M o n o e n e r g e t i c A c c e l e r a t i o n o f E l e c t r o n s b y L a s e r - D r i v e n P l a s m a W a v e

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1 USj-WS on HIF & HEDP at Utsunomiya 29 Sep, M o n o e n e r g e t i c A c c e l e r a t i o n o f E l e c t r o n s b y L a s e r - D r i v e n P l a s m a W a v e Kazuyoshi KOYAMA, Takayuki WATANABE 1), Masahiro ADACHI 2), Eisuke MIURA, Susumu KATO, Shinichi MASUDA 3), and Mitsumori TANIMOTO 4) National Institute of Advanced Industrial Science and Technology (AIST), 1) Utsunomiya University, 2) Hiroshima University, 3) National Institute of Radiological Science (NIRS), 4) Meisei University This work was financially supported by the Budget for Nuclear Research of the MEXT, based on the screening and counseling by the Atomic Energy Commission and the Advanced Compact Accelerator Development Program of the MEXT.

2 O u t l i n e Monoenergetic electron acceleration with a laser power of 2-TW. Moderately high density plasmas ( cm -3 ) were used for electron acceleration. E 7-15MeV Monoenergetic electron acceleration with a laser power of 3-4TW. Monoenergetic beams were produced at the density of approximately (4-5)X10 19 cm -3. The focal length 300mm (160mm at the previous experiment) E 18-25MeV Empirical scaling law of monoenergetic acceleration of electron.

3 Rf-linac Laser-plasma Accelerator Characteristic Scale Length Several-cm 10-cm Several- µm 100-µm Structure Metal electrodes Plasma (pulse) Field Gradient MeV/m GeV/m Length to get 1GeV m 1-10 cm Bunch length pico-second femto-second Present Status mature technology a lot of problems

4 Growth and Problem MAXIMUM ENERGY (MeV) Glass Lasers ( fs) RAL KEK LLNL RAL CUOS NRL KEK RAL RAL MPQ CUOS CUOS LULI RAL ILE LOA U.TOKYO AIST NRL YEAR Ti;S Lasers (<100fs) Detection Threshold Electron Energy (MeV) Maximum Acceleration Energy grows >100 times and reaches >200MeV in this decade. Number of electrons (/MeV/ sr) V.Malka, et al., Science 298,1596 (2002). However, the energy spectrum of electron bunch was very wide (Boltzman-like distribution).

5 Principle of Laser-plasma Particle Accelerator ω p τ L π ω p τ L > π Electrons are expelled by the ponderomotive force of the laser pulse. Electrons plasma wave is excited after the pulse. Laser Wakefield Accelerator For mono-energetic acceleration T. Tajima and J. M. Dawson, Laser Electron Accelerator, Phys. Rev. Lett. 43, (1979), A small number of ( single) potential well ωpτl π rapid decay ωpτl>π slow growth & rapid decay Localized trapping of electrons (local breaking) No wavebreaking during the acceleration Self-modulated Laser Wakefield Accelerator t 0 t 1 Electron Bunch t 2 Plasmawave Z

6 Another Acceleration Scheme of mono-energetic acceleration (Bubble acceleration / Bullet regime) ω p τ L π A. Pukhov, J. Meyer-ter-vehn, Appl. Phys. B 74, (2002)

7 Schematic Drawing of Experimental Setup Forward Scattering Spectrum Electron Energy Spectrum IR-Vis. Spectrometer Energy Angle Divergence E Gas Jet Mirror and/or Filter (0.2 T; 30MeV-max) Electron Spectrometer Self-scattering Image ( 800-nm ) 2TW-Laser Pulse Short Pulse Shadowgraph ( 50-fs)

