillations [26].


Chapter 6


Potential for applications, business and technology transfer

First of all, it should be noted that nowadays the exotic physics is far away from any imaginable application and a huge effort in this fundamen-tal research field has to be done. The ultra-high, ultra-intense laser pulses themselves will be the main tool for experimental studies relevant to QED. Ultra-intense lasers have a unique poten-tial as an effective source of electron-positron plasmas due to laser matter interaction and/or vacuum nonlinear effects. The positrons can be separated from the electron-positron pair plasmas and might be further utilised e.g. for positron emission tomography. The nature of the vacuum under extreme conditions can be also one of the most important subjects to be studied. At the certain intensity threshold the vacuum acts in a nonlinear regime as a dielec-tric or birefringence material due to the effects of the virtual particles.

The interaction of high intensity laser pulses with underdense and overdense plasmas pres-

ents a manifestation of one of the most basic nonlinear processes in physics: the high-order optical harmonic generation. High order optical harmonics have been observed in laser plasma interactions with radiation intensities ranging from moderate up to relativistic intensities. Non-linear orders as high as 300 have been report-ed recently [24]. In addition to its fundamental interest in the theory of nonlinear waves, this radiation presents unique properties of coher-ence and short pulse duration that makes high order harmonics a useful XUV source of short coherent radiation such as EUV for lithography, holography, etc.

The photon-photon collider is considered as the best tool for addressing and discovering new physics: Higgs physics, extra dimensions, supersymmetry, and top quark. In a photon col-lision any charged particles can be produced [24]. The cross sections for pairs are signifi-cantly higher than in the electron-positron col-

lisions. The γ-γ collider relies on the scattering of photons from a high-intensity laser by an ultrarelativistic electron beam. After scattering, photons have energy close to the electron en-ergy [24] and the process efficiency is excel-lent with one electron scattering one γ photon. The photon beams after focusing correspond approximately to the electron beam size. These are additional meeting points of laser and high energy charged particles. In some of these ap-plications it is possible to explore nonlinear QED and also to produce large amounts of high-energy γ-photons through the inverse Compton scattering process useful for high-energy and nuclear physics [27]. Tajima proposed the pro-cess for realizing a possible nuclear transmuta-tion in combination with efficient lasers such as the free-electron laser [28].

A gamma ray initiating an electron-positron pair production has a necessary condition of pres-ence of electric field which has to reach the


Schwinger limit. With the presence of the trident process (a process in which a nucleus plays the role of an additional photon in the interaction among electrons and photons), the electric field condition is relaxed [29-31] at an intensity level directly accessible (1022 W/cm2). γ-rays may be also used in photonuclear physics. For example, such a photon interacting with nuclear matter may lead to a new field of investigation which relies on the coupling between weak and strong interactions [32]. Moreover, polarized γ-photons may be used for creation a large flux of polar-ized positrons, which may have an important role in future collider-beam sources to enhance the signal-to-noise ratio of desired events [33].

In experiments conducted on solid targets high-energy electrons have been generated, leading to the creation of high-energy gamma photons in the solid by the bremsstrahlung process. These gamma rays might be possible to induce a nuclear transmutation [34]. The production of

energetic electrons as well as acceleration of other particles can be provided for the creation of substantial numbers of isotopes; this tech-nique has already been demonstrated [35-38]. Transmutation in the minor actinides may be carried out via a new fission decay mechanism in which the vibrational levels created by the hyperdeformation of nuclei (as in the formation of isomers) are populated [39]. Such a process may be initiated by gamma rays generated by inverse Compton scattering of the laser pulse with high energy electron beam. The produced gamma rays can induce various (γ,n) nuclear processes, as opposed to the more common (n, γ) processes which can be seen in nature.

A further advantage of high intensity laser field is the application in gravitational field effects that could be measurable in laboratory accord-ing to the main postulate of general relativity, the Einstein principle of equivalence. The effect of a homogenous gravitational field is equiva-

lent to that of a uniform accelerated reference frame. In the past various experiments have been performed in order to test the equivalence principle in its weak limit in the laboratory us-ing neutron beams with a spinning mirror [40]. By using ultra-high ultra-intense lasers it should be possible to test the equivalence principle in its strong limit. The theory of quantum gravity has been recently advanced [41-46], stating that gravitational effects, having extra dimen-sions, could be observed over macrodistances. Ultra-intense lasers could provide a new way to test extradimensional effects. Probably for a sufficiently intense laser field the distance of the electron to its horizon might become on the or-der (or even smaller) of the proper distance over which the effects of extra dimensions could be observed.

