Mono-energetic Electron Generation and Plasma Diagnosis Experiments in a Laser Plasma Cathode

Transcription

1 Nuclear Engineering Research Laboratory Graduate School of Engineering University of Tokyo Mono-energetic Electron Generation and Plasma Diagnosis Experiments in a Laser Plasma Cathode K. Kinoshita, T. Hosokai, A. Zhidkov 1, T. Ohkubo, A. Maekawa, K. Kobayashi and M. Uesaka Nuclear Professional School, School of Engineering, University of Tokyo 1. National Institute of Radiological Sciences JAPAN A. Yamazaki 2, H. Kotaki 2, M. Kando 2, K. Nakajima 2 and S. V. Bulanov 2 2. Advanced Photon Research Center, Japan Atomic Energy Research Institute Kansai The phy sics and A pplications of High B rightness Electron Beams, Erice (Italy), 10th-14th Oct

2 Superv isor Prof. Staff (Ex periment) Staff (Ex periment) S taff (Simulation) D3-student (Simulation) D3-student (Ex periment) M1-student (Experiment) M1-student (Experiment) Mitsuru UESAKA Tomonao HOSOKAI Kenichi KINOSHITA Alexei ZHIDKOV Takeru OHKUBO Atsushi YAMAZAKI Akira MAEKAWA Kazuyuki KOBAYASHI (Kyoto Univ) (Collaborators) 17TW-37fs Ti:Sappire Laser facility JAERI-APRC Masaki KANDO JAERI-APRC Hideyuki KOTAKI Sergei V. BULANOV JAERI-APRC JA ER I-A PRC Kazuhisa NAKAJIMA KEK

3 Research Goals -- Femtosecond Electron Beam High quality femtosecond electron beam - 10 fs pulse duration - 1 nc charge - E/E ~ 1 % 2nd pulse Colliding Pulse X-rays Gas jet Shock wave produced by prepulse 1st pulse Driv e Pulse - Jitter free Electron bunch, 40fs, 10pC, can be produced. Femtosecond pump-probe analysis Fast processes in radiation chemistry Electronic behaviorin THz devices Femtosecond X-ray generation through laser Compton scattering E ~ 1-10 kev, ( ~ 10 9 photon/s, within 1 deg )

4 Plasma Channel by Capillary DC 2-staged Acceleration ~20kV fs, quasi-mono energetic e-beam Low density Vacuum Wave-free Slit Jet Femtosecond Injector High density (~10 17 cm -3 ) (~ cm -3 ) Dephaseing ~10cm ~100µm Length Charge few ~pc huge ~nc Acc. Energy High Low Plasma wavelength ~100fs ~10fs Wake-fields Regular few cycles Optical guiding Effective??? Ti:sapphire Laser pulse ~10TW Requirements 2staged Acc. Injector : Plasma Cathode Further acc : Capillary DC High Charge Uitrashort High Energy How to overcome the contradictory? High density gas jet for injector Low density with optical guiding for further acc.

5 Laser Wakefield Accelerator (Tajima and Dawson Phys. Rev.Lett. 43, 267, (1979)) Transverse plasma oscillation w > wp StationaryIons E Z underdense plasma λ p E Electron bunch λ p F Pond E 2 Longitudinal plasma oscillation at ω p Multi- Tera Watt Ten s femtosecond laser pulse

6 Femtosecond Electron Injector by Plasma Wave Breaking Wave-breaking Rapid injection into correct acceleration phase Femtosecond e- bunch Wave-breaking field E B ~[2(ω/ω pl -1)] 1/2 mcω pl /e Density gradient λ pl N/(dN/dx)~1 ω :Laser frequency ω pl : plasma frequency λ pl =2πc/ω pl λ pl: plasma wavelength Pump Pulse Steep Density Transition Wakefield Density Density e e e e e e e e Gas Jet Plasma e e e Injection by wave-breaking e-bunch Reference : S.V.Bulanov, et al, Phys.Rev.E. 58, R5257 e Acceleration e e e ee e e e e e Pump Pulse

