M. Borghesi et al- Fast Ion Generation by High-Intensity Laser Irradiation of Solid Targets and Applications

FAST ION GENERATION BY HIGH-INTENSITY LASERIRRADIATION OF SOLID TARGETS AND APPLICATIONSM. BORGHESI,*† J. FUCHS,‡§ S. V. BULANOV,7 A. J. MACKINNON,# P. K. PATEL,#and M. ROTH**

†The Queen’s University, School of Mathematics and Physics, Belfast BT7 1NN, United Kingdom‡Laboratoire pour l’Utilisation des Lasers Intenses,

UMR 7605 CNRS-CEA-École Polytechnique-Université Paris VI, 91128 Palaiseau 3, France§University of Nevada, Physics Department, MS-220, Reno, Nevada 895577Kansai Research Establishment, APRC-JAERI, Kizu, Japan#Lawrence Livermore National Laboratory, Livermore, California

**Technical University Darmstadt, Darmstadt, Germany

Received March 22, 2005Accepted for Publication September 26, 2005

The acceleration of high-energy ion beams (up toseveral tens of mega-electron-volts per nucleon) follow-ing the interaction of short (t � 1 ps) and intense (Il2 �1018 W{cm�2{mm�2) laser pulses with solid targets hasbeen one of the most active areas of research in the lastfew years. The exceptional properties of these beams(high brightness and high spectral cutoff, high direction-ality and laminarity, and short burst duration) distin-guish them from the lower-energy ions accelerated inearlier experiments at moderate laser intensities. In viewof these properties, laser-driven ion beams can be em-ployed in a number of groundbreaking applications inthe scientific, technological, and medical areas. This paperreviews the main experimental results obtained in thisarea in recent years, the properties of the acceleratedbeams, the relevant theoretical and computational mod-els, and the main applications that have been imple-mented or proposed.

KEYWORDS: ion acceleration, laser-plasma interaction,inertial confinement fusion

I. INTRODUCTION

One of the most important results recently obtainedin laser-plasma interaction experiments is the observa-tion of very energetic beams of ions produced from laser-irradiated thin metallic foils. In a number of experiments,

performed a few years back with different laser systemsand under different interaction conditions, protons withenergies up to several tens of mega-electron-volts weredetected behind thin foils irradiated with high-intensitypulses.1–3 These high-energy proton beams have funda-mentally different properties from lower-energy protonsobserved in earlier work at lower laser intensity withlaser pulses in the nanosecond and tens-of-picosecondregime,4,5 which were accelerated from the coronal plasmaand emitted into a large solid angle. Beams producedduring these longer interactions exhibited strong trajec-tory crossing and a broad energy spectrum with typicalion temperatures of ;100 keV0nucleon. These unspec-tacular characteristics prevented major applications. Onthe contrary, beams accelerated by ultraintense laser pulsesexhibit a remarkable degree of beam collimation andlaminarity, high cutoff energy, and emission along thenormal to the unirradiated rear surface of the target. Sincethe first observations, an extraordinary amount of exper-imental and theoretical work has been devoted to thestudy of the characteristics and production mechanismsof these beams. Particular attention has been devoted tothe exceptional accelerator-like spatial quality of thebeams, and current research focuses on their optimiza-tion for use in a number of groundbreaking applications.

The scope of this paper is to report on the state ofthe art in this area of research, with a particular view topossible application of these sources in a fast ignitor0inertial confinement fusion ~ICF! context. The follow-ing sections will briefly outline the ion acceleration andtransport mechanisms from a theoretical point of viewand describe the main experimental results and the ap-plications of possible ICF relevance. Other importantapplications will be briefly mentioned for the sake ofcompleteness.*E-mail: [email protected]

412 FUSION SCIENCE AND TECHNOLOGY VOL. 49 APR. 2006

We will focus mainly on proton beams producedfrom irradiation of solid targets, as this is the most effi-cient and promising mechanism. Nevertheless, we notethat ion acceleration can also take place in underdenseplasmas, which can be obtained by ionizing gas jets tar-gets, or can be formed by ionization and evaporation ofthin solid targets due to the laser pulse pedestal and0or bythe prepulse6,7 ~even under conditions in which the targetis not exploded, a preplasma is generally present in frontof the target8!. During high-intensity propagation throughan underdense plasma, Coulomb explosion processes canaccelerate ions radially away from the laser axis,9,10 up toenergies of the order of the laser’s ponderomotive poten-tial. This process can be enhanced by collisionless shockacceleration.11

II. THEORETICAL AND COMPUTATIONAL MODELS

OF LASER-ION ACCELERATION

The ion acceleration processes have been exten-sively investigated theoretically and numerically, mainlyby means of particle-in-cell ~PIC! computer simulations.

There have been several theoretical mechanisms thathave been proposed to interpret the experimental obser-vations of ion beams accelerated by ultraintense lasersfrom solid targets. The mechanisms that have attractedthe most attention and appear to be more relevant tocurrently accessible experimental regimes are related tolarge electric fields set up by laser-accelerated electronsat target interfaces. A schematic of a typical charge andelectric field configuration following high-intensity laser-pulse interaction with a solid foil is shown in Fig. 1.Electrons escaping from the front target will set up an

electric field that will accelerate ions in the backwarddirection ~i.e., toward the laser!. Electrons propagatingforward into the target will set up fields in the interior ofthe target and at the target rear, which will accelerate ionsin the forward direction. The attention of researchers hasfocused mainly on forward-accelerated ion beams as theseexhibit higher beam quality and, typically, higher energies.

Electrons can be accelerated forward inside the tar-get by the laser through a variety of mechanisms thatdepend on the interaction and target conditions.12 Twocases may be distinguished depending on the plasma den-sity gradient present at the front surface of the target.When a steep density gradient is present at the targetfront surface ~i.e., in high-contrast laser interactions!,electrons are forward-accelerated mainly by vacuumheating13,14 or, for a , 1, by resonant absorption.15,16

However, these processes result in a smaller numberof accelerated electrons compared with the case of elec-tron acceleration in longer scale-length plasmas. Ex-tended plasmas can be produced at the target front surfaceby the laser pulse pedestal and0or by the prepulse17 thatcause evaporation and subsequent ionization of the targetmaterial. In this case, several electron accelerationmechanisms are relevant. Despite extensive work on thistopic, no clear picture has emerged yet owing to thecomplexity of the laser-plasma interplay. Indeed, duringhigh-intensity propagation through an underdense plasma,the laser pulse can undergo processes such as relativisticself-focusing18–21 leading to intensity increase22,23 orconversely detrimental processes such as filamenta-tion,24,25 all depending on the local plasma gradient,the laser intensity, and its spatial distribution. Amongthe processes coupling the laser energy into fast elec-trons, the main ones are j � B heating26,27 and betatronacceleration.28

Once accelerated inward, the fast electrons induce acharge-separation electrostatic field at the critical sur-face interface. This field will in turn result in accelera-tion of ions swept from the target front surface.29 Thetypical energies of accelerated fast electrons are such thattheir mean free path is much larger than the thickness ofthe targets typically used in experiments. The target ca-pacitance however allows only a small fraction of elec-trons to escape before the target is sufficiently charged sothat further escape is nearly impossible. The fast elec-trons that are electrostatically confined on the target rearsurface therefore set a charge-separation field over aDebye length.30,31 Typically, the Debye length is of theorder of 1mm, resulting in strong ~approximately teravolt-per-meter! electric fields. Such fields can ionize atomsand rapidly accelerate ions normal to the initially unper-turbed surface. It has been experimentally observed thatthis process is responsible for the acceleration of thehighest-energy ions, as will be detailed later. The ac-celerated ions form a dense bunch of short durationthat is charge neutralized by co-moving electrons. Theextremely short duration of the acceleration and the fact

Fig. 1. Schematic representation of the charge and electric fielddistribution following high-intensity laser interactionwith a solid foil. The arrows below the x axis show thedirection of fast ion motion.

Borghesi et al. FAST ION GENERATION BY LASER IRRADIATION OF SOLID TARGETS

FUSION SCIENCE AND TECHNOLOGY VOL. 49 APR. 2006 413

that it starts from an initially cold surface are essentialfacts that result in the unique characteristics of the ionbeam, as will be detailed in the following sections. Afterthis initial phase ions stream into vacuum with electrons,preceded by a Debye sheath of hot electrons, as illus-trated in Fig. 2.

The charge-separation structure at the expansion frontresults in a peak of the accelerating electric field at theion front, as shown in Fig. 3a ~Ref. 32!. The most ener-getic electrons always extend farther out into vacuum,maintaining the accelerating field as long as the electrontemperature is high. The acceleration from the target rearhas been described by several authors as an extension ofthe classical case of a plasma expanding into vacuum,33

driven by the ambipolar electric field generated in a nar-row layer at the front of the plasma cloud. There arehowever fundamental differences between the short-pulse and long-pulse cases. In the latter, bulk effects andcollisional ionization by thermal electrons in the coronalplasma are the dominant mechanisms. In the short-pulse-laser case, however, the ion generation and accelerationmechanisms are decoupled from the stochastic laserplasma. This general scenario has been confirmed by PICsimulations, as will be seen below.

Early theoretical models for the description of plasmaexpansion were based on the quasi-neutral behavior ofthe plasma and on the thermodynamical equilibrium ofelectrons on nanosecond timescales.33–35 Under these as-sumptions analytical solutions of self-similar type33 andin the framework of the renormalization group theory36,37

were found. In the subpicosecond laser irradiation case,the assumption of quasi neutrality should be abandonedsince as we have seen, the process of ion accelerationfinds its origin precisely in the strong charge separation,which is produced in the very early phase of the laser-solid interaction. The effect of charge separation has in-deed been the object of attention of recent theoreticalwork,32,38,39 which has updated the freely expandingplasma model to the case of a sudden burst of energeticelectrons but still in the framework of an isothermal ex-pansion. More realistic models that take into account thefinite size of the initial reservoir of particles and theadiabatic cooling and energy transfer between the ionsand electrons40 or the effect of two-electron tempera-tures41 have been more recently developed. Indeed, asthe beam expands, the fast electrons transfer progres-sively their energy to the ions, and the accelerating charge-separation field decreases until the ion energy saturates,which is a fact that is not accounted for in the isothermalmodels.

Charge neutralization of the ion beam is provided bythe initially hot electrons that expand into vacuum with

Fig. 2. Schematic representation of laser acceleration of ionsfrom the target rear surface. The position of the reartarget surface is indicated in the drawing ~the target islocated on the left!. The unneutralized Debye sheath ofhot electrons at the front of the expansion induces ac-celeration of ions co-moving with neutralizing elec-trons. ~Figure extracted from Ref. 30.!

