Beam optics of exploding foil plasma x-ray lasers

Abstract

In soft x-ray lasers, amplification is achieved as the x rays propagate down a long narrow plasma column. Refraction, due to electron density gradients, tends to direct the x-rays out of high density regions. This can have the undesirable effect of shortening the distance that the x ray stay within the plasma, thereby limiting the amount of amplification. The exploding foil design lessens refraction, but does not eliminate it. In this paper, a quantitative analysis of propagation and amplification in an exploding foil x-ray laser is presented. The density and gain profiles within the plasma are modeled in an approximate manner, which enables considerable analytic progress. It is found that refraction introduces a loss term to the laser amplification. The beam pattern from a parabolic gain profile laser has a dominant peak on the x-ray laser axis. The pattern from a quartic gain profile having a dip on-axis can produce a profile with off-axis peaks, in better agreement with recent experimental data.