Acceleration with Self-Injection for an All-Optical Radiation Source at LNF

L. A. Gizzi a,b) , M. P. Anania c) , G. Gatti c) , D. Giuliettia,b,d) , G. Grittania,b,d) , M. Kandoe) , M. Krus f ) , L. Labatea,b) ,T. Levatoa, f ,g) , Y. Oishi(g,b) , F. Rossii)

a)ILIL, INO-CNR, Via Moruzzi,1 56124 Pisa, Italyb)INFN, Sez. Pisa, Largo B. Pontecorvo, 3 - 56127 Pisa, Italy

c)Laboratori Nazionali di Frascati, INFN, Via E. Fermi, Frascati, Italyd)Dipartimento di Fisica E. Fermi, Universitá di Pisa, Italy

e)Japan Atomic Energy Agency (JAEA) Kyoto, Japanf )Fyzikální ústav AV CR v.v.i., Praha, Czech Republic

g)U. Tor Vergata, Roma, Italyh)CRIEPI, Kanagawa, Japan

i)Universitá di Bologna e Sez. INFN, Bologna, Italy

Abstract

We discuss a new compact γ-ray source aiming at high spectral density, up to two orders of magnitude higher than currentlyavailable bremsstrahlung sources, and conceptually similar to Compton Sources based on conventional linear accelerators. Thisnew source exploits electron bunches from laser-driven electron acceleration in the so-called self-injection scheme and uses acounter-propagating laser pulse to obtain X and γ-ray emission via Thomson/Compton scattering. The proposed experimentalconfiguration inherently provides a unique test-bed for studies of fundamental open issues of electrodynamics. In view of this, apreliminary discussion of recent results on self-injection with the FLAME laser is also given.

Keywords: ultra-intense laser-matter interactions, X-ray sources, γ-ray sourcesPACS: 52.38.Kd, 41.75.Jv, 52.25.Fi

1. Introduction

The impressive progress of high power laser technology ini-tiated by the introduction of the Chirped Pulse Amplification(CPA) concept [1] is now leading to the realization of new largelaser systems within the framework of the Extreme Light Infras-tructure (ELI) that, by the end of this decade, will start pavingthe way to the exploration of new physical domains, approach-ing the regime of electron-positron pair creation and the possi-bility to reach the critical field of quantum electrodynamics[2].At the same time, the control of ultra-high gradient plasma ac-celeration [3, 4, 5] is being pursued and advanced schemes arebeing proposed for the future TeV linear collider [6].

Meanwhile, existing laser-plasma accelerating scheme arebeing considered for the development of novel radiationsources. All-optical X-ray free electron lasers (X-FEL) are al-ready being explored [7] with encouraging chances of successin the short term. All-optical, laser-based bremsstrahlung X-rayand γ-ray sources have already been explored [8, 9] and suc-cessfully tested using self-injection electron bunches [10, 11]showing high efficiency and potential for laboratory applica-tions.

However, in order to positively enter the domain of nuclearapplications, significantly higher energy and spectral densityγ-rays are required. In this scenario, a very demanding ap-plication is the Nuclear Resonance Fluorescence (NRF) due

Email address: la.gizzi@ino.it (L. A. Gizzi a,b))

to the small spectral width of nuclear resonance transitions.New sources have been developed [12] based upon the use ofhigh energy LINACS and high power lasers or free electronlasers[13] to generate γ-rays via Thomson/Compton scattering.In this scenario, the use of laser-plasma accelerated electronshas also been explored [14] and is regarded as a possible wayto make nuclear sources far more accessible than current Linacbased sources.

In a pioneering experiment [15] carried out at the Jena laserFacility in 2006, all-optical Thomson scattering (TS) in the1keV X-ray region was demonstrated using a compact config-uration with a relatively low degrees of freedom for optimiza-tion and using poor quality laser-accelerated electron bunches,still affected by 100% energy spread. Since then, laser-plasmaacceleration has seen dramatic advances and laser acceleratedelectrons can now exceed 1GeV with energy spread well below10%, with record values close to 1%. Moreover, new schemesare being proposed to control injection and optimize accelera-tion, which are now being implemented to further improve thequality of laser accelerated electrons.