Fig. 3. ~a! Structure of the electric field along the expansiondirection of a plasma streaming into vacuum under theinfluence of a hot electron Debye sheath. ~b! Resultingspectra of the accelerated ions at different times in theexpansion. ~Figures extracted from Ref. 32.!



the ion beam. These electrons cool down as t�2 ~Ref. 42!within a few hundreds of microns of the expansion toreach a co-moving state with the protons. Since the char-acteristics of this sheath of hot electrons depend on thetransport properties of the fast electrons through the tar-get, transport processes have a crucial impact on thebeam properties of the ions accelerated from the rearsurface. This is an area of study that is under intensescrutiny at present.43– 49

Theoretical considerations based on computationaland experimental results indicating that the peak protonenergy increases in inverse proportion to the target thick-ness29,50 have highlighted the role of electron recircula-tion. Electrons that have time to recirculate between thefront and rear surfaces of the target within the temporalwidth of the pulse could give rise to an enhanced hotelectron density at each surface, leading to higher accel-erating electric fields and thus higher proton energies.This effect becomes less important as the target thicknessincreases.

Theoretical considerations predict a priori that front-side acceleration yields lower energetic protons than rear-side acceleration. At the laser-irradiated target frontsurface, the sweeping electrostatic potential that accel-erates ions at the front29 is balanced by the laser pon-deromotive potential, i.e., fsweeping ; fpond. Therefore,the maximum energy that can be extracted by the ionsafter passing through this potential is also of the order of

fpond � me c2@ ~1 � Ilmm2 01.37 � 1018 W{cm�2{mm2 !102 � 1# ,

where

me � electron mass

I � laser power density ~“intensity”! ~W{cm�2!

lmm � laser wavelength ~mm!.

The acceleration ~sweeping! time is however finite~tsweeping ; 100 fs for protons and a 1-mm wavelengthlaser!, and if it is shorter than the laser pulse durationtlaser , i.e., the time during which the photon pressure ismaintained, the maximum energy gained by the ions willbe reduced. Therefore, the maximum energy of the ions~with a charge state of Z! will be Emax0front ; Zfpond iftlaser � tsweeping and Emax0front; Zfpond~tlaser 0tsweeping!102

if tlaser � tsweeping.This has to be compared with the maximum energy

that can be gained by protons accelerated at the rear. Asimple estimate of the maximum energy that can be gainedby the accelerated ions based on a self-similar fluidtheory32 is

Emax0rear ; 2Zfpond{�ln�2{vpi t

M2e��2

,

where

t � acceleration time, i.e., roughly the laser pulseduration51

vpi � �ne0 Ze2

mi «0

� ion plasma frequency ~ne0 is the

initial electron sheath density, and mi is theion mass!.

Adirect comparison between Emax0front and Emax0rear showsthat higher ion energies can be a priori expected fromrear-surface acceleration. Note however that the energyof ions accelerated from the front can be boosted by therear-surface field if the target is thin enough that therear-surface field is still large when these ions crossthe rear surface.52

A different mechanism that has been studied theo-retically and can in principle lead to ion energies muchlarger than the ponderomotive potential is related to thelaunch of an electrostatic shock from the front of thetarget by the ponderomotive force of the laser. Theoret-ical and computational work53–55 describes how the re-cession of the surface, driven by the laser radiationpressure, can launch an ion acoustic wave into the plasma,which in turn can evolve into an electrostatic shock. Therecession of the target front surface is due to the pon-deromotive pressure of the laser pulse pL � 2I0c, whichis of the order of gigabar at 1 � 1019 W{cm�2. Thispressure pushes the critical surface inward at a velocity

u0c � @~ncr 02ne !� ~Zme0Mi !� ~Il201.37 � 1018 !#102 ,

~where ncr is the critical density and ne the electron plasmadensity! as described by Wilks et al.27

In turn, the collisionless shock can trap and acceler-ate ions to high energy. As these shock-accelerated pro-tons, after crossing the target, gain additional energy fromthe back-surface electrostatic field, they screen this fieldand reduce the acceleration for protons originating fromthe back.

Shock-accelerated protons can have higher energiesthan protons accelerated from the rear target surface forvery high laser intensities ~I . 1021 W0cm2, as inferredfrom the PIC simulations! and relatively thick targets. Asignature of the mechanism is a plateau in the spectrumnear the high-energy cutoff.55

These different mechanisms have also been studiedextensively and confirmed by means of PIC simula-tions.29,51,56– 65 A typical example of PIC simulation ofthe interaction between an ultraintense laser pulse and asolid target ~composed of deuterium and hydrogen! lead-ing to the acceleration of ions is shown in Fig. 4. Fig-ure 4a shows the density profiles of the different speciesat an instant in time in the expansion.65 The fast electronsprecede the protons, followed by heavier deuterons forwhich the highest field has been screened by the protons.



This compares favorably with the simplified picture shownin Fig. 1. Figure 4b shows a phase-space plot of theaccelerated protons along the laser axis. Note that theprotons taking off from the target front surface achievemuch lower energies than the ones accelerated from thetarget rear surface. Being able to discriminate betweenions accelerated at the target rear and at the target front,several PIC simulations confirmed that rear-surface ac-celeration produces higher-energy ions for laser intensi-ties in the range of 1017 to 1019 W{cm�2, showing alsothat rear-surface acceleration induces a higher conver-sion efficiency of the laser energy into the ion beamenergy than front-surface acceleration.29,57 Sentoku et al.obtain conversion efficiencies of up to 8% for the rear-side acceleration and up to 2% for front-side acceleration~depending on pulse length!.29 Figure 5 shows ion spec-tra from three-dimensional ~3-D! simulations57 high-lighting the different contributions of the front and rearmechanisms.

Three-dimensional PIC simulations have also beencarried out for the case in which light ions are contained

in a thin layer at the surface of a high-Z target.64 Such alayer can be realized in practice, for example, by coatingthe thin light ion layer onto a high-Z substrate. In thiscase a small number of light ions ~e.g., protons fromhydrogen contaminants! gain energy in a time-independentelectric field near the target surface, and their dynamicscan be described in the test-particle approximation. Thisregime of ion acceleration takes on special significancefor the problem of high-quality ion beam generation. Thetypical energy spectrum of laser-accelerated particlesobserved both in the experiments and in the computersimulations can be approximated by a quasi-thermal dis-tribution with a cutoff at a maximum energy. The effec-tive temperature that may be attributed to the fast ionbeams is only within a factor of few from the maximumvalue of the particle energy. On the other hand, as will bediscussed in Sec. IV, many applications require high-quality proton beams, i.e., beams with sufficiently smallenergy spread, DE0E �� 1. Such a beam of laser-accelerated ions can be obtained using a double-layertarget. As illustrated in Fig. 6, the use of a double-layertarget produces fast proton beams with controlled qualityand a narrow spectrum. In this scheme the target is madeof two layers with ions of different electric charge andmass. The first ~front! layer consists of heavy ions withelectric charge eZi and mass mi , followed by a second~rear! thin proton layer.

Particle-in-cell simulations also indicate future per-spectives for ion acceleration at the extremely high laserintensities that may be reachable in the future with pulses

Fig. 4. The electron, deuteron, and proton density profiles av-eraged near the laser axis at 462 fs. The dot-broken lineindicates the initial electron’s density profile. Plot ~b!shows the proton’s phase plot X versus Px at the sametime of plot ~a!. The dotted lines indicate the initialtarget surfaces. ~Figure extracted from Ref. 65.!

Fig. 5. Ion energy spectra from 3-D PIC simulations.57 Thelines represent ions accelerated inward at the frontsurface, ions accelerated at the rear surface, and theselection of ions escaping the simulating box within10�4-srad solid angle around the rear-surface normal.



in the multipetawatt power regime and beyond. An ex-treme ion acceleration regime, named the laser-pistonmechanism, has recently been highlighted by 3-D PICcode simulations, carried out for extreme laser intensities

~up to ;1023 W{cm�2! ~Refs. 39 and 63!. In this mech-anism, the radiation pressure of the electromagnetic waveis directly converted into ion energy via the space-chargeforce related to the displacement of the electrons in a thinfoil. In this regime the efficiency of the laser energyconversion into the fast ion energy can be very high. Thisacceleration scheme has a connection with a mechanismof ion acceleration proposed by Veksler66,67 as well aswith the “snow plow” acceleration mechanism suggestedin Ref. 68. Formulated in the mid-1950s, Veksler’s con-cept of the collective acceleration of ions in an electron-ion bunch moving in a strong electromagnetic wave hada great influence upon both particle accelerator technol-ogy and plasma physics. Up to now its direct realizationwas considered at moderate driving electromagnetic ra-diation intensities, when transverse instabilities impedethe acceleration.69 In this scheme the transverse instabil-ities are suppressed or retarded because of the following.First, the plasma layers become quickly relativistic, dur-ing one or more laser wave periods, in the first stage ofthe acceleration. Because of relativistic effects the trans-verse instabilities in the laboratory frame grow moreslowly than in the plasma reference frame. Second, theradiation pressure causes a stretching of the plasma mir-ror in the transverse direction, so the transverse instabil-ities can be retarded similarly to the slowing down of theJeans instability in the theory of the expanding earlyuniverse.70,71

The simulations show that the foil is transformedinto a “cocoon” where the laser pulse is almost confined~Fig. 7!. The accelerated foil, which consists of the elec-tron and ion layers, can be regarded as a relativistic plasmamirror copropagating with the laser pulse. The acceler-ated ions form a nearly flat thin plate with high density~see Fig. 7b! and energies in the giga-electron-volt re-gion. According to simulations for intensities ranging

Fig. 6. Computational results from Ref. 64. Distribution of theelectric charge inside the computation region at ~a! t �40 and ~b! t � 80 ~position of heavy @thick shell# andlight ions @thin shell# is indicated by arrows, whereasthe diffuse feature corresponds to electrons!.

Fig. 7. Computational results from Ref. 63. ~a! The ion density isosurface for n � 8ncr ~a quarter removed to reveal the interior!at t � 40 � 2p0v. ~b! The isosurface for n � 2ncr, green gas for lower density at t �100 � 2p0v. The black curve showsthe ion density along the laser pulse axis.



from 1021 to 1023 W{cm�2, the interaction exhibits acontinuous transition from the acceleration regimes de-scribed previously to the laser-piston regime as the in-tensity increases.

Although PIC codes have so far been the main nu-merical tool for ion acceleration investigations, recentlypublished work has reported on simulations of ion accel-eration from wire targets carried out using a differentnumerical method, namely, a gridless tree code employ-ing a finite size particle approach.52 Although being purelyelectrostatic and not including electromagnetic wave prop-agation, the method may present advantages over PICcodes in terms of inclusion of collisional effects and theabsence of geometrical boundaries.

