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8/3/2019 Patrizio Antici et al- Postacceleration of Laser-Generated High-Energy Protons Through Conventional Accelerator Linacs

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IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 36, NO. 4, AUGUST 2008 18

Postacceleration of Laser-Generated High-EnergProtons Through Conventional Accelerator Linac

Patrizio Antici, Mauro Fazi, Augusto Lombardi, Mauro Migliorati,Luigi Palumbo, Patrick Audebert, and Julien Fuchs

Abstract The postacceleration of laser-generated protonsthrough conventional drift-tube linear accelerators (linacs)(DTLs) has been simulated with the particle-code PARMELA. Theproton source is generated on the rear surface of a target irradi-ated by an high-intensity (10 19 W cm 2 ) short-pulse (350 fs)laser and focused by a microlens that allows selecting collimatedprotons at 7 0.1 MeV with root-mean-square unnormalizedtransverse emittance of 0.180 mm mrad. The simulations showthat protons can be accelerated by one DTL tank to more than14 MeV with unnormalized transverse-emittance growth of 8 and22.6 in different transverse directions when considering a total

proton charge of 0.112 mA. This rst numerical study shows thatcoupling between laserplasma accelerators with traditional accel-erators is possible, allowing a luminosity gain for the nal beam.

Index Terms Drift-tube linear accelerator (linac) (DTL),hybrid accelerator, laser-accelerated proton beams, low-emittancebeams.

I. INTRODUCTION

L ASER acceleration of intense collimated beams of mul-timegaelectronvolt ions is a promising and fast-growingarea of high-eld science [1]. Proton energies up to 55 MeVand ion energy> 10 MeV/u ([2] and [3] and references therein)with large particle numbers [4], [5], i.e.,1011 1013 ions pershot, havebeen measured with a very high beam quality [6], [7].Such a novel pulsed ion source could have a signicant impactfor a number of applications such as radiography [8], [9], ac-celerators [7], [10], fusion science [11], medical use [12][14],or transmutation of nuclear waste [15]. However, the requiredenergies for those applications are very high: up to 250 MeV formedical use, 1 GeV for nuclear spallation, and in the order of teraelectronvolts for high-energy particle physics. Such ener-gies are, nowadays, far from being achieved with laser-particleacceleration but are routinely obtained with large facilities that

Manuscript received November 15, 2007; revised June 3, 2008. This workwas supported in part by Euratom/Marie Curie actions and in part by SPARX.

P. Antici was with the Laboratoire pour lUtilisation des Lasers Intenses,cole Polytechnique, CNRS, CEA, UPMC, 91128 Palaiseau Cedex, France,and the Dipartimento di Energetica, Universit di Roma La Sapienza, 00161Rome, Italy. He is now with the Istituto Nazionale di Fisica Nucleare, 00044Frascati, Italy.

M. Fazi, M. Migliorati, and L. Palumbo are with the Dipartimento diEnergetica, Universit di Roma La Sapienza, 00161 Roma, Italy.

A. Lombardi is with ATreP, 38100 Trento, Italy.P. Audebert and J. Fuchs are with the Laboratoire pour lUtilisation des

Lasers Intenses Laboratory, UMR 7605 CNRS-CEA-cole Polytechnique-Universit Paris VI, 91128 Palaiseau, France.

Color versions of one or more of the gures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identier 10.1109/TPS.2008.2001412

use conventional acceleration devices such as linear accelerators (linacs) and cyclotrons. The laser-based ion source hashowever, some unique features: 1) transverse emittance, 2) versatility of ions (changing of targets), 3) compact accelerationup to a few tens of megaelectronvolts due to a strong electrield at the source, and 4) high number of particles per shotTherefore, if one could couple high-energy ions generated by table-top laser-particle accelerator to conventional acceleratingdevices such as linacs or cyclotrons, there could be a bene

in terms of compacting the overall structure and increasing thbeam luminosity. This hybrid accelerator would prot from thunique characteristics of the laser-generated-ion particle sourcand from its moderate cost and small size. Up to now, no studof a hybrid model proposal has ever been performed. In thipaper, we report on simulations that have been carried out withPARMELA code version 3.38 [16], a particle code used to simulate the transport and acceleration of particles in acceleratorsin order to study the postacceleration of laser-generated ions ba conventional linac design. In this paper, we have restrictedourselves to the study of the injection of protons in a singldrift-tube linac (DTL) as a rst stage of postacceleration. Thaccelerator is composed of a laser-accelerated proton source, drift-focusing section, and a nal accelerating structure using DTL tank (also called Alvarez) [17].