8 Layout in the Target Chamber

9 LASER E x p e r i m e n t a l C o n d i t i o n Ti:sapphire laser - Wavelength ; 800 nm - Power ; 2 TW - Pulse width ; 50 fs - Focus diameter ; 5μm (w0=4.3μm) - F & (focal length) ; 3.5 (165 mm) Focus Intensity ; 5x10 18 W/cm 2 (a0=1.5) TARGET Supersonic gas jet - Gas ; N2, He - Reservoir pressure ; 2-8 MPa (20-80 bar) - Mach number ; 3-4 (for N2) PLASMA DENSITY ( Neutral density & BSI-model) ( )x10 20 cm -3 ; N2 (5+ at IL >10 18 W/cm 2 ) ( )x10 20 cm -3 ; He

10 Spectrum of Monoenergetic Electron beam Divergence angle ± shots accumulation Number of Electrons(/MeV/sr/shot) Energy x MeV 30MeV 90-shots average Electron energy (MeV) I 0 = W/cm 2 ( a 0 1.5) λ L = 800nm n e = cm -3 Normalized emittance ε n = γσδθ 0.7π mm mrad E.Miura, K.Koyama, S.Kato, N.Saito, M.Adachi, Y.Kawada, T.Nakamura, and M.Tanimoto; Appl. Phys. Lett., Vol.86, ,

11 Spectra of Electron Energy and Forward Scattering Number of Electrons (/MeV/sr/shot} bar cm bar (2x20 cm -3 ) Number of electrons in a bunch (/MeV/sr/shot) 30 bar (1.5x20 cm -3 ) Number 10 5 of Electrons [/MeV/sr/shot] Electron Energy (MeV) Average of 90 laser shots Electron energy (MeV) Scattering Intensity (arb.unit) Intensity [arb. unit] 10 0 First Stokes Satellite Peaks Electron density (cm -3 ) 2.0x x x Wavelength [nm] E Main Pulse (50fs) Stokes satellite peaks grow large while the ne is increasing from to 1.5x10 20 cm -3. Stokes satellite become intense and broad at n e > 2x10 20 cm -3.

12 Plasma wave propagated 500μm (>> the dephasing length). Side Scattering Image and Side-view of Shadowgraph - A fishbone structure of the side scattering well correlated with Stokes satellites of forward scattering. - Side scattering is quite intense. 0.5 m Nozzle Exit Laser Beam Probe Pulse (50fs) Scattering of Main pulse Sidescattering(800nm) Main Pulse (50fs) E - Coherent scattering by the plasma wave. - Terminated by breakup of the laser beam. Shadowgraph

13 Electron Energy Spectra for Different Electron Densities I L = W/cm 2 Number of Electrons (/MeV/sr/shot} bar cm bar (2x20 cm -3 ) 60 bar (3.1x20 cm -3 ) 80 bar (4.4x20 cm -3 ) 30 bar (1.5x20 cm -3 ) Number 10 5 of Electrons [/MeV/sr/shot] Electron Energy (MeV) N2-gasjet Energy Gain of Monoenergetic Peak (MeV) He N Maxwellian Electron density (cm -3 ) High Density ; huge number of electrons Maxwellian Distribution Low Density ; no high-energy electron Monoenergetic electron beam is accelerated in the narrow region of the density.

14 Mono-energetic spectra at various electron densities AIST RAL 2TW-50fs Number of electrons in a bunch (/MeV/sr/shot) 1.0E+06 ω p τ L / 2π 5.5 Average of 90 laser shots Electron energy (MeV) ω p τ L / 2π 1.6 LBNL #e[/mev] 8TW-55fs 2-5 mrad Charge>100 pc Energy [MeV] 16TW-40fs LOA 30TW-30fs Guided ω p τ L / 2π 2.2 ω p τ L / 2π E E E E+05 PSL per relative energy spread 0.0E Energy (MeV)