2. Thomson scattering source

An all-optical scheme is proposed here in which a laser-driven electron accelerator based on the design of the self in-jection test experiment (SITE) [16] is used to deliver elec-tron bunches required to generate γ-rays in a Thomson back-

Preprint submitted to Nuclear Instruments and Methods B January 1, 2013

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scattering configuration[17]. TS from free electrons is a pureelectrodynamics process in which each particle radiates whileinteracting with an electromagnetic wave. From the quantum-mechanical point of view TS is a limiting case of the processof emission of a photon by an electron absorbing one or morephotons from an external field, in which the energy of the scat-tered radiation is negligible with respect to the electron’s en-ergy. If the particle absorbs only one photon by the field (thelinear or non relativistic quivering regime), TS is the limit ofCompton scattering in which the wavelength λX of the scat-tered photon observed in the particle’s rest frame is much largerthan the Compton wavelength λ = h/mec of the electron. Sinceλc/λX << 1, the TS process can be fully described within clas-sical electrodynamics both in the linear and nonlinear regimes.

TS of a laser pulse by energetic counter-propagating elec-trons was initially proposed in 1963 [18, 19] as a quasimonochromatic and polarized photons source. With the devel-opment of ultra intense lasers the interest on this process hasgrown and the process is now being exploited as a bright sourceof energetic photons from UV to γ-rays and atto-second sourcesin the full nonlinear regime.

As shown in Figure 1, the proposed source is based upona counter-propagating configuration of two ultraintense laserpulses focused in a gaseous target. The counter-propagatingconfiguration is obtained by splitting the main laser pulse in twopulses with controlled energy and independent focusing con-figuration. In this configuration, one of the pulses propagatestowards the electron bunches generated by the other pulse.

Figure 1: Schematic view of the counter-propagating configuration for all-optical X/γ-ray generation. The main FLAME laser pulse is split in two pulseswhich are focused in the proximity of a gas target. One of the two pulses (right)accelerates electrons. The other pulse is focused on the accelerated electronbunch to scatter off radiation which is emitted along the electron bunch propa-gation direction.

The other, counter-propagating pulse interacts with the ener-getic electrons generating radiation along the bunch propaga-tion direction. An additional laser pulse, the auxiliary "probe"pulse of the FLAME system is then transported to the targetchamber and is used to diagnose the plasma density before andduring the interaction.

2.1. Source parametersThe three main parameters of the Thomson scattering of a

laser pulse by a free electron are the particle energy γ0, theangle αL between the propagation directions of the pulse andthe electron and the laser pulse normalized amplitude a0 =

8.5× 10−10√

Iλ2 where λ is the laser wavelength in µm and I isthe laser intensity in W/cm2. The pulse amplitude a0 controls

the momentum transferred from the laser pulse to the electron,i.e. the number of photons of the pulse absorbed by the elec-tron. If a0 << 1 only one photon is absorbed and the quiveringis non-relativistic (linear Thomson scattering). For an electroninitially moving with γ0 >> 1 the resulting scattered radiationis emitted forward with respect to the electron initial motionwithin a cone of aperture 1/γ0 and is spectrally shifted at a peakwavelength given by:

λX,γ ' λ1 − β cos θ

1 − β cosαL(1)

where β = v/c is the particle velocity and θ and αL are definedaccording to the geometry of Figure 2. Among the possible

Figure 2: Thomson scattering geometry. The scattered radiation is emittedalong the z axis, in a small cone of aperture 1/γ0. When αL = π the backscat-tering geometry occurs.

interaction geometries, the case of backscattering αL = π isthe most suitable for a source as it produces radiation with thehighest energy:

EBack ' 4γ2E0, (2)

where E0 is the energy of laser photons. According to Equa-tion 2, for a laser wavelength of 0.8 µm, the electron energyrequired to achieve photons with energy of 50 keV, 500 keVand 1 MeV are 46 MeV, 145 MeV and 205 MeV respectively,which are within the accessible electron energy of laser-plasmaacceleration with self-injection. Moreover, the head-on config-uration allows the highest overlap of the electron beam and thepulse and minimizes spurious effects induced by the transverseponderomotive forces of the laser pulse.