II. PROTONSOURCE

High-current collimated multimegaelectronvolt beams ofions are generated by irradiating thin solid foils with ultraintense (> 1018 W cm 2 ) short-pulse lasers (30 fs10 ps)produced by the chirped-pulse-amplication technique [18]Currently, the dominant mechanism that leads to the acceleration of high-energy protons from solids in the forward directio(i.e., in the laser direction) occurs at thenonirradiated (rear) sur-

face [19], [20]. Laminar acceleration in the backward directio(toward the laser) of ions generated at the target front surfacexist, but it does require high-contrast laser pulses, whichadds complexity to the process. In this paper, we will restricourselves to the case of standard contrastlimited lasers whera laminar beam with very low emittance parameters, either inthe transverse or longitudinal direction, is only produced athe target rear surface. Recently, the use of an ultrafast lasertriggered microlens [21] has allowed to refocus or collimatthe divergent laser-accelerated proton beam for transport ovelarge distances and to select a small energy spread E/E 1out of the energy spectrum of the beam. The energy-selectiocapability of the microlens, that is tunable, is shown in Fig. 1(b)

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1844 IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 36, NO. 4, AUGUST

Fig. 1. (a) Basic scheme of the ultrafast laser-triggered microlens. (b) Typicalproton spectra as recorded (black) without microlens and (green) with themicrolens [21].

Such a relativistic laserplasma device can achieve the angularand spectral control of the high-current laser-accelerated ionbeams that is required in order to match the necessary injectionparameters for the linac.

In the simulation, proton energies after the cylinder werein the range of 6.97.1 MeV. This gives2 109 protonsand a total charge of 320 pC. Using a frequency of 350 MHzfor the downstream linac, we obtain an equivalent maximumtotal current of 112 mA that would run through the linac. The

choice of the frequency of 350 MHz (as well as the numberof cells in the DTL, see as follows) has been motivated by thedesign chosen for the Spallation Neutron Source facility andhas been tested to be a good compromise in terms of resultsand cost efciency. PARMELA is a 3-D code. By convention,we denex and y as the transverse axes, the choice of theseprojections (as will be shown later) being arbitrary (physicalquantities become asymmetric between the two axes only dueto asymmetry in the elements encountered by the beam on itspath as, e.g., is the case for the quadrupoles). The transversesource sizes of the beam used in this paper were, indicatingall values as full-width at half-maximum,x = y = 80 m

with x = y = 20 m and x = y = 40 mrad withx =y = 9 mrad. Regarding the calculation of the transverseemittance, for the transverse phasespace dimensions (inthis paper,x px for beam propagation alongz), the areaof the bounding phasespace ellipse equals N , where theroot-mean-square (rms) value of the normalized emittanceN , at a specic beam energy (or momentump), is expressed asN = ( p/mc )[ x2 x 2 xx 2 ]1 / 2 , wherem is the ion mass,c is the velocity of light,x is the particle position within thebeam envelope, andx = ( px /p z ) is the particles divergence inthex-direction. At a beam waist,N = x x , wherex andx are the rms values of the beamwidth and divergence angle.This leads to an unnormalized transverse emittance of source =0.180 mm mrad in bothx- andy-directions. Gaussian particledistribution has been used inx- andy-directions.

Fig. 2. Global scheme of the hybrid accelerator, showing the sizes for source, focusing, and accelerating sections.

III. POSTACCELERATINGSTRUCTURE

Since it is technically not foreseeable to couple an acceleating device directly to the lasertarget interaction point insidthe target chamber, a drift space of 60 cm (corresponding a typical target chamber radius) has been inserted before thproton beam is captured by the focusing structure. Adding tha conventional focusing section is necessary, since after thdrift space the proton bunch, although its divergence has bereduced by the microlens, has diverged, and therefore, it is neessary to refocus the beam. Five quadrupoles of different lengandgradient havebeen designed for this structure.The interplabetween two quadrupoles with gradients of opposite sign givan overall focusing structure that allows injecting the beam the accelerating structure. The acceleration section has beemodeled using a DTL tank which is a radio cavity operatinin the TM010 mode. An acceleration using the more compacand more expensive radio-frequency (RF) radiopole (RFQ) not possible for protons above7 MeV, since the accelerationefciency decreases with increasing energy and is not anymoof advantage for our considered energies. In the DTL, 48 Rcavities (cells) containing quadrupoles are placed, separateby a drift space (gap) for longitudinal beam acceleration. Ce

parameters inside the DTL have been calculated using thdriftkickdrift method. The calculated magnetic- and electrield structures inside the DTL have been computed with thcode SUPERFISH coder version 7.16 (and postprocessor SFan electromagnetic-eld solver which creates eld maps finput in PARMELA.

The total size of the structure, starting from the protosource, is 785 cm, divided into three sections, 60 cm from tsource to the focusing section, 123 cm focusing section, an602 cm DTL tank as shown in Fig. 2.

