Optimization of high energy x ray production through laser plasma interaction

Abstract

A standard technique for generating a burst of hard x rays (above 30 keV) is to use ultra high intensity lasers incident on a target. The strong laser field causes rapid electron oscillations which then generate hard x rays via bremsstrahlung. We have demonstrated a new technique for optimizing the conversion efficiency of laser light to hard x rays at moderate Iλ2 (mid 1013 W/cm2.µm2) assuming that the two plasmon decay plasma instability is the predominant acceleration mechanism. In this scheme, electrons are not directly accelerated by the laser field but by electron plasma waves. Experiments at the National Ignition Facility show the effect of a pre-pulse on the hard x ray spectrum and conversion efficiency. Different experimental configurations are investigated to optimize the conversion efficiency using various pre-pulse levels as well as different target designs (gold vs. silver, varying target thickness, presence of an ablator layer of CH). The conversion efficiency of laser energy into photon above 30 keV for a 100 ps short pulse scales as ∼ I1.23 for laser intensity ranging from 1 × 1016 to 1 × 1017 W/cm2 at 3ω for high Z target. A 1-ns-long pre-pulse pre-seeding an 88-ps Gaussian laser pulse coupled with a CH-coated thin Au target led the highest conversion efficiency above 30 keV of ∼ 3×10−4.