In the nonlinear regime (a0≥1) the strong exchange betweenthe laser pulse and particle momentum induces a relativisticelectron motion, consisting of a drift and a quivering havingboth longitudinal and transverse components with respect tothe pulse propagation. In turn, the time dependent longitudi-nal drifting results in a non harmonic electron motion, thus pro-ducing scattered radiation with a complex spectral distributiontoo. If the electron interacts with a laser pulse with a flat-toptemporal shape, the spectral distribution of the scattered radia-tion is organized in equally spaced harmonics. The peak energyof each Nth harmonics depends now on both the energy of theparticle and the pulse normalized amplitude a0 and is given byEN ∼ N4γ2

0E0(1 + 1/2a20)−1.

It can be shown that the minimum bandwidth of a γ-ray beamproduced by a back-scattering source depends upon the electron

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energy spread ∆γ0/γ0, the electron rms transverse emittance εn,the beam spot size at the collision point σx, the reduced laserintensity a0 and the laser spectral bandwidth ∆νL/νL accordingto the equation:

∆νγ/νγ ' 2∆γ0/γ + 2(εn/σx)2 + a20(1 + a2

0) + ∆νL/νl (3)

Taking typical values of a self-injected electron beam, as thosecurrently produced in the bubble regime [20, 21], at the exit ofthe plasma, we have that the first two terms in Equation 3 are inthe range 1-10%, the laser bandwidth is typically of the order of10%. A collision laser pulse with a0 < 1 will then be requiredto avoid further increase of the bandwidth for the γ-ray beam.Overall, we can therefore expect a bandwidth in the range of5-10% at the best for an optimal arrangement, corresponding to500 keV to 1 MeV bandwidth for 10 MeV photons. In thesecircumstances, the rate of photon generation is given by

Nγ = 2.1 × 108UL[J] Q[pC] hν−1[eV] σ−2x [µm] f , (4)

where UL[J] and Q[pC] are the laser pulse energy and the elec-tron bunch charge. Assuming a typical value for the bunchcharge Q = 1000 pC, a repetition rate f = 10 Hz, a matched fo-cal spot diameter σx = 8 µm and a laser pulse energy UL = 2 J,we obtain Nγ = 3.5 × 1010 photons/sec over the entire solid an-gle and spectral bandwidth. This corresponds to approximately108 photons/sec within a 10% bandwidth. Finally, based on cur-rent optimum performance of acceleration with self-injection,spectral intensity at 1 MeV bandwidth would be approximately102 photons/sec/eV.

If we consider other laser-plasma acceleration schemes, e.g.those based upon capillary discharge gas targets, we can expecta narrower bandwidth and lower transverse emittance [22]. Thisis done at the expenses of the charge available in the acceleratedbunch, typically in the few pC range. This is a crucial aspect ofthe proposed experiment and will require further development.

3. Towards high fields effects.

In the general description of the interaction of a charged par-ticle with an external e.m. field, the emission of radiation by theaccelerated particle gives rise to a back-reaction, the so calledRadiation Reaction (RR) also referred to as radiation friction.In the usual approach, the problem is solved in steps. In the firststep, the motion of an electron is calculated using the Lorentzforce for the given external EM fields. In the second step, theradiation emitted by the electron can be calculated given its mo-tion. This two-step process is non self-consistent because it ne-glects the back-reaction on the electron by the EM fields gen-erated by its motion. To make the electron motion consistentwith the emission of radiation which carries away energy, mo-mentum and angular momentum, it is found that an additionalterm, the RR force, must be added to the Lorentz force.

However, in most cases, the non self-consistent approach stillprovides a very accurate description of the particle dynamics.This is the case for example, of particle accelerators and ofmost laboratory and astrophysical plasmas. These conditions

are preserved provided that the energy radiated by the particleis small compared with the energy of the system. It can beshown that in the case of a charged particle oscillating in an ex-ternal field, radiation dominates when the motion of the particlechanges appreciably in a time t = 10−24 s, i.e. over a distancect = 10−13 cm. Interestingly, the latter distance is comparablewith the classical electron radius. These considerations imme-diately tell us that the possibility of accessing this regime in thelaboratory is extremely challenging. Ultraintense, femtosecondlaser pulses are regarded as a possible tool to enter this regimeand future laser systems currently under construction are ex-pected to make this opportunity even more realistic.