15 LASER ACC/MONOENERGY Empirical Scaling-law of Laser-Plasma Accelerator LOA ; 30TW, 30fs, 1.5x10 18 W/cm 2 RAL ; 16TW, 40fs, 1.5x10 18 W/cm 2 LBNL; 8TW, 50fs, 1.0x10 19 W/cm 2 AIST ; 2TW, 50fs, 1.5x10 18 W/cm 2 n e -1 LOA LBNL RAL AIST LBNL AIST RAL LOA

16 For increasing energy gain and number of electrons Energy gain Lower electron density (long dephsing length) Number of electrons Large beam diameter long plasma wavelength Large density modulation Solution is Large focal spot and Large laser power Long focus

17 Experiments with a long focus D L f λ L d D = λ L F r 1/2 f L R L R = πw 2 0 w λ 0 = r 1/2 / ln2 /2 1.7r 1/2 0 L dp = cπ γ 2 φ ω p * The focal spot diameter and the Rayleigh range are increased ( 2 times). * In order to keep the relativistic intensity of the laser (>10 18 W/cm 2 ), the laser power should be increased from 2 TW to 3-4TW. * For elongate the dephasing length, the plasma density is decreased from 1.5X10 20 cm -3 to (2-4)X10 19 cm -3.

18 Electron Density Dependence of Electron Energy Spectra 2.7 TW The monoenergetic peak with the energy of 18-25MeV was observed at ne = (4~5)X10 19 cm -3. The number of electrons is 4 X10 5 / shot in a peak. The divergence of the monoenergetic electron bunch is estimated at 20 mrad.

19 Single-shot Energy Spectrum of Electron Beam P L = 3.8 TW n e = 3.2X10 19 cm -3 * A few monoenergetic components are observed by the single shot measurement. * This might be caused by the excitation of a few potential wells, which are responsible for the electron acceleration. * Number of electrons within the monoenergetic peaks are more than 10 6.

20 Single-shot Spectrum of Forward-scattering of laser P L = 3.8 TW n e = 3.2X10 19 cm -3 The second harmonic laser light was observed for the laser power higher than 3TW. The plasma electron density estimated from the the Stokes satellite peaks is agree with the gas jet density measurement. The density modulation is 60-80%.

21 Energy gain of monoenergetic electrons at the new setup Energy Gain of Monoenergetic Peak (MeV) TW He He N Electron density (cm -3 )

22 LASER ACC/MONOENERGY Empirical Scaling-law of Laser-Plasma Accelerator n e -1 LOA LBNL RAL LBNL RAL LOA AIST AIST W max = η π π r L 2 n 1 2 e Pγ

23 Plasma Diagnostics / Thomson scattering To know the growth and dumping rate of the plasma wave. Pump Pulse Probe Pulse Plasma Spectrometer

24 Gas Jet Main Laser Pulse 10TW Plasma Collision Laser Pulse Electron Bunch 1TW Hard X-ray + Improvement of reproducibility / stability

25 E X σ σ T 4γ 2 E L 1+ ( γθ) 2 + 4γ E L / m e c γ E L m e c , γ E << m L e c2 σ T = cm 2 ( ) 4γ 2 E L ( ) 2 (Ψ = π, θ << 1) Δθ 1/γ 1+ γθ N x σ T cn e n L Vτ L Δθ 1/γ n e cm -3 ;(N e = ,V πr 2 cτ L ) n L cm -3 ;(I L = W/cm 2 ) V = cm 3 ;(r = 10µm,τ L = 50fs) 1.0 dσ 2 / πr de 0 x γ electron =100 ( 50MeV) λ laser =800 nm N x photons/5mrad 370ergs/5mrad 1erg/cm 2 20cmΦ at 20m Ex (kev) θ (rad)

26 C o n c l u s i o n Monoenergetic electron beams were obtained by the laser-plasma particle acceleration. The energy gain is approximately proportional to P/ne, however the density range is limited for the fixed laser power. The normalized emittance of the beam were less than 1 π mm mrad. The Stokes satellite peak in the forward scattering shows that the monoenergetic beam was accelerated by the self-modulated wakefield.