We present simulation results and experimental setup for multi-GeV electron generation by a laser plasma wake field accelerator (LWFA) driven by the Texas Petawatt (TPW) laser. Simulations show that, in plasma of density n(e) = 2 - 4 x cm(-3), the TPW laser pulse (1.1 PW, 170 fs) can self-guide over 5 Rayleigh ranges, while electrons self-injected into the LWFA can accelerate up to 7 GeV. Optical diagnostic methods employed to observe the laser beam self-guiding, electron trapping and plasma bubble formation and evolution are discussed. Electron beam diagnostics, including optical transition radiation (OTR) and electron gamma ray shower (EGS) generation, are discussed as well.Physic

Bernstein, A.

D'Avignon, E.

Ditmire, T.

Dong, P.

Downer, M.

Du, D.

Dyer, G.

Fazel, N.

Gaul, E.

Henderson, W.

Kalmykov, S.

Martinez, M.

Reed, S.

Shvets, G.

Wang, X.

Yi, S. A.

Zagdzaj, R.

English

X. Wang

D. Du

S. A. Yi

S. Kalmykov

E. D’avignon

N. Fazel

R. Zagdzaj

S. Reed

P. Dong

W. Henderson

G. Dyer

A. Bernstein

E. Gaul

M. Martinez

G. Shvets

T. Ditmire

M. Downer

Steven H. Gold

Gregory S. Nusinovich

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Multi-GeV Electron Generation Using Texas Petawatt Laser