III. EXPERIMENTAL RESULTS

III.A. General Features of Laser-Driven Ion Beams

The discovery of highly collimated proton beamswith multi-mega-electron-volt energies was indepen-dently published by three research groups: Clarke et al.,1

Maksimchuk et al.,3 and Snavely et al.2 and Hatchettet al.30 In each of these experiments, a well-defined pro-ton beam was observed with a roughly exponential spec-trum and mean energy in the mega-electron-volt rangeand a high-energy cutoff in the 10- to 55-MeV range. Thebeam was generally emitted with a low divergenceangle, with the most energetic protons having the lowestdivergence angle, along the normal to the rear targetsurface.

Following these first measurements, the process oflaser-driven ion acceleration has been investigated byseveral experimental teams under very different physicalconditions and using laser systems with different char-acteristics ~a review of the major laser facilities used inthis area of research is provided in Ref. 72!. The laserintensities used for these studies range from a few times1017 W0cm2 ~Refs. 73 and 74! up to 3 � 1020 W0cm2

~Ref. 2!. A wide range of target thicknesses has beenexplored, ranging from a few wavelenghths ~Refs. 3 and50!, up to hundreds of microns ~Refs. 2, 30, and 75 through78! or even millimeters ~Refs. 75 and 76!; monolayermetallic or plastic targets ~Refs. 2, 30, 73, and 76! andC8H8 ~Refs. 65 and 79! as well as a range of double-layer foils ~Refs. 3, 73, and 77 through 80! were irradi-ated with the laser. Ion emission has also been investigatedfrom metallic wire targets with diameters of a few tens ofmicrons ~Refs. 81 and 82!. Liquid droplets83,84 have alsobeen used.

Energetic ions have been collected in the forwarddirection with respect to the laser and in the backwarddirection ~that is, on the same side of the target irradiatedby the laser!, with the latter being in general less ener-getic than the former.

In these experiments, the energy spectra are gener-ally measured by means of absolutely calibrated radio-chromic film ~RCF! ~sensitive to X-rays, ions, andelectrons!,85,86 nuclear activation measurements,87 CR-39particle detectors88 ~sensitive to ions with energies.100 keV0nucleon!, magnetic spectrometers ~from fewhundred kilo-electron-volts up to 100 MeV!, and Thom-son parabolas. RCF provides, with a high dynamic range,continuous spatial readout of the proton fluence, incoarsely resolved steps of proton energy by means of therange-energy relationship of the stopping power.2 It ispreferentially sensitive to penetrating protons, which havea large specific energy loss and produce a high-contrastimage. Electrons and X-rays generally appear as a dif-fuse low-intensity low-contrast background. Typical ex-amples are shown in Fig. 8.

As mentioned above, since protons have the highestcharge-to-mass ratio, they are favored by the accelera-tion processes. The protons are present in the target assurface contaminants4,78,89 or as compounds of the targetitself or of the target coating. In a few cases,75,77,78 heat-ing of the target was performed prior to the experimentsin order to eliminate the hydrogen contaminants as muchas possible and to obtain a better, controllable ion accel-eration. In particular, removing the proton from the tar-gets, or choosing H-free targets, the acceleration of heavierions was favored.

A selection of the published data on proton acceler-ation under relatively similar conditions of laser pulseduration, laser energy, laser intensity, and target condi-tions is shown in Figs. 9 and 10. Typical parameters ofthe proton beams obtained at some of the major laserfacilities are also reported in Table I together with theparameters of the laser pulses used for these studies.

Figure 9 shows the evolution of the recorded maxi-mum proton energy as a function of either the pulse du-ration or laser irradiance. The data have been grouped sothat similar conditions ~e.g., similar pulse durations! inexperiments carried out in different laboratories could becompared. Figure 9a shows clearly that the maximumproton energy increases with the laser pulse duration,along roughly parallel lines for varying laser irradiances.This can also be seen in Fig. 9b, which shows the evo-lution of the maximum proton energy as a function oflaser irradiance. Interestingly, we observe that for thelonger pulse durations, the maximum proton energy in-creases as I 0.5, whereas the increase seems to followmore closely a proportion to I for shorter laser pulses.Using PIC simulations, a change from a scaling propor-tional to I to a scaling proportional to I 0.5 has been ob-served as the laser irradiance passes from the subrelativisticdomain ~i.e., I , 1018 W{cm�2{mm2! to higher intensi-ties.60 This can be understood if Emax @ Fpond since forI ,, 1018 W{cm�2, Fpond @ I and for I .. 1018 W{cm�2,Fpond @ I 0.5.

In the case of Fig. 10b, the picture could be morecomplex because of differences in the laser contrast ratio



Fig. 8. ~a! Spectrum of laser-accelerated protons obtained by a magnetic spectrometer at different angles from the rear surface ofa 100-mm Al foil irradiated by a petawatt laser pulse. ~b! through ~e! RCFs for the same experiment. Data from Ref. 2. Theenergy responses of the RCFs to the proton flux are shown in ~f !.

TABLE I

Parameters of Some of the Laser Systems Used for High-Energy ~.1-MeV! Proton Beam Acceleration Experimentsand Typical Parameters of the Proton Beams Produced as Reported in the Quoted References*

LaserSystem Reference

LaserPulse

Duration~ps!

LaserEnergy~J!

LaserIntensity~W0cm2 !

TargetThickness~mm!

MaximumProtonEnergy~MeV!

T~MeV!

ConversionEfficiency

into Protons

LOA 90 0.04 0.8 6 � 1019 6 8 — —CRIEPI, Tokyo 96, 99 0.06 0.1 0.7 � 1019 to

1 � 10195 1.2 0.2 0.2%

ASTRA 97 0.06 0.2 7 � 1018 20 1.5 — 0.7%JanUSP 50 0.1 10 1020 3 24 3.2 1%MPQ 95 0.15 0.7 1019 10 2.5 — —LULI 100 TW 82, 100 0.32 30 6 � 1019 20 20 3 1% ~. 5 MeV!CUOS 94 0.4 5 5 � 1019 12.5 12 — —GEKKO 75 0.45 25 5 � 1018 5 to 25 10 3.4 —NOVA PW 2 0.5 500 3 � 1020 100 58 6 12% ~.10 MeV!RAL PW 92, 101 0.7 400 2 � 1020 100 44 — 7% ~.13 MeV!RAL Vulcan 76 1 90 1020 10 36 4.5 5%

*Some of the information was not provided in the references.



and also because of different behavior of the laser prop-agation in the underdense plasma in front of the targetsfor different laser irradiances. A dedicated experimentusing exactly the same conditions would be needed toobtain a clearer picture and an understanding of the un-

derlying processes. Figure 10 shows that not only themaximum proton energy but also the efficiency of theacceleration process increases with the laser pulse dura-tion and the laser irradiance. Here, we have chosen notto calculate a global energy conversion from laser to

Fig. 9. ~a! Maximum proton energy from laser-irradiated thin ~5 to 10 mm, except for the data points marked “RAL PW” and“Nova PW” where the thickness is 100 mm! metal ~Al or Cu! targets for different experiments as a function of the laserpulse duration and for three different ranges of laser irradiances. References are as follows: LOA ~Ref. 90!, JanUSP~Ref. 50!, LULI ~Ref. 91!, RAL PW ~Ref. 92!, Nova PW ~Ref. 2!, RAL Vulcan ~Refs. 76 and 93!, Osaka ~Ref. 65!, CUOS~Ref. 94!, MPQ ~Ref. 95!, Tokyo ~Ref. 96!, ASTRA ~Ref. 97!, and Yokohama ~Ref. 98!. ~b! Same but as a function of thelaser irradiance and for three ranges of pulse durations. The two dashed lines are trend lines proportional to I and I 0.5,respectively.



protons since the data reported in the various experi-ments differ in terms of spectral energy range so that aconsistent number cannot be tabulated and compared forthe various experiments. The point of comparison hastherefore been chosen to be the number of protons with10 MeV of energy accelerated in the forward direction.We can assume that when this number increases, the totalnumber of protons, and hence the conversion efficiency,will increase.

The present published record in terms of highest pro-ton energy has been achieved using the Nova Petawatt

laser facility, i.e., using the highest achieved peak powerand laser energy. A well-collimated, forward-directedpicosecond bunch of 3 � 1013 protons with energies upto;60 MeV, and an energy content of up to 48 J ~12% ofthe laser energy! was generated as a result of the inter-action of 400 J, in 1-mm, 0.5-ps laser pulses, focused inan 8- to 9-mm focal spot, producing an intensity up to3 � 1020 W0cm2, with a 100-mm-thick CH target.2,30

Similar energies, but five times fewer protons, wereobserved using Au targets.2,30

III.B. Experiments on Acceleration Mechanisms

There have been conflicting interpretations of theavailable experimental evidence for what concernsthe location of the accelerating mechanisms, and twodifferent models of the origin of these protons haveemerged: one suggesting proton acceleration from thefront1,3,8 and the other the rear2,30,80 of the target. Theannular angular ring pattern of proton emission, ob-served in a number of experiments, has been variouslyinterpreted as supporting acceleration from the front ofthe target in Clark et al.1 or from the rear in Murakamiet al.75 and in 3-D PIC code simulations by Pukhov.57

Experiments on the Nova Petawatt laser2,30 wereexplained in terms of electrostatic sheath at the rear sur-face of the target. In particular, targets that had a wedgedrear surface were shown to produce two proton beams,one from each surface. This was interpreted as consistentwith the rear acceleration mechanism occurring at therear surfaces of the wedge.

A number of experiments aimed at isolating the gen-erating mechanism have been carried out. For instance,ion acceleration from laser irradiation of water dropletshas been described by Karsch et al.83 From time-of-flightneutron measurements, these authors concluded that sheathacceleration occurring at the rear surface of each dropletwas the dominant mechanism of ion acceleration in thisexperiment. Mackinnon et al.80 observed eradication ofthe high-energy component of the proton beam whenintroducing a small preformed plasma scale length on therear of the target, as is consistent with a rear-surfaceacceleration mechanism.31 Conversely, Nemoto et al.8

clearly observed front-surface accelerated energeticdeuterons using a nuclear activation technique; the sameexperimental data have also been reported in Ref. 3 alongwith different results obtained with a frequency-doubledlaser. Experiments using heated targets to drive offsurface contaminants have also investigated the sourceproton production. Zepf et al.77 measured proton fluxreduction by factors of 2 to 3 when targets with plasticlayers were heated above their melt temperature. Theconclusion inferred from this experiment was that themost energetic protons originated from the front ofthe target.