7 Experimental Setup at Univ. of Tokyo (BS) Ti:Sapphire laser (~11TW, 37fs) Probe pulse (~1%) Slit Gas Jet Photo Diode Gas OAP f /3.5 (f=178mm) BG39 Delay line ICCD Lens Ti-foil+DRZ ICT monitor Deflector Magnet e-beam Band pass filter Filters Lens 1.2mm Jet Pump Laser CCD 4.0mm Gas density up to 6x10 19 cm Supersonic Slit Nozzle

8 Y [µm] Intensity [a.u.] Laser Parameters (Ti:Sapphire 17TW, 37fs) Laser spot X [µm] 5.0µm (FWHM) 8.0µm@1/e X [µm] Contrast ratio W(z)[µm] Rayleigh Length Laser pulse L~53 µm Z[µm] Contrast Ratio 3rd order cross correlator SEQUOIA Time[ps] Focusing Parameters OAP f =177mm Beam size D~50mm F # ~3.5 Spot size ~8.0 2 Rayleigh length ~53 µm Power Density for Main Pulse (~11TW) ~2.2x10 19 Wcm -2 a 0 ~3.1 Contrast Ratio 1:5X10-7 Power Density for Pre-pulse 2ns~1.0x10 13 Wcm -2 few ps~1.0x10 16 Wcm -2

9 Experimental Setup (Gas, Focusing, Beam Generation) Channel Schlieren θ~2 o He Me=5 ρ/ρ 0(x10-2 ) Gas density profile Focus point Nozzle Exit 1.2 mm propagation Axis Density profile inside gas-jet Distance [mm]

10 Summary of Prepulse effects -1 Intensity[arb.units] Main pulse Pre-pulse (c) (b) (a) t2 t Time [ns] Imaging Plate 125mm Electron Distribution [arb.un] PIC simulation Electron Energy [MeV] Electron Signal [arb.un] Modification of density profile by ns prepulse (Hydrodynamic motion) Ti:Sapphire laser Ipre =10 13 Wcm -2 Cavity 0 300µm 300µm 2.8x10 19 cm -3 Rayleigh Length ~50µm Contrast ratio of ns Pre-pulse to main pulse 1: ~10 6 0cm -3 0 Plasma density, N(x)/N focus point Distance[µm] Longitudinal distribution Reference:T.Hosokai,et al.,phys Rev.E 67, (2003) Shock Strong wave front Rapid Injection ~40fs bunch Plasma wavelength

11 Summary of Prepulse effects -2 Prepulse effects in high density gas 80mm Wavebreaking Wavebreaking Strong Diffraction Laser Axis p e-spot on LANEX CCD Image of Plasma Ultra-short Lase pulse Plasma Cavity driven by laser prepulse Thomson scattering Reference:T.Hosokai,et al.,phys Plasms, 11, L57 (2003) L as e r p u ls e

12 Energy distributions of acceleratedated electrons. 100% Energy spread case Quasi-mono energy case Signal on the detector Signal on the detector (x10 4 ) (x10 4 ) Electron Signal [arb.units] Detector Position [mm] Laser Axis ~10pC /Shot * Single-shot measurement. Electron Signal [arb.units] Detector Position [mm] Laser Axis E/E~10% (Minimum case) ~10pC /Shot * Single-shot measurement Electron Energy [MeV] Electron Energy [MeV]

13 Channel Formation Inside Pre-plasma Cavity Shadowgraph Images (a) -2.0ps (d) +5.2ps (b) 0ps (e) Cavity (c) +1.2ps Channel I~11TW (37fs,790nm) Ne~4x10 19 cm -3 (Helium) * Polarization: parallel to the axis of probe pulse.

14 A Narrow Channel Formation Inside Pre-plasma Cavity Interferogram Laser Gas Jet 1.2 mm Schlieren Image +5.2ps I~11TW (37fs,790nm) Ne~4x10 19 cm -3 (Helium) Laser Gas Jet 1.2 mm +5.2ps * Polarization: parallel to the axis of probe pulse.

15 Shadowgraph Images overlapped with Thomson Scattering. 100% Energy spread case Shadowgraph + Thomson Scattering Laser Gas Jet 1.2 mm (+5.2ps) Quasi-mono energy case Laser Gas Jet 1.2 mm (+5.2ps) I~11TW (37fs,790nm) Ne~4x10 19 cm -3 (Helium) Focus & defocus in the channel Density ramp by shockwave * Polarization: perpendicular to the axis of probe pulse.