The question of a description of the RR force is a long-standing, highy controversial and, to a large extent, still un-resolved, facing issues of electrodynamics that ultimately dealwith the actual structure of the electron charge, its nature andthe role of quantum effects [23]. The perspective of probingRR effects with ultra-intense lasers has further stimulated theinterest in these problems and alternative models to describeRR have been proposed, revitalizing the long standing debate.Probably the simplest, but already effective approach is that ofLandau and Lifshitz (LL) [24] which is free from pathologiesapparent in other models. Currently, the effort is not limitedto a rigorous formulation of the RR force on the single elec-trons but it is also devoted to understand how RR may be im-plemented consistently and effectively in a many-body system,i.e. in a plasma. Finally, experimental validation is needed todiscriminate among available theoretical and numerical modelsand a road-map for this validation is now being established atthe main high-power laser facilites world-wide.

3.1. Seeking experimental evidence of Radiation Reaction

A theoretical study [25] of the motion of an ultrarelativisticelectron in an ultraintense EM field, based on the LL equation,has characterized a Radiation dominated regime as the systemin which the energy gain by the electron equals the radiationloss. As anticipated above, it can be shown that this condi-tion occurs when the dimensionless field amplitude a0 > 400which implies a laser intensity of IL > 1024 W/cm2. This isa very high laser intensity, 3 orders of magnitude above cur-rent capability of existing laser systems. However, ultraintenselaser technology is providing increasingly high electromagneticfield intensities. It is therefore foreseeable that the next gener-ation of high power laser systems will allow this regime to beaccessed relatively soon. On the other hand, according to re-cent models based upon LL equations, it is predicted that ex-perimental evidence of RR effects can be obtained in Thomsonscattering configurations at relatively lower laser intensities, be-low the foreseen threshold for the RR-dominated regime, andnot far from the maximum intensities available from PW-classlaser systems. In addition, other models [26] predict laser pulseintensification and shortening in a self-injection laser wakefieldacceleration configuration which could enhance the effect andmake RR accessible at existing laser facilities.

In a recent theoretical paper [27], the interaction betweena superintense laser pulse at 5 × 1022 W/cm2 and a 40 MeV

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counter-propagating electron was investigated looking at the ef-fect of Radiation Reaction on the spectrum of Thomson Scat-tered radiation. According to this study, the angle and fre-quency resolved spectra show signatures of RR dependent ef-fects on the angular distribution of scattered radiation. Thesestudies confirm that, in principle, anomalies in the Thomsonemission due to RR are not far from the capabilities of currentlaser systems. In view of this, feasibility studies towards theexperimental realization may be already investigated, startingfrom a dedicated, start-to-end simulation of the entire interac-tion configuration. Moreover, from an experimental viewpoint,control of the laser-plasma acceleration process is necessary toestablish parameters of the accelerated electron bunch to be in-cluded in the simulations.

From the modelling viewpoint, this approach uses numeri-cal tools capable of describing the dynamics of electron beamacceleration and interaction with the counter-propagating pulsein a realistic geometry and incorporating RR effects. Recently,several self-consistent simulation studies incorporating RR inlaser-plasma interactions via the LL force in a Particle-In-Cell(PIC) code have been performed (see e.g. [28]). Numericalefficiency is of a paramount importance for such simulations(which require large supercomputers) because in a PIC codethe run time mostly depends of the calculation of particle ac-celeration, hence on details of the force term. The numericalimplementation of Ref.[28] proved to be efficient enough to al-low fully three-dimensional simulations and was thus suitablefor full-scale simulations for the experiment proposed in [29].A similar approach is being followed for the full modelling ofthe proposed radiation source.

4. Acceleration with Self-injection at FLAME

In the framework discussed above, the FLAME laser sys-tem has recently been commissioned at LNF and experimen-tal runs dedicated to laser-plasma acceleration experiment withself-injection (SITE) have already been carried based on previ-ous pilot experiments [30]. The first run was carried out duringthe early commissioning stage in 2010, at relatively low laserpower (≈10 TW), and showed successful generation of mono-energetic, high energy electron bunches with a moderate to highdegree of collimation down to the 6 mrad level [31]. A secondexperiment at higher laser power (≈100 TW) was carried out inJuly 2012 and enabled us to further explore the planned exper-imental configurations [16, 32]. A detailed description of theseresults will be given elsewhere. Here give an overview of theexperimental set up with some preliminary highlights of the re-sults, with focus on the stability and the reproducibility of theobserved acceleration process in view of the application to theproposed TS radiation source.