Hegelich et al.78 also used resistive heating to driveoff contaminant hydrocarbon layers on targets coated

Fig. 10. ~a! Number of accelerated protons in a 1-MeV energybin at 10 MeV of energy as a function of the laserpulse duration and for two different ranges of laserirradiances. The references the data are extracted fromare the same as in Fig. 8. ~b! Same as ~a! but as afunction of the laser irradiance and for two ranges ofpulse durations.



with calcium flouride ~CaF2! layers. In this case the tar-gets were heated enough to drive off all detectable sourcesof contaminants but remained below the melting temper-ature of the CaF2 layer. The light ion spectrum producedby these heated targets indicated that a rear-surface mech-anism was responsible for accelerating ions to multi-mega-electron-volt energies in this experiment.

Most recently, another method of removing contam-inants has used an ion gun to sputter off oxide layerspresent on both target surfaces continuously up until thelaser shot is fired, as described by Allen et al.89 Theadvantage of this technique is that the ion gun action canbe localized so that either the front or rear of the targetare cleaned, thus allowing the experimenter to isolate thelocation of the proton beam. When the front surface ofthe target was cleaned, no discernable effect was ob-served on the proton beam; however, when the rear sur-face was cleaned, the proton beam was essentiallyeradicated. This experiment showed conclusively that therear target sheath acceleration was the dominant mecha-nism. The same conclusion was reached in another ex-periment91 that used nuclear activation measurements oflaser-accelerated deuterons ~deposited on a single targetsurface! and protons. This allowed direct and quantita-tive comparison of front- and rear-side acceleration underidentical laser conditions. It was shown that for protons.3.5 MeV, front-side acceleration accounts for ,3% ofthe total energy of the accelerated protons, the rest beingprovided by rear-side acceleration.

III.C. Emission Characteristics of Proton Beams

As mentioned in Sec. III.B, most of the availableexperimental evidence indicates that the main source ofhigh-energy protons originates from rear-surface accel-

eration. In this section, we therefore concentrate on thespatial and angular characteristics of the rear-surface ac-celerated protons, the front-surface accelerated ones being,as mentioned before, widely divergent and poorly lami-nar since they originate from the laser hole–bored criticalinterface. Some of the unique and most interesting char-acteristics of the laser-accelerated proton beams are dis-cussed in this section.

Since the ions are accelerated by the electron sheathon the target rear surface, their spatial and angular char-acteristics will be determined by the electron sheath spa-tial distribution. Such distribution depends not only onthe target rear-surface condition but also on the distribu-tion of the electrons generated at the laser-target front-surface interface and on their subsequent evolution in thetarget. Therefore, the protons’ spatial and angular char-acteristics will depend on the target material, the targetsurface roughness, the target shape, and the laser focaldistribution on the target.

In the case of controlled, ideal conditions for thelaser and the target ~i.e., a conductor target, a mirrorlikesmooth target surface, and a smooth laser focal distribu-tion!, it is observed, as shown in Fig. 11, that the protonbeam exhibits a smooth angular distribution49 with asharp boundary.2 The proton angular distribution is hereobserved on RCF dosimetry media.85 The spatial distri-bution of the protons in a given RCF layer gives theangular emission pattern within a specific proton energyrange. If one imagines that the electron sheath follows ageneric bell-shaped spatial distribution, the existence ofa sharp angular boundary in the proton angular distri-bution is easily understood. Indeed, protons will be ac-celerated everywhere in the sheath, normal to the localiso-density contour. As sketched in Fig. 11a, the centralregion, as well as the wings of the sheath, will produce

Fig. 11. ~a! Schematic of a Gaussian sheath formation on a solid target rear surface with the image of the laser focal spot on targetat full power; protons are accelerated normal to iso-density contours in the sheath, defining a maximum emission anglethat corresponds to the inflexion point in the sheath. ~b!A sharp boundary is consistently observed, at each proton energy,in the angular distribution of the protons observed on an RCF: The experiment uses a 48-mm Au target irradiated by a350-fs, I; 2 �1019 W0cm2 laser pulse at normal incidence.49 ~c!An electrostatic model49 for the acceleration of protonsthat uses a Gaussian sheath reproduces well the experimental observation of ~b! as shown in ~d!.



protons normal to the surface while intermediate zoneswill accelerate protons at an angle. The presence of aninflexion point in the sheath will therefore produce amaximum angle of acceleration. A simple electrostaticmodel of acceleration assuming a bell-shaped sheath49

reproduces well this behavior, as illustrated in Figs. 11cand 11d. The precise angle of the boundary depends onthe particular parameters of the sheath.

Deviations from the ideal conditions mentioned abovewill perturb the proton angular distribution. A roughenedtarget surface will induce spatial modulations in the elec-tron sheath and therefore local random orientation of theprotons during the initial acceleration that will result inlocal angular modulations in the proton dose, i.e., filament-like structures.82 Similarly, an insulator target or a mod-ulated laser focal spot will produce the same result. Indeed,even starting from a smooth distribution of fast electronsat the laser-target interface, it has been experimentallyshown that the transport is disrupted in insulators49: Mod-ulations appear in the electron beam, which forms thesheath when reaching the target rear surface, inducingfilaments in the proton angular distribution as in the caseof a roughened target surface. Fast electron generationand transport in solid matter are a complex topic that isstill not well understood ~see, e.g., Ref. 102!. However,it appears that at least for conductor targets, the electrondistribution at the source can be imprinted by the laserfocal distribution.49 A modulated laser irradiating a con-ductor target will produce a modulated electron beam. Ifthis beam is transported smoothly through the conductortarget, it will produce a modulated sheath resulting in afilamented proton angular distribution. Small modula-tions in the laser focus are difficult to diagnose and canvary shot to shot unless there is a dynamic correction forthe laser wave front. They are therefore a likely expla-nation for the transverse profile dishomogeneities fre-quently observed in proton beams.1,2

A modulation of the proton beam angular distribu-tion can be obtained purposefully by micromachining onthe surface shallow grooves like on an optical grating.This produces a periodic modulation of the beam angularenvelope, as observed in Fig. 12 ~Refs. 49 and 51!. Asprotons are first accelerated normal to the surface, thegrooves on the surface induce a modulation of the take-off angle. Because of the global sheath expansion, anoverall near-linear divergence is added to this initiallyimprinted angular modulation of the beam. Projected ona film stack far away, this results in a modulation of theproton dose, as observed. Using such modulations of thebeam intensity, it is possible to image the proton-emittingsurface and thus to measure directly the source size. Inthe example of Fig. 12, the measured diameter of theemission zone is of the order of 50 mm @full-width athalf-maximum ~FWHM!# at 4.5 MeV and is ;30 mm~FWHM! for .9-MeV protons. Alternate ways to deter-mine the source size that rely on beam trajectory recon-struction by projection of patterned objects103 or knifeedges104 are possible.

Other interesting information that can be retrievedreadily from Fig. 12 is the dependence of the divergenceangle versus the source position of the accelerated pro-tons. One observes easily that within the central portionof the beam, the phase-space correlation is almost ex-actly linear. This remarkable result suggests that the rel-ativistic electron sheath has a nearly Gaussian radialdistribution in its density profile. Indeed, as the acceler-ating field is E � �¹F� �~kThot 0e!~¹nhot 0nhot !, wherenhot ; exp@eF~r!0kThot # is the Boltzmann distributionof the hot electrons in the sheath, a bell-shaped sheathwill result in a nearly linear relation between the radialposition and the radial electric field in the sheath. Thisconfirms the assumption of a bell-shaped sheath usedto explain the result of Fig. 11. A further confirmationhas been recently provided by the detection of the field

Fig. 12. ~a! Experimental angular distribution of 10-MeV protons accelerated from a thin Al solid density foil of 60-mm thicknessirradiated by a laser with a Gaussian intensity distribution. The rear surface of the foil has sinusoidal microgrooves in onedirection. ~b!Normalized lateral momentum distribution obtained with an effective simulation.107 ~Figure extracted fromRef. 51.!



associated with a bell-shaped sheath driving the expan-sion of a laser-driven proton beam.105 These measure-ments, obtained with high spatial and temporal resolution,have been performed by using the proton probing tech-niques that will be described in Sec. IV.A.

The proton beam angular distribution can also bemodified by curving the target surface in order to focusdown the protons to a tight spot. This has been theoreti-cally predicted31,58 and experimentally demonstrated106

via enhanced heating of a secondary target, as will bediscussed in Sec. IV.B. Figure 13 shows computationalresults from 3-D PIC simulations of the acceleration of abeam of protons from a spherical shell target,58 clearlyindicating the focusing effect due to the spherically de-formed Debye sheath.

Smooth or filamented, the proton beam has an angu-lar envelope that decreases with energy.2,107 Using theaforementioned technique of directly imaging the proton-emitting surface by imprinting grooves on the target rearsurface, it has been shown107 that for protons.4.5 MeV,the decrease in the angular envelope with energy is due toa decrease of the emitting zone. Such a decrease of theemission zone is expected for a transversally bell-shapedelectron density distribution. In such a sheath, the highest-

energy protons are accelerated in the central, high-density portion of the sheath, whereas lower energiescome from the wings of the sheath distribution and thusare emitted at larger angle.49

Ring structures have also been observed in the pro-ton angular distributions. These are of two distinct types:a ring that appears inside the proton beam for low protonenergies108 and a ring that appears on the edge of theproton beam at all energies.1 The latter ring is observeddirectly on the CR-39 nuclear track detector, as can beseen in Fig. 10 of Ref. 77. This feature is not visible inraw RCF data but can be revealed by a subtractive analy-sis method109 that allows the retrieval of the angularlyresolved spectrum of the beam.110 The ring at the protonbeam edge appears to decrease with the proton energy,and although it has been attributed to the effect of mag-netic fields inside the target,1 it may be also due to thefact, mentioned above, that the proton angular envelopedecreases with energy.57 Regarding the other ring thatappears inside the proton beam for low proton energies,it is due to proton acceleration in the wings of the sheath:The lower the proton energy, the larger will be the accel-eration zone and the larger the emission angle0divergence,as seen above. However, when the proton energy is low

Fig. 13. Three-dimensional PIC simulation of proton acceleration from a thin, dense, spherical plasma shell ~22-mm radius and5-mm thickness! by a 70-fs petawatt laser pulse: ~a! 3-D proton distribution ;400 fs after the interaction ~subboxboundaries: 5, x, 25mm, 5, y, 25mm, and 4, z, 32mm!; ~b! and ~c! cross section of the fast proton beam densityat ~a! z �19 mm and ~b! z � 26 mm. The geometric center of the spherical target is located at z � 30 mm. ~Adapted fromRef. 58.!



enough, the emission zone reaches the wings of the sheathwhere the emission angle reduces again. This producesan accumulation of dose, in the shape of a ring.

