Mitigation of hot electrons from laser-plasma instabilities in high-Z, highly ionized plasmas

Hard x-ray measurements are used to infer production of hot electrons in laser-irradiated planar foils of materials ranging from low- to high-Z. The fraction of laser energy converted to hot electrons, fhot, was reduced by a factor of 10³ going from low-Z CH to high-Z Au, and hot electron temperatures were reduced from 40 to ∼20 keV. The reduction in fhot correlates with steepening electron density gradient length-scales inferred from plasma refraction measurements. Radiation hydrodynamic simulations predicted electron density profiles in reasonable agreement with those from measurements. Both multi-beam two-plasmon decay (TPD) and multi-beam stimulated Raman scattering (SRS) were predicted to be above threshold with linear threshold parameters that decreased with increasing Z due to steepening length-scales, as well as enhanced laser absorption and increased electron plasma wave collisional and Landau damping. The results add to the evidence that SRS may play a comparable or a greater role relative to TPD in generating hot electrons in multi-beam experiments.