In the self-injection scheme proposed here, electron bunchesare generated from laser-plasma interaction with a rectangu-lar gas-jet of a few millimeters in the so-called bubble regime[20, 21]. In this regime, a short (cτ < λp/2) and intense (a0> 2) laser pulse rapidly ionizes the gas [33, 34] and expelsthe plasma electrons outward creating a bare ion bubble. The

Figure 3: Schematic layout of the laser-gas-jet interaction for the production ofenergetic electron bunches in the self-injection configuration used in the SITEexperiment at LNF. We used two different gas-jet length of 10 mm (longitudinalpropagation) and 1.2 mm or 4 mm (transverse propagation).

blown-out electrons form a narrow sheath outside the ion bub-ble and the space charge generated by the charge separationpushes the electrons back creating a bubble-like wake. For suf-ficiently high laser intensities (a0 ≥ 3-4) electrons at the back ofthe bubble can be injected in the cavity, where the longitudinalaccelerating field is of the order of 100 ∆n [cm−3]V/m, where∆n is the amplitude of the local electron density depression inthe wake.

The FLAME laser meets both conditions of short pulse du-ration and high intensity required to achieve this condition.When the laser pulse impinges onto the gas-jet it promptly ex-cites (without significant pulse evolution) a bubble wake whereelectrons are readily injected leaving the almost entire gas-jet length for the acceleration process. A proper choice ofplasma and laser parameters to ensure an optimized acceler-ation process can be obtained using a phenomenological the-ory [35] to account for dephasing and depletion of the laserpulse. The basic working point currently under considerationfor the self-injection configuration at FLAME is the one de-scribed in Table 1. In this case, following the phenomenologicaldescription, at the optimum laser performances we can expecta quasi monochromatic (few % momentum spread) bunch witha charge of ≈ 0.6 nC and an energy of approximately 1.0 GeVafter 4 mm propagation.

Figure 4: A possible working point of the self-injection test experiment atFLAME for laser-acceleration in the GeV scale.

This scenario is confirmed by 3D PIC simulations performedwith the fully self-consistent, relativistic, electromagnetic PICcode ALaDyn [36, 37]. Infact, simulations predicts a bunchwith an energy of 0.9 GeV, a momentum spread (rms) of 3.3%,a bunch charge of 0.6 nC, a bunch length of 1.8 µm (the averagecurrent is 50 kA) and a beam divergence (rms) of 2.8 mrad.

More recently these results were basically confirmed by sim-ulations carried out using the 3D GPU particle in cell code. Infact, advanced numerical tools in the modelling scenario usecomputer architectures based upon graphical processing units(GPU) and are proving to provide much faster simulations [38].Indeed, by using a 32-GPU cluster it was possible to performnumerical simulations of a 4 mm gas-jet in approximately twodays, reaching the same accuracy as previously obtained withthe code Aladyn [36]. The plots of Figure 5 show the longi-tudinal field (left) and the electron energy gain (right) for the

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full SITE case of laser intensity of 5×1019 W/cm2 on a matchedplasma electron density of 3×1018 cm−3. As shown in Figure

Figure 5: Simulation results of the numerical code JASMINE showing the out-put for self-injection for optimized performance of the FLAME laser systemand a gas-jet intensity of 3×1018 cm−3. on a 4 mm gas-jet.

5(left) the accelerating field exceeds the 0.3 TV/m in an ac-celerating structure of approximately 30 µm in diameter. Theelectron energy after the 4 mm acceleration peaks at approxi-mately 830 MeV, with an energy spread of 6%. In addition, alow energy component is also visible as a well separated com-ponent. The simulation also yields the angular divergence andthe charge of the high energy component which are found to beapproximately 8 mrad and 0.6 nC respectively.

