ИСТИНА

Femtosecond laser plasma is a brilliant source of ultra-short bursts of high energy electrons, which lead to the generation of hard X- and gamma-rays. Electron acceleration mechanisms depend significantly on the laser pulse parameters, such as: intensity, duration and contrast. So, the energy and flux of gamma-rays generated in a plasma can be significantly increased by using a subrelativistic intensity (7*10^18W/cm2) pulse with a low (10^-5) ASE contrast compared with a case of high ASE contrast (10^-8). Moreover, there was an increase of gamma-radiation yield with the increasing of laser pulse duration (from 45 to 350fs) at a constant energy (intensity decreases). For studying the mechanisms of electron acceleration we used a nanosecond duration (~10ns) pulse to simulate an ASE pedestal for a short (50fs) high-contrast (10^-7) main pulse of relativistic intensity (2*10^18W/cm2). In this work we present experimental and numerical simulations results on gamma-emission from plasma in the case of different main pulse durations (up 1700fs) and nanosecond artificial prepulse as ASE pedestal. It was shown that when the prepulse is ahead of the main pulse by ~20ns, the gamma-ray yield can be increased more than 10 times by increasing the main pulse duration from 50 to 1700fs. Parametrical processes developing in the plasma lead to electron acceleration. However, the required plasma gradient is formed due to electron impact ionization in the field of the main pulse. In our experiments we used Ti:Sa laser system (p-pol, pulse duration – 50-1700fs, energy on target – 25mJ, wavelength – 800nm, repetition rate – 10Hz, ASE level – 10^-7). Pulse duration was changed by the grating compressor detuning. The controlled pre-plasma layer was created using Nd:YAG laser (8ns, 130mJ, 1064nm) locked with Ti:Sa laser system. Numerical simulations were done using fully relativistic PIC code Mandor.