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 γ-rays. Preplasma layer created by prepulses or ASE have a significant influence on electron acceleration mechanisms. In our experiments, for changing of the preplasma length and density we used a nanosecond duration (~10ns) artificial prepulse with a short (50fs) high-contrast (10^-7) main pulse of relativistic intensity (2*10^18W/cm^2). Two gamma-ray yield maxima were observed depending on the time delay between the pulse and prepulse (0-10 ns – “first” and 20-40 ns – “second”). In maxima gamma-ray temperature and maximum energies increased up to 1.6 and 7.5 MeV. Also dependence of gamma-ray yield on main pulse duration was observed. Presence of the artificial prepulse is critical for the described phenomena of the increasing gamma-rays yield or energies, but the ASE impact is also significant. In this work we present experimental results on gamma-emission from plasma in the case of different ASE contrast levels (10^-9, 10^-7, 10^-6-10^-5) and different coincedence of the artificial prepulse and main pulse beams on the target surface. In particular, it was shown, that the “second” gamma-ray emission maximum disappears at a high ASE contrast (10^-9). For lower ASE contrast (>10^-7) γ-ray yield is minimal when the beams coincide and increases when they are separated. Comparison of the obtained data with the interferometry results clarified the laser-plasma interaction processes.