Fast electron temperature and conversion efficiency measurements in laser-irradiated foil targets using a bremsstrahlung x-ray detector

Abstract

Measurements of fast electron temperature and conversion efficiencies using bremsstrahlung x-rays emitted from laser-produced, fast electrons are presented. Experiments were carried out using the Titan laser (150 J, 1.5 ps) at the Lawrence Livermore National Laboratory. The maximum intensity was 2 × 1020 W/cm2 on 250 μm thick silver foil targets. The emission of bremsstrahlung x-rays from the fast electrons in the target was measured using a filter-stack based detector. The conversion efficiency of laser energy into fast electrons and the electron temperature were studied as a function of incident laser energy. Several models of the electron divergence angle were investigated, and the effect of the assumed divergence angle on the inferred conversion efficiency was quantified. This allows for upper and lower bounds on the conversion efficiency to be established for a range of possible divergence angles. The value for upper bound is 60% (from a 75° divergence angle model) and for the lower bound is 25% (from a 0° divergence angle model).