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Multi‐GeV Electron Generation Using Texas Petawatt LaserX. Wang, D. Du, S. A. Yi, S. Kalmykov, E. D’avignon, N. Fazel, R. Zagdzaj, S. Reed, P. Dong, W. Henderson, G.Dyer, A. Bernstein, E. Gaul, M. Martinez, G. Shvets, T. Ditmire, and M. Downer  Citation: AIP Conference Proceedings 1299, 209 (2010); doi: 10.1063/1.3520315 View online: http://dx.doi.org/10.1063/1.3520315 View Table of Contents: http://scitation.aip.org/content/aip/proceeding/aipcp/1299?ver=pdfcov Published by the AIP Publishing  Articles you may be interested in Numerical modeling of multi-GeV laser wakefield electron acceleration inside a dielectric capillary tube Phys. Plasmas 20, 083120 (2013); 10.1063/1.4819718  Quasimonoenergetic GeV electron bunch generation by the wake-field of the chirped laser pulse Phys. Plasmas 17, 033103 (2010); 10.1063/1.3339908  Generation of above 10 10 Temporal Contrast, above 10 20   W / cm 2 Peak Intensity Pulses at a 10 HzRepetition Rate using an OPCPA Preamplifier in a Double CPA, Ti:sapphire Laser System AIP Conf. Proc. 1153, 3 (2009); 10.1063/1.3204551  Ag ion generation irradiated by Nd:YAG laser onto solid target for use of direct plasma injection schemea) Rev. Sci. Instrum. 79, 02B311 (2008); 10.1063/1.2816235  GeV Wakefield acceleration of low energy electron bunches using Petawatt lasers Phys. Plasmas 12, 093104 (2005); 10.1063/1.2010347   This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to  IP:128.83.205.78 On: Fri, 20 Mar 2015 15:49:20Multi-GeV Electron Generation Using Texas Petawatt LaserX. Wang, D. Du, S.A. Yi, S. Kalmykov, E. D’avignon, N. Fazel, R. Zagdzaj, S. Reed, P. Dong, W. Henderson, G. Dyer, A. Bernstein, E. Gaul, M. Martinez, G. Shvets, T. Ditmire and M. DownerDepartment of Physics, University of Texas at AustinAustin, TX 78712, USAAbstract  We present simulation results  and experimental  setup for multi-GeV electron generation by a laser  plasma wake field accelerator (LWFA) driven by the Texas Petawatt (TPW) laser.  Simulations show that, in plasma of density ne = 2 – 4 ×  1017 cm-3, the TPW laser pulse (1.1 PW, 170 fs) can self-guide over 5 Rayleigh ranges, while electrons self-injected into the LWFA can accelerate up to 7 GeV. Optical diagnostic methods employed to observe the laser beam self-guiding,  electron  trapping  and  plasma  bubble  formation  and  evolution  are  discussed.  Electron  beam  diagnostics, including optical transition radiation (OTR) and electron gamma ray shower (EGS) generation, are discussed as well.Keywords: Multi-GeV, Laser plasma accelerator, Texas petawatt laser systemPACS: 52.38.Kd, 41.75.Jv, 52.70.KzINTRODUCTIONWith the development of chirped pulse amplification (CPA), multi-TW laser peak power and focused intensity over 1019  Wcm-2 have been reached. In this relativistic regime, laser pulses can generate a nonlinear “bubble” structure in underdense plasma, from which electrons are evacuated and an approximately spherical region full of ions is formed [1, 2]. This plasma bubble, with electric field gradient on the order of GV/cm, is a high-quality table-top laser-plasma accelerator (LPA) that has generated quasi-monoenergetic electron bunches up to 1GeV [3,4,5], with many potential applications in science and technology. However, unless external guiding mechanism is applied [6], the acceleration length of a TW-laser-driven LPA, determined by the laser focusing geometry, dephasing length and pump depletion length, is usually around several millimeters, which limits the energy gain to < 1GeV. Furthermore, tightly focusing the  laser  pulse  (<  5μm)  into  plasma  of  density  1019cm-2 can  cause  strong  self-phase  modulation  (SPM)  and filamentation [7], which destroys the stability of bubble propagation and further limits energy gain. Recently recipes [8, 9] for high-energy electron generation with LPA have shown that electrons can be accelerated to multi-GeV by using ~PW laser in more tenuous plasma (1017cm-3). Under these conditions, the dephasing length and depletion length are increased to tens of centimeters, and nonlinearities such as SPM and filamentation are weaker . PW laser power is required to satisfy P ≥ Pcr,  where Pcr = 17ω02/ωp2GW is the critical power for relativistic self-focusing, so that the drive pulse self-guides. The Texas Petawatt (PW) laser pulse (1.1PW, 170fs [10]), the shortest among all PW lasers in operation, with f#/40 focus satisfies the conditions P ≥ Pcr and 1 <  ωpτl  < 2π for relativistic self-focusing in plasma of density ne ~1017cm-3 without SPM instabilities. It reaches high intensity (~1019Wcm-2) at large focal spot (r0  = 80μm) to reduce radial plasma wave effect and increase laser Rayleigh range up to centimeters. Therefore, laser wakefields generated by the TPW laser are well suited for multi-GeV electron acceleration.In the first section of this paper, we summarize the main results of particle-in-cell (PIC) simulations of LPA driven by the TPW laser, using the code WAKE. Further details of these simulations are presented in a paper by S. Kalmykov in this volume. In the second section of this paper, the anticipated experiments are described. 209 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to  IP:128.83.205.78 On: Fri, 20 Mar 2015 15:49:20FIGURE 1. Evolution of TPW laser pulse and plasma structure in tenuous plasma (1017cm-3). Top row shows laser intensity profiles at four different positions (Z = 1.145cm, 1.53cm, 5.72cm, 9.0cm) in the gas cell. The dot line shows the original laser spot size and the arrow indicates laser propagation direction. Bottom row shows the plasma structure profile at the corresponding positions. The dotted line indicates the intensity profile of the laser pulse.SUMMARY OF SIMULATION RESULTSFig.1 shows simulations results for the propagation of the TPW laser pulse (top row) and the bubble structure (bottom row) in plasma of density ne = 1017cm-3.  