From an experimental perspective, the working point givenabove requires the optimized performance of the FLAME lasersystem, with special attention to the transverse phase and, there-fore, to the quality of the focal spot. This is of a particularconcern when operating the laser at the maximum output laserenergy of 7 J and will require installation of an adaptive optics.In the mean time, preliminary experimental runs were carriedout a maximum output laser laser energy of 4 J. Up to this en-ergy level of energy no significant phase front distortion wasfound to occur and the Strehl ratio was measured to be > 65 %.Taking the flat top beam size of 90 mm diameter (correspond-ing to a 120 mm diameter aperture) and assuming an M2 = 1.5,we have a maximum nominal laser intensity on target of 2×1019

W/cm2.

5. First experimental run on self-injection

A stable regime of production of collimated bunches was es-tablished during the first SITE run in 2010 using the transversegas-jet configuration of Figure 4 with nitrogen, working with aback pressure of about 17 bar corresponding to a maximum gasdensity of approximately 1 × 1019 atoms/cm3. These prelimi-nary data were obtained using an F/10 focusing optics, at a fixedpulse duration of about 30 fs and for an energy per pulse rang-ing from a minimum of about 300 mJ up to 1 J, corresponding toa laser intensity on target ranging from 3×1018 W/cm2 to a max-imum of 9×1018 W/cm2. Figure 6 shows a sample of the data inwhich electron bunches with a divergence in the range between5 and 30 mrad were systematically accelerated. Preliminaryelectron energy measurements obtained initially with a stackof radiochromic films using a technique discussed in [39] con-firmed that the energy of electron accelerated at 1 J laser energy

Figure 6: Images of the accelerated electron bunch on a scintillating screen(Lanex) showing the production of collimated electron bunches of minimum (5mrad) and maximum (30 mrad) divergence. Images were recorded during the1st SITE run.

were already in the 100 MeV range. In addition, a spectrome-ter consisting of a magnetic dipole was used to obtain a morequantitative estimate of the electron energy. It was based upona couple of permanent magnets mounted on a C-shaped ironstructure to form a closed magnetic loop with a 5 mm gap witha quasi uniform magnetic field of 1 T. Electrons were set topropagate across this magnetic field region where are deflectedaccording to their energy and then land on a position on theLanex screen according to the plot of Figure 7. According tothis plot, electrons of 50 MeV experience a deflection of ap-proximately 80 mm on the screen and this deflection becomes10 mm at 500 MeV.

Figure 7: Dispersion curve of the permanent magnet spectrometer showing theenergy of electrons vs. landing position on the Lanex scintillating screen. Thevertical arrow indicates the minimum detectable deflection.

By using this spectrometer it was possible to observe themain spectral features of the accelerated electrons, increasingthe laser energy at fixed pulse duration and focal spot. Analy-sis of these spectra showed that the highly collimated electronbunches have a typical electron energy of 100 MeV, consistentwith the value measured using the RCF technique. Data werealso taken at higher laser energy, up to the value of 2.5 J be-fore compression. These additional measurements enabled usto identify the role of phase front distortions affecting the laserpulse during the first FLAME commissioning phase. In fact,further increasing the laser energy above the 1 J level showedevidence of beam break up in the focal region which preventedlaser intensity to increase as expected. As discussed below,these issues were identified and partially corrected during thesecond SITE run.

6. Second experimental run on self-injection

A second SITE run took place in July 2012 and was fo-cused on a more systematic study of the acceleration process

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for different gas type, gas density using both the transverse andlongitudinal gas-jet configurations of Figure 4. We used nitro-gen and helium with a back pressure ranging from 1 to 17 bar,corresponding to a maximum gas density approximately in therange from 6× 1017 and 1019 atoms/cm3 and a maximum laser-intensity on target of 2×1019 W/cm2. A total of 3000 shots withe-bunch production was recorded and data were taken with anoptical imaging system and Lanex screen placed at 475 mmfrom the gas-jet to measure both the electron bunch transversesize and, with the insertion of the 50 mm long magnetic dipoleat 132.5 mm from the gas-jet, the energy spectrum. In addition,a shadowgraph of the image was taken to measure the longi-tudinal extent and the transverse size of the interaction region.The imaging system was set to view the interaction region inthe vertical direction, i.e. along the axis perpendicular to thelaser polarization plane to image out the Thomson scattered ra-diation. The plot of Figure 8 shows the typical Thomson imageobtained from measurements with the longitudinal gas-jet con-figuration. As shown by the arrow, the laser propagates fromleft to right showing a clear Thomson emission (red-yellow inthe color figure) in the first 3.2 mm (FWHM) of the propaga-tion. Taking into account the laser beam parameters, we expecta depth of focus of approximately ± 260 µm. Therefore, ac-cording to the image of Figure 8, we find that laser propaga-tion occurs over a propagation length which is several times thedepth of focus.

