Short-pulse-heated Plasma Dynamics near Solid Density via X-ray Lineshapes

Abstract

Intense, short-pulse lasers interacting with solid materials tend to produce particles in two regimes: relativistic particles, which quickly exit the interaction region and are well-studied, and bulk, thermal particles, which are highly collisional, near solid density, and have been difficult to diagnose. The bulk population is nonetheless important as an x-ray source for radiography and as a highly collisional platform for fusion and electron equilibration studies. The work presented here sheds light on this challenging regime via x-ray spectroscopy, measuring thermal line emission from highly charged ions near the laser focus with high-resolution Bragg crystal spectrometers. Three central advances have made these studies possible: (1) ultrahigh-contrast laser technology that heats solid-density material to high temperatures; (2) laser targets with thin Ti tracer layers embedded in Al foils, used to distinguish emission from a fine-grained series of depths relative to the laser-facing surface; and (3) high-resolution, spherically bent Bragg crystal x-ray spectrometers, developed for magnetically confined plasmas and here applied to plasmas heated by short-pulse lasers. This thesis relies on laser technology developed externally to generate novel data with newly applied targets and spectrometers. These experiments capture well-resolved fine-structure line emission from solid-density plasmas with reduced spatial gradients, mapping their spectral characteristics in space. Analysis of the observed x-ray lineshapes yields two contributions that enhance our understanding of short-pulse-heated plasmas. Firstly, Doppler-shifted lineshapes imply the velocity distributions of solid-density ions, a new measurement that reveals heretofore unobserved rapid acceleration of bulk ions. Secondly, Stark-broadened lineshapes show both double-peaked structure and quantifiable redshifts; both are hallmarks of emission from high-density plasmas which jointly constrain both electron density and ion temperature. These developments showcase the usefulness of high-resolution x-ray spectroscopy for observation of dense plasmas, here put in service of identifying new physics phenomena in laser-heated solids. These techniques are potentially applicable to a wide variety of high-energy-density systems.