Laminarity is another remarkable characteristic ofthese proton beams. The degree of laminarity of charged-particle beams can be expressed in terms of their emit-tance, which is proportional to the area of the boundingellipsoid of the distribution of particles in phase-space.The highest-quality ion beams have the lowest valuesof transverse and longitudinal emittance, indicating alow effective transverse ion temperature and a high de-gree of angle-space and time-energy correlation. Usingthe groove imaging technique discussed previously, it ispossible to fully reconstruct the transverse phase-spaceand thus determine the transverse emittance.107 In thisway, it has been experimentally shown that for protons ofup to 10 MeV, the transverse emittance is as low as0.004 mm{mrad, i.e., 100-fold better than typical radio-frequency accelerators and at a substantially higher ioncurrent ~kiloampere range!. Indeed, for typical protonaccelerators ~e.g., the CERN SPS!, the emittance fromthe proton injector linac is ;1 mm{mrad ~normalizedroot-mean-square! and up to 3.5 mm{mrad within thesynchrotron, with 1011 protons per bunch. It has alsobeen shown107 that the removal of the co-moving elec-trons after 1 cm of the quasi-neutral ~protons and elec-trons! beam expansion did not significantly increase themeasured proton transverse emittance. This last observa-tion is important since in order to take advantage of theexceptionally small proton beam emittance in future ap-plications, e.g., to capture them into a postaccelerator,removal of the co-moving electrons without significantlyperturbing the protons is crucial. The ultralow measuredemittance stems from the extremely strong, transient ac-celeration that takes place from a cold, initially unper-turbed surface and from the fact that during much of theacceleration the proton space charge is neutralized by theco-moving hot electrons. Such an acceleration processrepresents a potentially near-ideal kind of ion diode ascompared either to the ion beams generated from plasmaplumes,4 i.e., from the laser-heated turbulent plasma onthe front side of the foils, or to the conventional plasmadischarge ion sources used in accelerators. It should benoted that transverse emittance can also be determinedby alternate means, like using projection, on a far distantfilm, of objects such as knife edges111 or meshes103 placedin the ion beam path, although these methods are lessprecise.

In terms of applications using the ion beam as aprojection source, having a low-emittance beam is equiv-alent to projecting from a pointlike source located infront of the target, with much smaller transverse extentthan the ion-emitting region on the target surface.103 Thisallows high resolution ~point projection! in charged-particle radiography or ion patterned lithography. Re-garding the longitudinal emittance, the energy spread ofthe laser-accelerated proton beam is large, from zero to

tens of mega-electron-volts; however, because of the ex-tremely short duration of the accelerating field ~,10 ps!,the longitudinal phase-space energy-time product is prob-ably ,10�4 eV{s. PIC simulations show that longitudi-nally the acceleration is extremely laminar31 in the sensethat the spread of proton energies in a given longitudinalslice is very small. A good longitudinal velocity “chirp”of the beam is important since it could allow producingmonochromatic beams by coupling the beam to the fieldgradient of a postaccelerator.

III.D. Heavy Ion Beams

As mentioned previously, regardless of the materialused in laser-ion acceleration experiments, protons havealways been observed as the dominant ion species.2 Theorigin of the protons is surface contaminants, e.g., watervapor or hydrocarbons, providing a layer containing pro-tons at the target surface. Because of the low ionizationpotential and high charge-to-mass ratio, hydrogen isamong the first ion species produced and most efficientlyaccelerated. The cloud of accelerated protons then screensthe electric field generated by the electrons for all theother ion species. The key for the efficient acceleration ofheavy ions is the removal of any proton or light ioncontaminants. Heavy ions have also been observed back-ward to the laser originating from the front side.108 But,these ions, even though high in energy ~4 MeV0u!, do nothave the high-quality beam characteristic as those in thelaser direction from the rear surface. For the latter, recentexperiments have demonstrated heavy ion accelerationup to.5 MeV0u, which corresponds to ion energies usu-ally available at the end of conventional accelerators ofhundreds of meters in length.78

First attempts to remove the hydrogen contaminantsused resistively heated Al targets up to temperatures of afew hundred degrees. Even a partial removal of the hy-drogenous contaminants strongly enhanced the acceler-ation of heavier ions ~i.e., carbon!.82 A reduction by afactor of 10 of the hydrogenous contaminants increasedthe energy of the carbon ions by a factor of 2.5 and theirnumber by two orders of magnitude. Using tungsten as athermally stable target resist and coating the rear surfacewith the material of interest, the target could be heated to.12008C. Spectra from such targets show no acceleratedprotons at all and a strongly increased heavy ion compo-nent. The maximum energy could be enhanced by a fac-tor of 5 compared to Al targets and the conversionefficiency by a factor of 10 ~Ref. 78!. In cases whereohmic heating of target materials of interest is not pos-sible because of a low evaporation point, laser heatinghas been demonstrated to be an appropriate option toremove the contamination layers. In this case the inten-sity of the laser heating the rear surface and evaporatingthe proton layers and its timing, with respect to the shortpulse, have to be matched carefully.



The energy spectrum of the heavy ions and theircharge state distribution also provide detailed informa-tion about the accelerating electric field at the rear sur-face. It was shown that in a typical experiment collisionalionization and recombination in flight are negligible, andso the detected charge states directly image the electricfield strength due to the field ionization process. Theresults that have been obtained match very well the es-timated field strength, also predicted by theory,30 andrange from 1011 V0m up to a few 1012 V0m. The accel-erating field deduced from the ion acceleration is consis-tent with the observed proton energies with nonheatedtargets. For example, in typical experiments fluorine ionswere accelerated up to 100 MeV, i.e.,.5 MeV0nucleon ata maximum charge state of 7� ~see Fig. 14!. This cor-responds to an electric field of 2 TV0m, which wouldhave accelerated protons, if present, to energies up to25 MeV, i.e., the maximum energies found in experi-ments with nonheated targets under similar experimentalconditions. The conversion efficiency in heavy ions isvery high. Similar to the results obtained for proton beams,conversion efficiencies of up to 4% from laser to ionbeam energy have been measured. Since the accelerat-ing mechanism for protons and heavy ions is the same,excellent beam quality can be expected also for theheavy ions.

Experiments using structured targets have recentlyprovided measurements of the emittance of heavy ionbeams, indicating a beam quality comparable to that ofthe laser-accelerated proton beams and superior to con-ventionally accelerated ion beams. The future prospectsfor laser-accelerated heavy ion beam applications are aspromising as those for protons. Recently, the initiation offusion reactions of laser-accelerated heavy ions withsecondary target material has been demonstrated result-

ing in a variety of short-lived radioisotopes, some ofthem relevant for medical applications.112 Furthermore,because of the high intensity, short pulse length, excel-lent beam quality, and directionality, the idea of light ionfast ignition has also been developed.113 One of the mostpromising applications of laser-driven ion beams is theiruse as next-generation ion sources for conventional ac-celerators. Thanks to the high particle number ~more than1012 ions per pulse! and the excellent beam quality, thismight greatly enhance the luminosity of particle accel-erators. Laser-accelerated heavy ions exhibit, as the pro-tons, a broad energy distribution up to a characteristiccutoff energy. If matched appropriately, the ion contentin a single charge state can be as high as 80% of the totalnumber of ions. The challenge for applications as next-generation ion sources lies mainly in designing a match-ing section into a conventional accelerator that can accepthigh beam currents and simultaneously rotate the longi-tudinal phase-space in order to achieve a monochromaticbeam.

IV. APPLICATIONS

In this section, the main applications of laser-drivenion beams ~either achieved or proposed! will be re-viewed, together with a description of the requirementsof each application in terms of beam parameters. A sum-mary of the beam properties required by some of theseapplications is given in Table II.

IV.A. Radiography and Imaging with

Laser-Accelerated Protons

The use of ion beams, and particularly proton beams,for radiographic applications was first proposed in the1960s. Quasi-monochromatic beams of ions from con-ventional accelerators have been used for detecting arealdensity variations in samples, with spatial resolution.This was achieved by exploiting the energy depositionproperties of the particles in matter through a number ofdifferent methods: differential stopping radiography; mar-ginal range radiography, which is based on the enhancedsensitivity of ions to areal density variations toward theend of their range; and scattering radiography, whichexploits the intensity pattern created via scattering in theion beam intensity cross section for samples with thick-ness smaller than the stopping range. Radiography withvery high energy protons ~;1 to 10 GeV! is being de-veloped as a tool for weapon testing.114 Ion beams fromaccelerators have also been employed on some occasionsfor electric field measurements in plasmas via the detec-tion of the proton deflection, e.g., in Ref. 115. In practice,the difficulties and high cost involved in coupling exter-nally produced particle beams of sufficiently high energyto laser-plasma experiments ~or indeed magnetic confine-ment experiments! and the relatively long duration of ion

Fig. 14. F7� spectra from heated and unheated targets ~fromRef. 78!. More than 5 MeV0nucleon are achieved forF7� ions when the targets are heated.



pulses produced from conventional accelerators has lim-ited the application of such diagnostic techniques.

The unique properties of protons from high-intensitylaser-matter interactions, particularly in terms of spatialquality and temporal duration, have opened up a totallynew area of application of proton probing0proton radi-ography. Several experiments have been carried out inwhich laser-driven proton beams have been employed asa backlighter for static and dynamic target assemblies, insome cases a secondary target irradiated by a separatelaser pulse. As seen in Sec. III.C, the protons emittedfrom a laser-irradiated foil can be described as emittedfrom a virtual, pointlike source located in front of thetarget.103 A point-projection imaging scheme is thereforeautomatically achieved. The magnification of the systemis determined by M �1 � L0h, with L and h, respectively,

the object-to-detector and source-to-object distances. Den-sity variations in the target probed can be detected viamodifications of the proton beam density cross section,caused by differential stopping of the ions in the case ofthick targets or by scattering in the case of thin targets.Similarly, electric or magnetic fields in the sample regioncan be revealed by the proton deflection and the associ-ated modifications in the proton density pattern.

The detector employed in proton radiography0probing experiments consists mainly of RCF films,often arranged in a multilayer package. The multilayeredarrangement offers the possibility of energy-resolved mea-surements despite the beam’s broad energy spectrum asin each layer the dose deposited is mainly due to theprotons within a narrow range dE, which reach the Braggpeak in the layer. Typical values for RCF packages andprotons with 5- to 10-MeV energies are dE; 0.5 MeV. Inexperiments with low proton fluxes, track detectors~CR39! are a viable alternative to RCF, and the protondensity distribution is obtained directly from the densityof tracks after etching.