Figure 8: Images of the Thomson emission from propagation of the laser pulsein a Nitrogen gas-jet at 10 bar valve pressure. The laser propagates from left toright. Also visible in the image is the plasma self-emission.

Propagation length was found to be dependent on the gasdensity and pressure, ranging from approximately 1.3 mm(FWHM) for 70% Nitrogen gas mixture (air) at 1 bar pressure toless than 2 mm (FWHM) for Helium at 15 bar pressure. Theseresults indicate that propagation length increases at higher elec-tron density where stronger refraction effects may occur on thepropagation of the laser pulse in the plasma. At this stage wecan anticipate possible contribution of self-focusing effects tothe observed behavior of the laser pulse. In fact, according tothe well known expression for the critical power for relativisticself-focusing, Pcr ≈ 17(ω/ωp)2 GW and taking into account theestimated maximum electron density given above, we find thatthe critical power in our experimental conditions ranges from3 TW for the highest density case to approximately 50 TW ofthe lowest density case. In the case of 1J of laser energy ontarget, corresponding to a laser power exceeding 30TW, we ex-pect the interaction in the higher density case to be affected byself-focusing that could set the conditions for a moderate chan-neling of the laser pulse, thus effectively extending the propa-

gation length. Detailed numerical simulation will be necessaryfor a confirmation of this result.

Information about the accelerated electron bunches was ob-tained by using the LANEX scintillating screen to measure boththe angular divergence and the energy spectrum. The imageof Figure 9 (left) shows the typical image of a single electronbunch obtained from optimized acceleration in Nitrogen. Ac-cording to this image, the single bunch exhibits a divergenceof approximately 1 mrad FWHM, a value significantly smallerthan that measured during the first SITE run and among thesmallest values measured in similar experiments. The image ofFigure 9 (right) shows instead the same image integrated over30 laser shots which gives an overall cone of emission of ap-proximately 10 mrad HWHM. The latter measurements givesan indication of the shot-to-shot pointing stability of the elec-tron bunch. As for the origin of this fluctuation, it is unlikely tobe affected by the laser pointing stability which was measuredto be within the µrad range. The oscillations of the electronbunch inside the accelerating structure [40] is instead being ex-plored as a possible explanation of the observed limit to thebunch pointing stability.

Figure 9: Left: Image the scintillating LANEX screen at 47 cm downstreamthe interaction point showing the bunch transverse size of approximately 0.5mm and a corresponding bunch divergence of 1 mrad. Right: integrated imageof the electron bunch over 30 laser shots showing a total pointing stability ofapproximately 10 mrad. The bottom-left plot shows the lineout of the imageacross the vertical and horizontal directions.

Finally, information about the energy of the electron bunchwas obtained by inserting the permanent magnet dipole down-stream from the laser focal position. The image of Figure 10shows a typical spectrum of acceleration in Nitrogen in thesame conditions of Figure 8 and Figure 9 above. The white dot

Figure 10: Raw electron spectrum of a typical bunch accelerated from Nitrogengas-jet. The spectrum shows a main component above 100 MeV and a lowenergy tail extending down to 60 MeV. The white dot indicates the landingposition of undeflected electron.

in the centre of the image indicates the average landing positionof the electrons without the magnetic dipole. In the presence ofthe magnet, electrons will be deflected on the l.h.s. of the screenaccording to the dispersion curve of Figure 7. According to this

6

image, the spectrum shows a main component around 150 MeVand a low energy tail extending down to 60 MeV. This generalbehavior was quite reproducible from shot to shot and accuratedeconvolution of all the spectra is currently in progress.