IV. RESULTS

PARMELA gives outputs of the relevant beam parameteat xed steps during the simulation, allowing tracing the beabehavior during its capture, transport, and acceleration. Fig.shows the result of the PARMELA simulation along the entistructure without considering space-charge effects. As one csee, the oscillating character of the energy spread and bealength is due to the focusingdefocusing property of the struture. This means that the beam is focused by one quadrupole one transverse direction (e.g.,x) but, therefore, defocused in theother direction (e.g.,y). Conversely, with the next quadrupole,the beam is focused and defocused in the other direction (i.focused iny and defocused inx). Since the beam enters theaccelerating structure while one of the axis is focused and tother defocused, the emittance and beam sizes will not va



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ANTICIet al. : POSTACCELERATION OF LASER-GENERATED HIGH-ENERGY PROTONS 18

Fig. 3. Results of the PARMELA simulation for the hybrid accelerator andwithout considering space-charge effects. (a) Proton energy. (b) Unnormalizedrms transverse emittance. (c) Normalized rms transverse emittance. (d) Longi-tudinal bunch length. (e) Transverse beam size. (f) Energy spread.

in a symmetrical way. Since simulations shown in Fig. 3 areperformed without space-charge effects, the variation of the

emittance and of the beam size are decoupled.Several simulations have been run in trying to optimize thedifferent parameters in the acceleration such as acceleratingeld gradient, drift-tube size, and phase. We are aware thatthe proposed structure might still be improved to reduce thehigh transverse-emittance growthat theentrance of the focusingsection.

Space-charge effects have been added to the proposed ac-celerator, starting from an initial proton-beam current of I =0.112 mA, i.e., reducing by a factor of 1000 the number of protons that are emitted from the proton source. We have thenincreased the beam current to see up to which point the designof the hybrid accelerator still yields reasonable results.

For currents< 0.66 mA, the nal energy is always closeto 14.54 MeV, similar to the nal energy without space-

charge effects. When using currents< 0.66 mA, transverse-emittances values increase by less than a factor of two as compared to the transverse-emittance values without consideringspace charge effects. However, for proton-beam currents ove0.66 mA, the transverse emittance increases strongly, particularly when the beam enters the focusing section. ForI =

1.12 mA, the transverse-emittance values are too high as com-pared to the existing standards. We can, therefore, not use thicurrent as entrance current for the current-accelerator designA further simulation using 11.2 mA (which corresponds tousing a tenth of all particles produced by the source within thchosen energy range) has nally shown that, with this beamcurrent and even accepting bad values of transverse emittancethe acceleration is not any more ensured.

In conclusion, the PARMELA simulations performed in thispaper show that a hybrid accelerator, coupling laser-generatedprotons with a traditional linac, is feasible and offers ways tboost the particle energy while maintaining an emittance lowethan found in current accelerators, usually found to be betwee28 mm mrad for unnormalized emittance. In addition, thebeam current is comparable with the one used in existing applications, such as positron emission tomography and spallatio(on the order of 0.15 mA). However, the simulations alsoreveal a strong limitation, i.e., that with thecurrent structure, weare not able to use all the protons generated by the ion sourceAn optimization regarding that issue could be found keepingthe beam larger than it is currently, in order to reduce spacecharge effects and to improve the adaptability to the entrance othe focusing section as said in the previous paragraph.

ACKNOWLEDGMENTThe authors would like to thank the LULI laser teams for

their expert support. They would also like to thank T. Cowanand J. Rosenzweig for useful discussions.

REFERENCES[1] M. Borghesiet al. , Fast ion generation by high-intensity laser irradia-

tion of solid targets and applications,Fus. Sci. Technol. , vol. 49, no. 3,pp. 412439, Apr. 2006.

[2] M. Hegelichet al. , MeV ion jets from short-pulse-laser interaction withthin foils,Phys. Rev. Lett. , vol. 89, no. 8, 085002, Aug. 2002.

[3] P. McKenna, Characterization of proton and heavier ion acceleration ultrahigh-intensity laser interactions with heated target foils,Phys. Rev. E, Stat. Phys. Plasmas Fluids Relat. Interdiscip. Top. , vol. 70, no. 3,036405, Sep. 2004.

[4] E. L. Clarket al. , Measurements of energetic proton transport throughmagnetized plasma from intense laser interactions with solids,Phys. Rev. Lett. , vol. 84, no. 4, pp. 670673, Jan. 2000.

[5] R. A. Snavelyet al. , Intense high-energy proton beams frompetawatt-laser irradiation of solids,Phys. Rev. Lett. , vol. 85, no. 14,pp. 29452948, Jan. 2000.