As mentioned above, backlighting with laser-drivenprotons has intrinsically high spatial resolution, which isdetermined by the size d of the virtual proton source andthe width ds of the point spread function of the detector~mainly due to scattering near the end of the protonrange!. For large magnification, the spatial resolution isa few microns. However, resolution degradation due tomultiple scattering affects this technique when the objectprobed is not very thin, basically increasing the size ofthe effective proton source. Energy dispersion providesthe technique with an intrinsic multiframe capability.In fact, since the sample to be probed is situated at afinite distance from the source, protons with differentenergies reach it at different times. As the detectorperforms spectral selection, each RCF layer contains,in first approximation, information pertaining to a par-ticular time. Depending on the experimental conditions,two-dimensional proton deflection map frames spanningup to 100 ps can be obtained in a single shot. The ultimatelimit of the temporal resolution is given by the durationof the proton burst t at the source, which is of the orderof the laser pulse duration. However, other effects arealso important: The finite energy resolution of the detec-tor layers and the finite transit time of the protons throughthe region where the fields are present normally limit theresolution to a few picoseconds.

Several radiographic applications of laser-producedprotons have been reported to date. Roth et al.82 havedemonstrated proton radiography of macroscopic, thickcompound targets using laser-produced protons with en-ergy of 5 to 10 MeV, in a projection arrangement withM ; 1. Because of proton stopping, the RCF exposureprovided a negative image of the areal density of thetarget. Radiography of thick masks with large magnifi-cation and approximately micron resolution has beenshown at Los Alamos National Laboratory employing

TABLE II

Parametric Requirements of Some of the Applicationsof Laser-Driven Proton Beams

Application Beam Requirements

Radiography~density detection!

Low emittanceShort durationHigh energy~for dense matter probing!

Imaging0deflectometry~electromagneticfield mapping!

Low emittanceShort durationBroadband for multiframing

Isochoric heating~warm dense matter!

Low emittance0focusabilityHigh fluxSpectral tailoringShort duration

Fast ignition Low emittance0focusabilityHigh flux

Protontherapy Small energy spreadHigh energy ~50 to 250 MeV!High repetition ~duty cycle!Removal of co-moving

electrons

Industrial application~implantation,lithography!

Monochromatic beamHigh repetitionHigh currentRemoval of co-moving

electrons

Injection intoaccelerators

Ultralow emittanceHigh fluxSmall energy spreadHigh repetition

PET High energyHigh fluxHigh repetition



sub-mega-electron-volt protons produced at moderateirradiance.116

Radiography of thin objects, i.e., with areal densitymuch smaller than the proton range, has also been dem-onstrated.103 Because of multiple small angle scatteringby the ions inside the object under test, protons that prop-agate through the object suffer a larger lateral spreadcompared to protons that propagate in vacuum. At theedges of the object, the difference in the lateral spreadsproduces a perturbation in the beam intensity profile. Ifthe transverse size of the object is larger than the pertur-bation width, then the perturbations due to different edgesdo not superimpose, and only the contour of the object isvisible in the image. If instead the transverse size of theobject is smaller than the perturbations’ width, then theperturbations due to two adjacent edges can superimposeproducing deep minima in the intensity profile. Radio-

graphs of thin objects ~e.g., 5-mm Cu meshes irradiatedby 15-MeV protons, or 7-mm CH spherical shells; seeFig. 15a! have been obtained by this technique.103,117

Density diagnosis via proton radiography has poten-tial application in ICF. Protons of 50 to 100 MeV pro-duced by a petawatt laser would be sufficiently energeticto propagate through cold, compressed National IgnitionFacility cores. A preliminary test studying the compres-sion of empty CH shells under multibeam isotropic ir-radiation at the moderate irradiance of 1013 W0cm2 hasbeen carried out in an experiment at the Rutherford Ap-pleton Laboratory.117 Radiographs of the target at vari-ous stages of compression were obtained ~see Fig. 15!.Modeling of proton propagation through target and de-tector using Monte Carlo codes permits the retrieval ofdensity and core size at maximum compression ~3 g0cm3

and 80 mm! in good agreement with hydrodynamic

Fig. 15. ~a! A 7-MeV proton radiograph of undriven 500-mm-diam, 7-mm-wall-thickness shell. ~b! Proton radiograph taken ofhighly asymmetric implosion, caused by mistimed heater beams. Beams from lower left side of figure are 1 to 2 ns earlierthan beams from upper right. ~c! Proton radiograph, in 7-MeV protons, of a 500-mm-diam microballoon with a 3-mm wallat a time close to stagnation. The core is slightly elliptical and has assembled below the center point of the shell. ~d! Radiallineout taken through center of proton radiograph data and Monte Carlo simulation output for fixed peak density andvarying core size ~FWHM!.



simulations. Preliminary work on experiments aiming touse protons as a shock diagnostic in laser-irradiated sol-ids has also been carried out.117–119

Probably the most important applications to date ofproton probing are related to the unique capability of thistechnique to detect electrostatic fields in plasmas.120,121

This has made possible obtaining for the first time directinformation on electric fields arising through a numberof laser-plasma interaction processes.117,122,123 The hightemporal resolution is here fundamental in allowing thedetection of highly transient fields following short-pulseinteraction.

When the protons cross a region with a nonzero elec-tric field, they are deflected by the transverse componentE4 of the field. The proton transverse deflection at theproton detector plane is equal to

Dr4 � eL�0

b

~E4 0mp vp2! dl ,

where

mp vp202 � proton kinetic energy

e � its charge

b � distance over which the field is present

L � distance from the object to the detector.

As a consequence of the deflections, the proton beamcross-section profile undergoes variations showing localmodulations in the proton density. Assuming the protondensity modulation to be small, dn0n0 �� 1, where n0 anddn are, respectively, the unperturbed proton density andproton density modulation at the detector plane, we ob-tain dn0n0 ��6div~Dr4!60M, where M is the geometricalmagnification. The value of the electric field amplitudeand spatial scale can then be determined if a given func-tional dependence of E4 can be inferred a priori, e.g.,from theoretical or geometrical considerations.

Thin meshes inserted in the beam ~e.g., between theproton source and the object! are sometimes used as“markers” of the different parts of the proton beam crosssections, in a proper proton deflectometry arrangementparticularly suited to revealing relatively large-scale

fields.117 As mentioned earlier, the meshes impress a mod-ulation pattern in the beam before propagating throughthe electric field configuration to be probed. The beam iseffectively divided in a series of beamlets, and their de-flection Dr4 can be obtained directly from the deflectionof the impressed pattern. A proton moiré technique em-ploying two grids to generate a set of moiré fringes hasalso been proposed as a way to increase the sensitivity tosmall electric fields.124,125

A general analysis method, applicable to both protonimaging and proton deflectometry data, consists of usingparticle-tracing codes to follow the propagation of theprotons through a given 3-D field structure, which can bemodified iteratively until the computational proton pro-file reproduces the experimental ones. State-of-the-arttracers allow realistic simulations including experimen-tal proton spectrum and emission geometry, as well asdetector response.

Typical data obtained with this technique are shownin Fig. 16. The data show, with picosecond resolution,the evolution of the electric fields surrounding a 50-mmTa wire irradiated by the Vulcan, 1-ps, 25-TW pulse.122

The RCF layers shown ~second layers of the radiochro-mic stacks, corresponding to proton energies of 6 to 7MeV! have been obtained in separate shots for differentdelays between the two chirped pulse amplification pulses~one interacts with the wire; the other generates the pro-tons used to radiograph!. When the proton probe arrivedon target before the interaction pulse ~Fig. 16a!, only theshadow of the Ta wire, thick enough to slow down andscatter the protons, is visible, with some small effectvisible in the interaction region due to preplasma presentahead of the interaction. However, when the probe isroughly coincident with the interaction ~Fig. 16b!, adramatic effect is observed, with the protons beingdeflected away from the surface of the wire, whichcharges up because of hot electron expulsion. At the sametime an ion front appears to be driven from the target,as also observed on irradiated foils.105 The charge isseen to decay in a few tens of picoseconds, as filamentarystructures ~horizontal striations! appear, likely to be as-sociated with the development of an electromagnetic

Fig. 16. Proton projection images of a 50-mm Ta wire for different interaction-probing delays: ~a! �12 ps, ~b! �2 ps, ~c! 8 ps,~d! 18 ps, and ~e! 30 ps ~the absolute timing precision was ;5 ps!. The energy of the protons employed was 6 to 7 MeV.



instability. From the deflection surface, fields of ;1010

V0m are deduced. Evidence of an ultrafast current flow-ing through the wire and progressively neutralizing thecharge lost is also provided by the data.122 The wire evo-lution under these interaction conditions has been de-scribed as a “laser-driven Z-pinch.”126

Beside fields associated with fast electron dynamics,the technique has provided novel information ~via theirassociated electrostatic field! on late-time ionic struc-tures developing from nonlinear phenomena after high-intensity interactions, including ~a! the first observationsof “postsolitons,” arising from merging of solitons cre-ated in the wake of the laser pulse in near-critical plas-mas,123 and ~b! the first observation of a quasi-stationaryionic modulation arising from a two-stream instability inthe wake of a 30-fs pulse propagating in a tenuousplasma.127

Application of the technique to nanosecond-producedplasmas of ICF interest has also yielded interesting re-sults. Filamentary structures tentatively associated withelectron transport instabilities have been seen in plasmasproduced by moderate irradiation ~1013 to 1014 W0cm2!at the Rutherford Appleton Laboratory.120 In experi-ments carried out at Laboratoire pour l’Utilisation desLasers Intenses ~LULI!, electric fields associated withthe pressure gradients have been detected for the firsttime in plasmas produced by 1015 W0cm2 irradiation ofmetal wires and foils by using the deflectometry tech-nique.117 By particle-tracing analysis of the pattern, fieldsof the order of 109 V0m have been obtained. LASNEXsimulations, combined with particle tracing through thefield structure predicted, reproduce in magnitude and ge-ometry the experimental observations.

The magnetic field can also deflect protons, and thereis some scope for the application of proton probing tech-niques to this purpose.

IV.B. Proton Heating

The high-energy flux and short temporal duration oflaser-generated proton beams has recently led research-ers to investigate their capability for heating solid matterto high-temperature plasma states. The heating of mate-rials to a plasma state can be achieved by many means;however, for studies of fundamental properties such asthe equation of state or opacity, it is often desirable tocreate large-area, or large-volume, plasmas in a uniform,single temperature and density state.

Heating of materials with ions has already been dem-onstrated with accelerator-based and electrical-pulsed ionsources. The heavy ion synchroton at GSI Darmstadtproduces more than 1010 ions ~Ar18�! in a 250-ns pulsecapable of heating several millimeters of lead to almost1-eV temperatures ~1 eV � 11 604 K! ~Ref. 128!. Pulsedelectrical sources such as the PBFA-II facility in SandiaNational Laboratories have produced 1017 protons in 20-nspulses, focused to 1-cm diameter, with subsequent heat-

ing of initially solid aluminum to 35 eV ~Ref. 129!. Onefacet of these sources is that because of the relativelylong pulse durations, the materials undergo significanthydrodynamic expansion during the heating period. Laser-generated proton beams with temporal durations of justa few picoseconds may provide a means of very rapidheating, on a timescale shorter than the hydrodynamictimescale.