7. γ-RESIST conceptual set up

As shown in Figure 1, the experimental set up is basedupon two counter-propagating laser pulses focused on a gastarget using two off-axis parabolic mirrors. The two counter-propagating pulses are obtained by splitting the main laser pulseusing a very thin splitter to minimize detrimental effects on thelongitudinal phase of the transmitted pulse. The reflected pulsewill be sent to the “self-injection” arm (right) of the counter-propagating configuration, while the transmitted pulse will besent to the “Thomson/Compton scattering” arm. Given thesmall thickness of the splitter, it will be likely affected by flat-ness distortions which may induce phase distortions in the re-flected pulse. The planned full beam adaptive optics will beused to compensate for this distortions and optimize both focalspots. The “self-injection” arm will be based upon the currentset up and will use the existing 1 m focal length, F/10 off-axisparabolic mirror which will enable a maximum intensity on tar-get up to 2×1019 W/cm2. The specific self-injection acceler-ation regime to be ultimately used in γ-RESIST will dependupon the detailed full modeling currently in progress. However,the starting configuration will likely consist in the experimentalconfiguration already tested during the self-injection test exper-iment (SITE) and summarized above. The “Thomson/Comptonscattering” arm will consist of an off-axis parabolic mirror witha 0.5 m focal length, F/5 numerical aperture which will enablea maximum intensity on target up to 4×1019 W/cm2. Opti-mization in the design of this arm will include the possibilityto define the exact location in which overlapping of the laserpulse and electron bunch will occur. A delay line on one ofthe two beam lines will be used to control the effective positionof the scattering laser pulse relative to the accelerating pulse.This control will be crucial for the identification of the Thom-son emission and will be used to explore coupling of the laserpulse with the electron bunch along its propagation trajectoryfrom the injection point to the exit of the plasma.

7.1. Control of self-injection

As described, in the typical experimental conditions of laser-plasma acceleration, its possible to evaluate the maximum en-ergy gain in term of available power (in focus) and plasma den-sity. In a simplified description we can say that energy gainincreases as plasma density decreases as a result of longer ac-celerating distance [41]. A preliminary test of this behavior wascarried out during both runs described above. The pressure scancarried out during the first run, at the maximum laser energyof 1 J, showed injection and acceleration for plasma densitydown to minimum gas-jet pressure of 7 bar in the case of Nitro-gen. Similar measurements taken during the second run, at thehigher laser energy of 2×1019 W/cm2 showed injection at pres-sures as low as 1 bar, corresponding to approximately 6 × 1017.

Ionization induced injection has been proposed as a possiblesolution to further enhance injection [42] and will be exploredin future tests. Additional options to enhance the control of theinjection process it to rely on the colliding laser pulses scheme[43, 44] which exploits the large ponderomotive force associ-ated to the beat-wave produced at the overlapping region of twocounter-propagating pulses in order to pre-accelerate and injecta bunch of electrons into the bubble. However, in this case, onlya limited amount of charge is injected, typically around 10 pC,which makes this approach not suitable in view of an efficientradiation source.

8. Conclusions

The progress of laser-plasma acceleration with self-injectionis motivating the development of secondary radiation sourceswith unique properties, easily accessible for a wide range ofapplications. The γ-Resist project aims at demonstrating thegeneration of X/γ-rays using laser-accelerated electrons andThomson/Compton scattering. Here an overview of the pro-posed scheme has been given, with attention to the expectedperformances and with a look at the possible use of the pro-posed experimental scheme for advanced studies of dynamicsof electrons in intense fields. Finally, a preliminary presenta-tion of recent experimental results on self-injection at FLAMEwas also given showing the achievement of effective accelera-tion of highly collimated, high energy electrons with moderatereproducibility and good pointing stability, a first step for theproposed radiation source.

9. Acknowledgements

We thank the staff of the LNF Accelerator and TechnicalDivisions for the support during the the SITE operations atLNF. "SITE" and "γ-RESIST" are funded by INFN throughthe CN5. The work was carried out in collaboration withthe High Field Photonics Unit (MD.P03.034) and X-rayPhotonics (MD.P03.006.006) at INO-CNR partially funded byCNR through the ELI-Italy project and by the MIUR-FIRBSPARX respectively. We acknowledge the CINECA Grant N.HP10CZX6QK2012 for the availability of high performancecomputing resources and the INFN APE project for theavailability of the QUonG cluster.

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