[6] S. V. Bulanovet al. , Feasibility of using laser ion accelerators in protontherapy,Plasma Phys. Rep. , vol. 28, no. 5, pp. 453456, May 2002.

[7] T. E. Cowanet al. , Ultralow emittance, multi-MeV proton beams from alaser virtual-cathode plasma accelerator,Phys. Rev. Lett. , vol. 92, no. 20,204801, May 2004.

[8] M. Borghesiet al. , Electric eld detection in laserplasma interactionexperiments via the proton imaging technique,Phys. Plasmas , vol. 9,no. 5, p. 2214, May 2002.

[9] M. Rothet al. , Energetic ions generated by laser pulses: A detailed studyon target properties,Phys. Rev. Spec. Top., Accel. Beams , vol. 5, no. 6,061301, Jun. 2002.



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[10] M. Borghesiet al. , Multi-MeV proton source investigations in ultra-intense laserfoil interactions,Phys. Rev. Lett. , vol. 92, no. 5, 055003,Feb. 2004.

[11] P. K. Patelet al. , Isochoric heating of solid-density matter with anultrafast proton beam,Phys. Rev. Lett. , vol. 91, no. 12, 125 004,Sep. 2003.

[12] M. I. K. Santalaet al. , Production of radioactive nuclides by energeticprotons generated from intense laserplasma interactions,Appl. Phys. Lett. , vol. 78, no. 1, p. 19, Jan. 2001.[13] J. Fuchset al. , Laser-driven proton scaling laws and new paths towardsenergy increase,Nature Phys. , vol. 2, no. 1, pp. 4854, 2006.

[14] E. Lefebvreet al. , Numerical simulation of isotope production forpositronemission tomography withlaser-accelerated ions, J.Appl. Phys. ,vol. 100, no. 11, 113308, Dec. 2006.

[15] P. McKennaet al. , Broad energy spectrum of laser-accelerated protonsfor spallation-related physics,Phys. Rev. Lett. , vol. 94, no. 8, 084801,Mar. 2005.

[16] L. M. Young and J. Billen, 2004, PARMELA LANL (LA-UR-96-1835).[17] T. Wangler,Principles of RF Linear Accelerators . New York: Wiley-

Interscience, 1998.[18] D. Strickland and G. Mourou, Compression of amplied chirped optical

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Patrizio Antici received the M.S. degree in electricalengineering from Universit di Roma La Sapienza,Rome, Italy, in 1999 and the Ph.D. degree fromcole Polytechnique, Paris, France, and Universitdi Roma La Sapienza, in 2007.

He is currently with the Istituto Nazionale diFisica Nucleare, Frascati, Italy, where he workswithin theSPARX-FEL project andcollaborateswithLOA, Palaiseau, France, for the Extreme Light In-frastructure Project. His research interests includehigh-power laser interaction with matter, inertial

connement fusion physics,andphysics of high-energy laser-drivenionsourcesand applications.

Mauro Fazi received the degree (cum laude ) inelectronic engineering from Universit di Roma LaSapienza, Rome, Italy, in 2006. His thesis was onthe acceleration of laser-generated proton beam.

He is currently with the Dipartimento di Energet-ica, Universit di Roma La Sapienza, and also witha company working in the information technologyeld.

Augusto Lombardi , photograph and biography not available at the time opublication.

Mauro Migliorati , photograph and biography not available at the time opublication.

Luigi Palumbo received the Laurea degree in elec-tronic engineering in 1979.

He wasa Fellow with CERN in 1982,a Researcherin 1984, an Associate Professor in 1992, and a FullProfessor in 2000. He currently teaches physics withUniversit di Roma La Sapienza, Rome, Italy. Hismain eld of research is related to particle acceler-ators for high-energy physics and for interdiscipli-nary applications. He is currently responsible of theSPARX-FEL project in Rome.

Mr. Palumbo is an Associate Member of the Isti-tuto Nazionale di Fisica Nucleare and a member of the EPS AG Board.

Patrick Audebert , photograph and biography not available at the time opublication.

Julien Fuchs received the M.S. degree in opticalengineering from Ecole Suprieure dOptique, OrsayCedex, France, in 1992 and the Ph.D. degree fromUniversit du Qubec, Quebec City, QC, Canada,in 1998.

He has been a Researcher (since 1998) and aProfessor (since 2006) with the Laboratoire pourlUtilisation des Lasers Intenses Laboratory, CNRS(Ecole Polytechnique), Paris, France, after a sab-batical from 2002 to 2004 at General Atomics,San Diego, CA. His research interests include high-

power laser interaction with matter, inertial connement fusion physics, aphysics of high-energy laser-driven ion sources and applications.

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