Experimental measurements of the conversion effi-ciency of laser energy into protons have ranged between2 to 7% ~Refs. 1, 2, and 50!, dependent on the preciselaser parameters, interaction conditions, and target ma-terial and thickness. The temporal duration of the protonpulse near the source has been shown to be of picosecondorder both experimentally105,122 and in extensive mod-eling.29 The proton energy spectrum can often be approx-imated as a single temperature exponential distributionwith kT of a few mega-electron-volts. Most measure-ments have been made above a minimum threshold pro-ton energy of 3 to 4 MeV; thus, the spectrum andconversion efficiency into protons at the lowest energiesis rather uncertain. If one assumes that the spectrum con-tinues in an exponential form, one can estimate the en-ergy deposition of the beam in a secondary target.

The first demonstration of laser-generated proton heat-ing was obtained by Patel et al.106 In this experiment a10-J pulse from the 100-fs JanUSP laser at LawrenceLivermore National Laboratory was focused onto an Alfoil producing a 100- to 200-mJ proton beam. A second10-mm-thick Al foil was placed in the path of the protonbeam a distance of 250 mm from the first. A time-resolved single-wavelength measurement was made ofthe rear-surface emission in a narrow band at ;570 nm.If the target is heated volumetrically, one would expectto see a thermal emission trace with a sharp, almostinstantaneous, rise followed by a gradual fall as the sur-face expands outward and cools. This pattern is indeedobserved in the data, as shown in Fig. 17. The absolutemeasured intensity at this wavelength provided an esti-mate of the initial temperature of the rear surface, whichwas in this case 4 6 1 eV. A focused proton beam, pro-duced from a spherically shaped target, was seen to heata smaller region to a significantly higher temperature,;23 eV.

In these experiments the isochoric heating by theprotons is volumetric but not uniform. The exponentialform of the proton energy spectrum will result in a sig-nificant temperature gradient along the direction of thebeam, with the highest energy deposition, and thus tem-perature, at the front surface of the heated foil. Furtherwork will be required, optimizing the proton beam spec-tral characteristics, or the target design, to minimize thesegradients in energy deposition. For some applications,such as proton fast ignition ~PFI!, this feature of protonheating may not be of primary concern; however, it isof some consequence for the achievement of the ideal-ized single temperature and density states so desired for



precise quantitative measurements of material propertiesin extreme, strongly coupled plasma regimes.

IV.C. Proton Fast Ignition

Since the discovery of intense, short, energeticbursts of ions with excellent beam quality generatedby the short-pulse-laser–solid-matter interaction, theidea of using those beams for fast ignition was intro-duced.113,130,131 Protons have several advantages com-pared to other ion species and electrons. They can penetratemore deeply into a target to reach the high-density re-gion, where the hot spot is to be formed, because of thequadratic dependence of the stopping power on the chargestate. Also, as with all ions, they exhibit a characteristicmaximum of the energy deposition at the end of theirrange ~in the Bragg peak!, which is desirable in order toheat a localized volume efficiently.As sketched in Fig. 18,

the basic idea is to use multiple, short-pulse lasers irradi-ating a thin curved foil. The protons are accelerated fromthe rear surface of the foil and, because of the parabolicgeometry, are focused into the compressed fuel. Withrespect to electrons, the higher mass and the neutralizedspace charge make them less likely to be subject to in-stabilities. As has been shown in Sec. III.C, laser-drivenion beams can have an excellent beam quality in terms ofemittance ~permitting to focus them down into a smallvolume!, the pulse duration is short, and the particlenumbers are high.

According to current experimental and theoreticalunderstanding, the prospects for PFI are quite promising,although a number of issues still need to be addressedquantitatively in the parameter range that is relevant tofast ignition.

One of the requirements for PFI is the possibilityof focusing the proton beams into a small volume. As

Fig. 17. ~a! Experimental setup for flat and focusing target geometries. Each target consisted of a flat or hemispherical 10-mm-thick Al target irradiated by the laser and a flat 10-mm-thick Al foil to be heated by the protons. ~b! Corresponding streakcamera images showing space- and time-resolved thermal emission at 570 nm from the rear side of the proton-heated foil.The streak camera images an 800-mm spatial region with a 1-ns temporal window.



discussed in Sec. IV.B, it has recently been demonstratedthat proton beam focusing is indeed possible and spotsizes of ;50 mm have been achieved.106 This is stilllarger than required by PFI; however, in principle theultralow transverse emittance reported for these protonbeams should allow focusing down the beam to spotsizes orders of magnitude smaller than what has beenachieved so far. In practice, there are several issues thatneed to be considered.

The first issue is the degree of neutralization of thebeam, which is composed of protons but also of co-moving and neutralizing electrons. If the beam is notperfectly neutralized or if there are instabilities that de-velop in the flow and modulate the electron and ion den-sity along the propagation axis, an emittance-limited focusmay not be achieved because of Coulomb forces increas-ing near the focus.

The second issue concerns the geometry of the fo-cusing device and of the accelerating electron sheath.The aforementioned 50-mm spot size was achieved usingprotons accelerated from a hemispherical target. The con-cave rear surface of the hemisphere, where the protonacceleration takes place, induced a curvature of the sheath.However, this may have only partially compensated thenatural Gaussian-like curvature of the sheath, which isresponsible, for example, for the variation in divergenceangles for different proton energies observed in the ex-periments. The final divergence of the accelerated beamresults from the combined and competing effect of thetarget curvature and of the sheath curvature. This mayexplain why a relatively large focal spot was achievedcompared to what could be in principle assumed from thedetermination of the beam emittance. However, one hasto take into account the fact that larger irradiated areas onthe target front surface as required for PFI ~see below!would flatten the electron distribution at the rear surface.

This would result not only in a single divergence anglefor different energies but also in a much smaller initialdivergence angle, for mostly all of the energies, whichcould be more easily compensated in order to reach therequired focal spot diameters.

The proton pulse length is of the right order of mag-nitude for PFI. The protons are not monochromatic butrather have an exponential energy distribution. Thisseemed to be a concern at the beginning for two reasons.First, because of the different energies, the dispersion ofthe proton pulse from the source to the target lengthensthe pulse. Therefore, a maximum source-pellet distanceof the order of a few millimeters is necessary for theenergy deposition of the PFI beam to take place withinthe disassembly time for the hot spot region. On the otherhand, this relatively short distance raises the concern ifthe thin metallic foil, which is to be the source of theprotons, can be kept cold enough not to develop a densitygradient at the rear surface, which would diminish theaccelerating field. A second concern was related to thestopping power. Because of the difference in initial ve-locity, the energy deposition of protons with differentkinetic energy is spread over a larger volume. Slowerprotons are stopped earlier and do not contribute to thecreation of the ignitor spark.

Recent numerical simulations have partly relievedthose concerns. Simulations by Basko et al. ~presented atthe Workshop on Fast Ignition of Fusion Targets, 2002,Tampa, Florida! have shown that the protective shieldplaced in front of the source can withstand the X-ray fluxof the pellet compression and keep the rear surface of thesource foil cold enough for efficient acceleration fromthe rear surface. In the simulation the drive pulse causedthe pellet stagnation at 48 ns after the beginning of theirradiation. The thickness of the protective foil was foundto be crucial not only with respect to thermal shielding ofthe proton source but also because of the closure of theacceleration gap. For a foil too thin, the displacement ofthe rear side ~the one facing the proton source! due tothermal expansion and ablative foil acceleration can be-come quite substantial. A thickness of a few tens of mi-crons on the other hand provides thermal shielding aswell as sufficient mechanical stability.

A monochromatic proton beam is actually not theoptimum to heat a hot spot in a fusion target. Numericalsimulations have shown that one has to take into accountthe decrease of the stopping power of the nuclear fuelwith increasing plasma temperature.113 Therefore, an ex-ponential energy spectrum, like the one that is generatedby target normal sheath acceleration, is the most favor-able one. The protons with the highest energies arrivefirst and penetrate deeply into the fuel. By the time theproton number increases and the target temperature rises,the stopping power is reduced, thereby compensating forthe lower initial energy of the incoming protons. As aresult, the majority of the protons deposit their energywithin the same volume.

Fig. 18. Schematic view of PFI in indirectly driven ICF ~not toscale!. The rear surface of the laser target is shaped tofocus the proton beam into the spark volume.



In contrast to conventional fast ignition, the PFI canbe easily implemented into indirectly driven scenarios,necessary for example in heavy ion ICF. For practicalreasons, there is another advantage of PFI. Since focus-ing of the ion beam is provided by the geometry of thesource foil, the pointing and focusing requirements forthe high-energy short-pulse-laser beams are significantlyrelaxed. In fact, the laser beam foci can be as large as afew hundred microns, which as mentioned previouslywill flatten the shape of the electron sheath driving theprotons and enhance their focusability. As a conse-quence, the final optics of the short-pulse lasers can bemounted relatively far from the reactor center, whichmay increase their lifetime.

Existing short-pulse lasers have already demon-strated intensities that are sufficient for generating pro-ton energy spectra required for PFI. Regardless of thenature of the ignitor beam, calculations show a minimumdeposited energy requirement for fast ignition of the orderof 10 kJ. Measured and published efficiencies for con-verting the primary laser energy into ion beams appearfavorable for PFI. While experiments employing low-energy short-pulse lasers have conversion efficiencies,1%, the efficiency changes to a few percent for systemsof a few joules of energy and increases up to .10% forsystems having hundreds of joules at comparable fo-cused intensities. Extrapolating the conversion efficien-cies to multikilojoule laser systems conversion efficienciesof.10% can be expected, which would result in the needfor a few hundred kilojoules of short-pulse-laser energyfor PFI.

The most detailed theoretical analysis of PFI require-ments so far has been published by Temporal et al.113 forproton beams with an exponential energy spectrum. Fol-lowing their assumptions a total proton energy of;26 kJat an effective temperature of 3 MeV is required. Thesevalues have been found to be consistent with a source-to-target distance of 4 mm, which would allow for appli-cation in indirectly driven ICF. It is interesting to notethat the protons, which effectively heat the hot spot, con-tain only 10 kJ of the total energy and range from 3 to10 MeV. Techniques for shaping the energy spectrum ofthe laser-accelerated protons could lead to a reduction ofthe required laser beam energy. The simulations by Tem-poral et al. not only refer to an effective ion spectrumrather than to a monoenergetic ion beam but also takeinto account pulse lengthening due to velocity dispersionas well as range lengthening due to temperature effects.

