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12ELECTRON ACCELERATION BY PARAMETRICAL PLASMA WAVES CREATED IN RELATIVISTIC LASER-PLASMA INTERACTIONS
Shulyapov S.A.,1 Ivanov K.A.,1 Tsymbalov I.N.,1 Krestovskih D.A.,1 Savel'ev A.B.,1
2 2 2 KsenofontovP.A., BrantovA.V., Bychenkov V.Yu.
1Faculty of Physics and International Laser Center of Lomonosov Moscow State
University, Moscow, Russia P.N. Lebedev Physical Institute, Russian Academy of Sciences, Moscow, Russia
Plasma created by femtosecond laser pulse of high intensity can be used as the brilliant source of high energy electrons, ions and X- or y-rays. In most cases, laser pulses with high contrast are used for acceleration particles. But, it has been shown, that changing parameters of pre-plasma layer on the surface of the target can significantly increase electron energies [1]. In this work we present the results of the experimental and numerical studies of the abnormally hot electron generation mechanisms in the case of long scale pre-plasma layer subcritical density.
In our experiments we used Ti:Sapphire laser system (p-polarized, X - 800nm,
vpulses - 10Hz, Emax - 40mJ, Tmin - 45±5fs and Imax - 5 1018W/cm2). Laser pulse in this
8 8 system has a good contrast (ASE level - 10-, ns pre-pulse - 5 10- (-12.5ns)) and could
not create long pre-plasma. For creation controlled long and dense pre-plasma layer we used Nd:YAG laser (X - 532nm, E - 30mJ, t - 6ns, I - 1012W/cm2) locked with Ti:Sapphire laser system. Nd:YAG pulse, having a long duration, simulated bad ASE level on the front of the Ti:Sa pulse. Were performed optical and y-ray plasma diagnostics at different delays between the pulses and the focus positions of the main pulse. We observed several laser-plasma interactions regimes, in some of them average energies of hot electrons increases more than 7 times from 330keV in the case of clear Ti:Sa pulse (ponderomotive acceleration) up to 2.4MeV with artificial pre-pulse. Moreover, there is a strong correlation between the y-ray yield and three-halves harmonic (3co0/2) generation, it indicates the parametric processes participation in electron acceleration.
For clarification of the electron acceleration mechanisms numerical simulations were done using fully relativistic 3D3V PIC code Mandor (in 2D3V regime). A laser pulse (p-polarized, X - lum, t -
50fs, I - 1018 W/cm2) was focused onto the targets with different pre-plasma layer parameters (corresponding to the experiments). The typical simulation box size was 90x15um2 with periodic boundaries and spatial resolution of X/100. Simulation showed that relativistic self-focusing, parametrical processes, wavebreaking, stochastic heating, its combinations and variations, define the abnormally hot electron generation.
Acknowledgements: The reported study was supported by RFBR, research projects Nos. 14-02-31871, 13-02-00337, 13-02-00494. Simulations were made using Moscow State University Supercomputing Center Facility Lomonosov.
1. Ivanov K. A., et al. / / Physics of Plasmas 2014, V.21, No. 9, P.093110.