16 Optical guiding channel formation process 1st Stage Cavity formation by ns pre-pulse ns pre -pulse Hydrodynamic Expansion I~10 12 Wcm -2 (~2ns) Shock 3rd Stage Main pulse propagation through the channel. 2nd Stage Narrow channel formation inside the cavity by ps pre-pulse Refraction by density effects ps pre -pulse Electron evacuation by ponderomotive force I~10 16 Wcm -2 Shock Channel (~few ps) Chanel guiding through the narrow channel Electron evacuation by ponderomotive force Channel becomes deeper ~TW main -pulse Rapid Injection Shock Optical guide I~10 19 Wcm -2 Mono-energetic electrons (~35 fs)

17 Density structure inside cavity Thomson Scattering Cavity Channel Deisity modification by prepulses Transverse Ne R Z Focus & defocus by optical guide in the channel front Ne Longitudinal Focus Point R Z

18 PIC Simulation N e O Initial density consition (Preform channel) 20µm 5µm 4x10 19 cm -3 1x10 19 cm -3 y 35fs Laser x Quasimonoenergetic 10µm 1x10 19 W cm -2 Focus & Wave breaking Defocus & Regular wakefield Focus again Overfocus in adensity channel & Rapid injection by wave breaking y x-ct

19 Energy Spectra, (Experiment and PIC Simulation) (x10 4 ) Single-shot spectrum PIC Simulation Electron Signal [arb.units] Electron Signal [arb.units] ~6MeV Electron Energy [MeV] Electron Energy [MeV] Electron Signal[arb.units] ~12MeV [mm] Detector position (Energy) 5-shot accumulated spectrum

20 Further acceleration by capillary discharges, Optical guiding by Fast Z-pinch discharges Long Plasma Channel by Fast Z-pinch discharges in capillary Streak Image of Discharge 0 400µm He 1Torr 4.8kA D = 1mm Laser Pulses OAP Capillary D=1mm L=20mm Plasma Channel D=30~70µm Gas inlet DC ~5nF ~20kV axis Streak Camera Wall D= ~1mm Bθ ~10 4 T/m Channel D<30µm ~5 x10 17 cm -3 Typical e-density profile in the plasma column produced by fast Z-pinch. Plasma channel parameters can be controlled by discharges Ref. T.Hosokai et.al,opt.lett.25,10(2000) Channel 0 10ns 10 Guided 10ns µm Axis 40µm 400µm Channel 0 No. 0 Without Guide 400µm Gate CCD Images of Ti:sapphire Lase pulse 0 10ns µm Axis Streak Images of He-Ne Lase (CW) 50

21 Summary 2-staged acceleration using a gas-jet injector with capillary discharges is one of the most prom ising approach to produce high quality electron bunch with tens M ev, tens fs, and quasi-m ono energetic distribution. Injector -- Laser plasm a cathode Cavity form aton & Density steepening Expanding shock by ns pre-pulse N arrow channel Form ation inside the cavity Focusing of ps-pulse due to density effects inside the w all? Optical guiding through pre-channel inside the cavity Quasi-m ono energetic electrons by LW FA N ext Step Further acceleration using capillary discharges.

22 Approach to quasi mono-energitic femtosecond electron bunch Staged Acceleration A plasma channel can serve as a media for perfect wake-field for further acceleration generated via wave-breaking Selfinjection To make self-injection the cavity length should be longer than the pulse length d=λ p a 0 >cτ and 2 v g c 1 1/ γ ; γ = 1+ a 2 0 / 2 Capillary discharge Further A cceleration High density Low density v =v g ( wave-guide) Gas jet Ti:sapphire Laser Femtosecond Injector (Laser Plasma Cathode) e-injection Self-injection is possible for a laser pulse with τ=50 fs and intensity I=10 19 W/cm 2 in a wave-guide with diameter D~ µm Mono-energy Eemax=mc 2 ao 2 /2 A. Zhidkov, et.al Phys. Rev. E 69, (R) (2004)