The laser pulse enters the He gas cell with radius r0  = 80μm, then over-focuses down to radius r0 = 25μm within a Rayleigh range (ZR = 1.9cm) due to relativistic self-focusing (P/Pcr  ~ 7). It then expands to r0  = 40μm and propagates stably up to 5ZR, without significant variation of beam size, with relativistic self-focusing balancing vacuum diffraction. The laser pulse self-guides to 9cm with no filamentation or beam divergence. We confirmed that these favorable propagation characteristics are insensitive to imperfections --- e.g. hot spots, super-Gaussian profiles --- in the incident laser pulse profile.  Fig.1 also shows that  a plasma bubble forms immediately after the laser pulse over-focuses (bottom row, left panel). This bubble structure subsequently contributes to stable self-guiding of the laser pulse. However, the initial over-focusing, subsequent expansion and re-focusing of the drive laser pulse causes corresponding small oscillations in the length of the bubble that are evident in the remaining bottom row panels of Fig. 1. These bubble length oscillations provide a key mechanism for injecting electrons from the ambient plasma into the bubble [11]. By contrast, a laser pulse that was better mode-matched to the plasma bubble, and experienced weaker oscillations, would be unable to capture electrons from such tenuous plasma.  Thus drive laser  over-focusing and bubble length oscillations  are a fundamental component of the design of the TPW-LWFA experiment.  Bubble expansion triggers electron injection, while  subsequent  bubble  contraction  terminates  self-injection,  enabling  the  TPW-LWFA  to  produce  quasi-monoenergetic electrons. Simulations  of  electron acceleration [9]  (not  shown in  this  paper)  predict  that  1nC,  7GeV,  small  divergence electron beam (6mrad.) with energy spread less than 10% can be generated within a 9cm self-guided TPW-LWFA in a tenuous plasma. 210 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to  IP:128.83.205.78 On: Fri, 20 Mar 2015 15:49:20EXPERIMENTAL SETUP FOR TEXAS PETAWATT LASER WAKEFIELD ACCELERATOR Fig. 2 shows the setup of the TPW LWFA experiment. An f#/40 concave spherical dielectric mirror (not shown) focuses the 1.1PW, 170fs, 1058nm laser pulses to r0  = 80μm at the entrance to the He gas cell, located inside the experimental chamber. The intense (~1019 Wcm-2, a0 ~ 4) laser pulse creates and interacts with a low density, uniform He plasma (ne ~ 1017cm-3), generating an oscillating bubble that simulations predict will trap and accelerate electrons to several GeV, as described in the previous section.FIGURE 2. A schematic of the PW LWFA experiment setup.To provide additional control over laser pulse over-focusing, bubble oscillation and electron self-injection, we have also designed an alternative gas cell with two chambers, as shown in Fig. 3, based on simulation results [9]. Chamber I is 5mm long and filled with mixture of He and N2 gas, chamber II is 9cm long and filled with pure He gas. The five 2p shell electrons of nitrogen and 1s shell electron of a helium atom will be fully ionized by the PW laser at its incident intensity (~1019Wcm-2). The dense plasma slab in chamber I will thus act as an enhanced nonlinear plasma lens to over-focus the  laser  beam more  strongly  into  the  lower  plasma  density  in  chamber  II.  By selecting  the  proper percentage of nitrogen in the gas mixture, the focal spot can be optimized without filamenting. Chambers II and I are filled to  equal  pressures,  thus  minimizing  interdiffusion and  plasma density  nonuniformity  that  can distort  laser propagation in chamber II. FIGURE 3. A gas cell for the TPW LWFA experiment.A probe laser pulse, split off from laser beam in the front-end optical parametric amplifier (OPA) of the TPW system, will be used for optical diagnostics of laser-plasma interactions. The probe pulse, initially 1058nm, 30nm bandwidth, and chirped (2ns), passes through a delay line, is compressed to 50fs, then frequency doubled pulse (to 529nm) in a type I betas barium borate (BBO) crystal. The green probe is split into two beams, one for transverse diagnostics of the plasma, the other for frequency-domain holographic (FDH) visualization of the plasma bubble.211 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to  IP:128.83.205.78 On: Fri, 20 Mar 2015 15:49:20As shown in Fig.2, the transverse diagnostic beam is expanded to 9cm to cover the length of the self-guided laser path and formatted in a Milchelson interferometer to measure on-axis plasma density profile during the laser plasma interaction.  The FDH beam, formatted into two pre-chirped 1ps laser pulses, reference and probe, with 3ps temporal separation, propagates at a 4.5 degree angle to the main PW beam, as shown schematically in Fig. 2 and 4(a). The plasma bubble sweeps across the probe beam and leaves a phase shift “streak” in it. The oscillations of the temporally evolving bubble can then be reconstructed from the interferogram generated by reference and probe pulse in frequency domain  using  Fourier  transform methods  described elsewhere  [12].  Fig.4  (b)  and  (c)  present  simulations  of  the bubble’s  reconstructed  phase  streak  based  on  the  experimental  conditions  and  configurations  described  above. Oscillations of the bubble’s tail, which are critical to electron self-injection, are clearly visible.  See the paper by Z. Li in this volume for further details of this frequency-domain streak camera method.  