The total number of protons needed for ignition isclose to 1016, and a typical proton beam temperature of3 MeV is commonly obtained in experiments at 5 �1019

W0cm2. Assuming a pulse length of 4 ps ~which wouldincrease the damage threshold of modern dielectric com-pressor gratings! and a conversion efficiency of 10%, atotal laser energy of 260 kJ would be needed. To matchthe temperature conditions, the lasers would have to ir-radiate a proton source surface of 13 mm2, correspond-

ing to a diameter of ;4 mm. The question arises if thereare enough protons available at the nonirradiated rearsurface of the source. Early experiments have shown anincrease in proton numbers of about an order of magni-tude when irradiating bulk plastic2; however, this was atthe cost of beam quality due to electron beam breakup.Later experiments have demonstrated undisturbed elec-tron transport ~and consequently the production of a highlylaminar ion beam! through hydrogen-containing layersof up to 1000-Å thickness, coated at the rear of metallictargets. Assuming a CH layer at the rear surface with400-Å thickness, an area of little less than 13 mm2 wouldcontain the 1016 protons required for igniting the hotspot. A similar area would need to be irradiated at thefront surface. Both areas would be tolerable in a geom-etry of 4-mm source to hot spot distance, as proposed inthe simulation by Temporal et al., and still allow forefficient focusing.

Currently, there are plans to build multikilojoule short-pulse laser systems in the United States, Japan, UnitedKingdom, and France. These facilities will allow exper-iments in a more relevant regime for fast ignition than theexisting ones and provide the opportunity to better gaugethe feasibility of PFI as an alternative way to achieveignition. However, several issues need to be studied be-forehand, even with the existing, smaller-scale facilities.There is a need to determine the exact degree of neutral-ization of the accelerated proton beam; the stopping powerof protons in dense, hot plasmas ~this could be addressedusing lasers coupled to ion accelerators!; techniques toachieve smaller proton focal spots; and ways of achiev-ing energy selection within the proton beam. It will alsobe necessary to enhance simulation capabilities so thatrealistic models of proton acceleration and focusing canbe used for predictive capability. Simulations of implo-sion and fuel assembly need also to progress so that morerealistic geometry of compressed fuels can be used forcalculations of proton stopping and beam requirementsin order to refine the parameters required for PFI.

IV.D. Nuclear Reactions Initiated by

Laser-Driven Ions and Applications

The interaction of laser-driven high-energy ions withsecondary targets can initiate nuclear reactions of vari-ous types, which, as mentioned before, can been usedas a tool to diagnose the beam properties. For example,63Cu~ p, n! 63Zn reactions in copper stacks are used toquantify the proton numbers through measurement ofb� decay of 63Zn nuclei using a NaI detector-based co-incidence counting system.93,132 More recently, tech-niques employing a single Cu layer have been used inwhich a range of isotopes resulting from a proton-induced nuclear reaction is analyzed in order to recon-struct the proton spectrum.133

This also presents the opportunity to carry out nu-clear physics experiments in laser laboratories rather than



in accelerator facilities and to apply the products of thereaction processes in several areas. Reactions initiatedby laser-accelerated high-Z ions have also been studied.McKenna et al.92,112,134 have shown that fusion reactionsbetween fast heavy ions from a laser-produced plasmaand stationary atoms in an adjacent “activation” samplecreate compound nuclei in excited states, which deexcitethrough the evaporation of protons, neutrons, and alphaparticles. The emission of precise energy gamma raysfrom the residual nuclei in the activated samples wasmeasured and used with calculated reaction cross sec-tions to make quantitative measurements of heavy ionacceleration from a laser-produced plasma. Nuclear re-actions of interest for spallation physics have also beeninvestigated by employing the multi-mega-electron-voltproton beams.101 The broad energy distribution of thebeams is in this case advantageous for the determinationof residual nuclide generation arising from specific spall-ation processes such as evaporation.

Multi-mega-electron-volt proton beams, generatedfrom intense laser-plasma interactions, have also beenused to induce nuclear reactions in low-Z materials suchas 11B and H2

18O, in order to produce short-lived positron-emitting isotopes of medical interest,90,135,136 i.e., for pos-itron emission tomography ~PET!. In PET, the patient isinjected with a pharmaceutical labeled with a short-livedpositron-emitting isotope. This radiopharmaceutical ismetabolized at specific sites in the body. Positrons anni-hilate with electrons to produce two counterpropagatinggamma rays. After their detection, specific sites of highuptake of the pharmaceutical may be imaged. PET hasproven to be extremely useful in medical imaging ofblood flow and amino acid transport and in the detectionof tumors. The radioisotopes used in PET are typicallyshort-lived positron emitters such as 11C, 13N, 15O, and18F. These reactions are carried out by using up to 20-MeV protons or similar energy deuterons from cyclo-trons with the concomitant problems of large size andcost and extensive radiation shielding. Production of short-lived isotopes via laser-driven proton beams may be eco-nomical in the near future with the possibility of employingmoderate-energy, ultrashort, high-repetition tabletop la-sers. Extrapolations based on present results point to thepossibility of reaching the gigabecquerel activities re-quired for PET therapy when laser systems capable ofdelivering 1-J, 30-fs pulses, focused at 1020 W0cm2, withkilohertz repetition will become available.90

Neutrons are an important product of the nuclearreactions produced above, with potential applications incancer therapy ~boron neutron capture therapy!, neutronradiography, and transmutation of nuclear waste. Thepotential for laser-driven neutron sources is considerableand offers advantages over accelerator- and reactor-driven sources in terms of cost, compactness, brightness,and short duration, particularly advantageous for fast neu-tron radiography137 and studies of impulsive damage ofmatter.138 In view of these potential applications, several

experiments have studied the production of neutrons ini-tiated by laser-driven proton beams on secondary targets.Experiments137,139 carried out at the Vulcan laser facilityhave studied neutron generation via 11B~ p, n! 11C reac-tions and via the 7Li~ p, n! 7Be reaction revealing neutronyields up to 4 � 109 sr�10laser pulse ~result obtained fora laser intensity of 3 �1020 W0cm2! ~Ref. 139!. Neutronproduction has also been observed during interactionswith targets containing deuterium, either solid140–145

or gaseous,9,146,147 directly irradiated by high-intensitylaser pulses. In this case the neutrons are produced inthe course of fusion reactions of the type D~d, n! 3Heinvolving laser-accelerated deuterium ions. In recentsolid-target experiments, the neutron spectra producedthrough this process have been used as a diagnostic ofthe laser-driven deuterium ions inside the target142–144 inorder to provide information on acceleration taking placeat the front surface or in the bulk of the target.

IV.E. Hadrontherapy

Hadrontherapy148–152 is a form of radiotherapy thatuses protons, neutrons, or carbon ions to irradiate cancertumors. The use of protons and carbon ions in radiother-apy has several advantages to the more widely used X-rayradiotherapy. First, the proton beam scattering on theatomic electrons is weak, and thus, there is less irradia-tion of healthy tissues in the vicinity of the tumor. Sec-ond, the slowing-down length for a proton with givenenergy is fixed, which avoids undesirable irradiation ofhealthy tissues at the rear side of the tumor. Third, thewell-localized maximum of the proton energy losses inmatter ~the Bragg peak! leads to a substantial increase ofthe irradiation dose in the vicinity of the proton stoppingpoint.

After.40 yr of experimental investigations, severaldedicated hadrontherapy centers are operating, and oth-ers are at present under construction ~see Ref. 153!. Thesecenters have accelerators specialized for medical appli-cations from which the proton beams are delivered intothree to five procedure rooms equipped with treatmentunits.Anecessary and costly treatment unit of the hospital-based proton therapy centers is the gantry, which is adevice that provides multidirectional irradiation of a lyingpatient.

The proton energy window of therapeutical interestranges between 60 and 250 MeV, depending on thelocation of the tumor. Proton beams with the requiredparameters are currently obtained using conventionalcharged-particle accelerators such as synchrotrons, cy-clotrons, and linacs.154 The use of laser-based accelera-tors has been proposed as an alternative,155–159 whichcould lead to advantages in terms of device compactnessand costs.

A laser accelerator could be used simply as a high-efficiency ion injector for the proton accelerator or couldreplace altogether a conventional proton accelerator.



Because of the broad energy and angular spectra of theprotons, an energy selection and beam collimation sys-tem will be needed.160,161 Typically, DE0E ; 10�2 isrequired for optimal dose delivery over the tumor region.All-optical systems have also been proposed in which theion beam acceleration takes place in the treatment roomitself and ion beam transport and delivery issues are sominimized.155 In this case the beam energy spread anddivergence would have to be minimized by controllingthe beam and target parameters.58,64

The required energies for deep-seated tumors~.200 MeV! are still in the future but appear withinreach considering the ongoing developments in the field.A demanding requirement to be satisfied is also the sys-tem duty factor, i.e., the fraction of time during which theproton beam is available for use that must not be ,0.3.

IV.F. Technological Applications

There is certainly scope for application of laser-driven ion sources in areas of industrial0manufacturinginterest, particularly if high-repetition tabletop, moderate-cost laser systems with the required parameters for effi-cient ion generation will be developed. Areas of obviousinterest are ion implantation in substrates and manu-facturing techniques such as deep proton projectionlithography and micromachining.162–164 While surfaceimplantation of moderate-energy ions accelerated by sub-nanosecond pulses has been already studied,165,166 thehigher ion energies attainable with high-intensity pulsescould allow deep implantation via ion projection ap-proaches,167,168 which is of interest for semiconductordevice manufacturing. The beams’ exceptional qualityin terms of emittance could play a crucial role in thistype of application.

In addition, the short duration of laser-driven ionbursts opens up to investigation a whole new regime ofphysical conditions relating to the interaction of ionizingradiation with matter and the induced material modifica-tions. This is a highly topical area for many technologicalapplications, including ion implantation or control of ma-terial damage in fusion reactors. Since the response ofradiation-damaged materials takes place typically overtimescales in the picosecond range, impulsive ~picosec-ond! particle irradiation of matter provides now favor-able conditions to carry out pump-probe experiments withunprecedented temporal resolution.

V. CONCLUSIONS

Research in the area of laser-driven ion sources hasgrown at a phenomenal pace since the first experimentsreporting multi-mega-electron-volt proton accelerationfrom laser-irradiated foils, as testified by the high num-ber of publications appearing in the scientific literature.Most of the present research is motivated by the excep-

tional spatial and temporal qualities of the beams, thehigh-energy and high particle numbers achievable, andthe application of these properties to a number of appli-cations, both in plasma physics and in interdisciplinaryareas. Work is undergoing worldwide aimed at the opti-mization and control of the acceleration mechanism andof the beam properties. Results in this area, coupled withthe continuous development in laser technology, are boundto further expand the field of applicability of these sourcesto include medical or technological applications.

ACKNOWLEDGMENTS

The authors acknowledge discussions with F. Pegoraroand M. Lontano and support from the QUB-IRCEP scheme.

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M. Borghesi et al- Fast Ion Generation by High-Intensity Laser Irradiation of Solid Targets and Applications