The entire universe began with an explosion of energy, the Big Bang, and accelerator physicists have witnessed the conversion of energy into


Chapter 6


matter (virtual photons) by smashing atoms to-gether. But such virtual photons are not under the direct control of physicists; these photons arise as part of a complex chain of events start-ing with a collision of two particles of matter. The conversion of light into matter could give particle physicists a new source of positrons that are exceptionally uniform in energy and momentum. Moreover, an electron-positron pair beam can be accelerated and focused by a properly shaped ponderomotive potential. The high-field experiments at ELI facility could shed light on phenomena at the surface of neutron stars, where magnetic fields are very strong, as well as in other exotic astrophysical settings.


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Facts and Figures

Facts and Figures

1. Members of Consortium ELI-CZ

2. Strategic location of the ELI facility in Europe

Existing and projected facilities as of September 2009

171www.eli-beams.euFacts and Figures

Facts and Figures


3. Overview of the high-power laser facilities worldwide

Worldwide High Power Laser facility overview. Existing and projected facilities as of September 2009

Facts and Figures

4. Overview of the high-power laser facilities in Europe

Existing and projected facilities as of September 2009


Facts and Figures


5. Positioning of high power laser facilities with respect to pulse duration and peak power

Facts and Figures

6. Positioning of high power laser facilities with respect to pulse energy and peak power


Facts and Figures


7. World Optics and Photonics Clusters

World Optics and Photonics Clusters as of September 2009

Facts and Figures

8. European Optics and Photonics Clusters

European Optics and Photonics Clusters as of September 2009

Facts and Figures

List of Abbreviations

PFS Petawatt Field SynthesizerOPCPA Optical Parametric Chirped Pulse AmplificationDPSSL Diode Pumped Solid-State LasersPLC Programmable Logic ControllerPCF Photonic Crystal FiberKDP Monopotassium phosphateDKDP Deuterated potassium dihydrogen phosphateCW Continuous WaveYAG Yttrium Aluminium GarnetCPA Chirped Pulse AmplificationASE Amplified Spontaneous EmissionOPA Optical Parametric AmplificationYCOB Yttrium Calcium Oxyborate LBO Lithium triborateOPO Optical Parametric OscillatorPPF Pulse-to-Pulse FluctuationI/O Input/OutputLAN Local Area NetworkUPS Uninterruptible Power SupplyHW tools Hardware toolsSW tools Software toolsMLD Multi-Layer DielectricXFEL X-ray Free Electron LaserFEL Free Electron LaserUV UltravioletIR InfraredVIS VisibleDNA Deoxyribonucleic AcidGRIP Grazing Incidence PumpingPMMA Poly(methyl methacrylate)HSQ Hydrogen SilsesquioxaneCNTs Carbon NanoTubesLLNL Lawrence Livermore National LaboratoryFWHM Full Width at Half Maximum

EUV Extreme Ultra-VioletRIXS Resonant Inelastic X-ray Scattering PTFE Polytetrafluoroethylene (teflon)FEP Fluorinated Ethylene PropyleneNIR Near InfraredR&D Research and DevelopmentLCLS Linac Coherent Light SourceEPR Electron Paramagnetic ResonanceVUV Vacuum UltravioletXUV Extreme Ultra-VioletICF Inertial Confinement FusionCPA Chirped Pulse AmplificationHEDP High Energy Density PhysicsKEEN waves Kinetic Electrostatic Electron Nonlinear wavesKEIN waves Kinetic Electrostatic Ion Nonlinear wavesSRS Stimulated Raman ScatteringSBS Stimulated Brillouin scatteringPIC simulation Particle-in-Cell simulationRPA Radiation Pressure AccelerationFCT Fast Current TransformerICT Integrating Current TransformerOTR Optical Transition RadiationTNSA Target Normal Sheath AccelerationQED Quantum ElectrodynamicsWDM Warme Dense MatterLANL Los Alamos National LaboratoryLANSCE Los Alamos Neutron Science CenterGANIL Grand Accélérateur National d’Ions LourdsSNS Spallation Neutron SourceSLAC Stanford Linear Accelerator CenterICS Inverse Compton ScatteringQGP Quark-Gluon Plasma

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