FIGURE 4. Schematic configurations for FDH (a) and simulation results of amplitude (b) and phase reconstruction (c). Black arrow in (a) denotes the propagation of main laser pulse. In (c), red dot line marks possible boundary of plasma bubble and black arrow denotes the direction of temporal evolution of bubble.Incoherent side Thomson scattering (TS) of the main laser beam, as shown in Fig. 2, will be imaged to characterize laser self-guiding. The on-axis variation of TS light intensity, resulting from sequential diffraction and self-focusing, gives experimental evidence for laser self-guiding effect [13, 14]. Two 1 cm long sections, in the middle and at the end of the plasma, are selected and imaged on CCD cameras to study the laser beam evolution. Another image system is employed to image a 5mm section of the laser channel at the over-focused spot, which can be used to study the enhanced self-focusing effect caused by the nonlinear plasma lens described above.As shown in Fig.2, the electron beam emerging from the gas cell propagates immediately through a 1Tesla dipole magnet to deflect sub-GeV electrons to Lanex1 and Lanex2 to measure up to 200MeV and up to 1GeV electrons,  respectively.  A 25μm Al deflector,  which reflects the main laser pulse to a beam dump and transmits > 1 GeV electrons, separates the electron beam from main laser pulse. The electron beam penetrates another thin Al foil for optical transition radiation (OTR) and dumps into a calorimeter, described further below, which measures average electron energy in initial experiments. Two integrated current transformers (ICT) are located on the electron beam path to measure total  charge of electrons and charge of high-energy electrons.  Scattered main laser light  from the Al deflector is collected and imaged to a spectrometer to measure spectral red shifts that evidence wakefield and guiding structure generation [15].212 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to  IP:128.83.205.78 On: Fri, 20 Mar 2015 15:49:20Optical  transition  radiation  (OTR)  occurs  when  the  high-energy  electrons  transit  the  aluminum foil.  In  this experiment, visible and near infrared light (400nm-1100nm) emission from OTR is used to diagnose the electron beam. As shown in Fig.2, the incoherent OTR is split into two beams, one to image the electron bunch transverse spatial distribution, the other to image the angular distribution of the OTR, from which electron energy information can be retrieved. If the electron pulse contains structures shorter than OTR emission wavelength, the OTR can be coherent or partially coherent. Therefore the OTR spectrum is the frequency domain interferogram of coherent OTR light pulses and give temporal information of electron bunch structure [16]. As shown in Fig.2, the OTR emission is also delivered to a spectrometer for electron bunch diagnostic. To measure the average energy of multi-GeV electrons, a calorimeter, consisting of a periodic structure of He gas chambers and Pb plates, is employed. A high energy electron incident on calorimeter deposits its energy in the Pb plate by generating an electron gamma ray shower (EGS) as well as positrons. These low energy particles enter the He gas chamber, ionize He gas and generate ionization pairs. Applying bias voltage on the He chamber, electronic signals from these ionization pairs can be measured. Simulation results for the ionization pairs produced in the EGS process in a calorimeter are shown in Fig.5. By comparing the variation of the numbers of ionization pairs produced in each He chamber, energy of the electron can be deduced. This calorimeter will be independently calibrated with monoenergetic electron beams of known energy.FIGURE 5. EGS simulation of the calorimeter.CONCLUSIONSSimulation results show that the TPW laser beam can be self-guided up to 9cm in the plasma "bubble" regime in plasma of density ~1017 cm-3, whereupon the bubble oscillates in length, captures ~1nC electrons  and accelerates them to  7GeV  with  less  than  10%  energy  spread.  Optical  diagnostics,  i.e.  side  TS  of  main  laser  beam,  transverse interferometry and FDH, are very promising to show experimental evidence for laser self-guiding, and plasma bubble evolution. Electron beam diagnostics, including OTR and calorimetry, can provide very useful information of the electron beam energy, spatial and temporal profiles.ACKNOWLEDGMENTSThis work was supported by U.S. DoE grants DE-FG03-96ER40954 and DE-FG02-07ER54945. The authors wish to thank Vladimir Khudik and Laura Loiacono for very useful discussions.213 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to  IP:128.83.205.78 On: Fri, 20 Mar 2015 15:49:20REFERENCES1. J. Rosenzweig, B. Breizman et al., Phys. Rev. A 44, R6189 (1991).2. A. Pukhov and J. Meyer-ter-Vehn, Appl. Phys. B 74, 355-361 (2002).3. C. G. R. Geddes et al., Nature 431, 538–541 (2004).4. J. Faure et al., Nature 431, 541–544 (2004).5. W. P. Leemans and E. Esarey, Phys. Today, 44-49 (March 2010).6. W. P. Leemans et al., Nature Phys. 2, 696-699 (2006).7. A. G. R. Thomas et al., Phys. Rev. Lett. 100, 255002 (2008).8. W. Lu et al., Phys. Rev. ST Accel. Beams 10, 061301 (2007).9. S. Y. Kalmykov et al., New J. Phys. 12, 045019 (2010).10. E. Gaul et al., Appl. Opt. 49, 1676-1681 (2010).11. S. Y. Kalmykov et al., Phys. Rev. Lett. 103, 135004 (2009).12. N. H. Matlis et al., Nature Phys. 2, 749-753 (2006).13. P. Monot et al., Phys. Rev. Lett. 74, 2953-2956 (1995).14. R. Wagner et al., Phys. Rev. Lett.78, 3215-3218 (1997)15. N. E. Andreev et al., New J. Phys.12, 045024 (2010).16. Y. Glinec et al., Phys. Rev. Lett. 98, 194801 (2007).214 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to  IP:128.83.205.78 On: Fri, 20 Mar 2015 15:49:20

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Multi-GeV Electron Generation Using Texas Petawatt Laser

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