The effects of ASE contrast and beams mutual alignment on the γ-ray yield in laser-plasma interactions with artificial prepulse

Abstract

We present experimental study of the bremsstrahlung γ-rays generation in a plasma interacting with 50 fs laser pulse in slightly relativistic regime (intensity ∼2-1018 W/cm2). A pre-plasma layer on the surface of the molybdenum target is formed by an additional laser pulse with a duration of 8 ns and an intensity of ∼2-1012 W/cm2. The energy and intensity of the artificial pre-pulse exceed those of the amplified spontaneous emission (ASE) pedestal of the main pulse by ∼103 and 10 times (for contrast 10−7), respectively. It was shown that the low ASE contrast (>10−7) is the crucial condition for increasing (in comparison to the case without artificial pre-pulse) of the integral γ-rays yield when the nanosecond pulse is ahead of the femtosecond one by >20 ns. Interferometry data show that the reason of the γ-rays yield increasing is a pre-plasma layer initially produced by artificial pre-pulse and re-created by the ASE. The optimum conditions for γ-rays yield achieved if (i) the pre-pulse comes ∼25 ns in advance, (ii) ASE contrast is 10−7 and lower and (iii) the femtosecond focal point is shifted by ∼100 μm from the center of the nanosecond pre-pulse focal spot and by ∼100 μm above the target surface.