Radiation Hydrodynamics Experiments on Large High-Energy-Density-Physics Facilities that are Relevant to Astrophysics

Abstract

This thesis presents the design of three radiation hydrodynamics experiments at the largest high-energy-density-physics facilities in the United States. Two of these experiments explore the first measurements of photoionization fronts, in the laboratory on two independent facilities. The third is a radiative shock experiment at the National Ignition Facility, which is novel in its use of temperature measurements of shocks in foam targets. The results also use a novel analysis of streaked self-emission data and x-ray Thomson scattering measurements to understand the post-shock electron temperature.

The design and execution of a photoionization front experiment at the Omega laser facility resulted in the characterization of a platform to perform measurements of photoionization fronts using absorption spectroscopy as the primary diagnostic. These experiments use laser to produce ionizing radiation that drives a photoionization front into a gas cell that contains a mixture of nitrogen with argon as a spectroscopic dopant. The data allows for a characterization of the flux emitted from the rear surface of the gold drive source, the capsule implosion backlighter, and the geometry of the absorption measurement. An analysis of the capsule implosions introduces a new metric for comparing the implosion performance based on the initial laser irradiation pattern.

A computational study to design a photoionization front experiment at the Z-Machine provides a complementary experiment to those at Omega also using nitrogen gas but utilizing optical emission spectroscopy as the primary diagnostic. This design provides a comparison between the laser-driven foil source on Omega and the wire array z-pinch source to produce a photoionization front. The simulation study explores the parameter space for the experiment, suggests experimental conditions that should create a photoionization front and varies the physics detail in the model to validate the approximations used in the design. Estimates of the streaked visible spectroscopy show evolution of the line structure as the front passes the diagnostic viewing window before continuum emission dominates the signal.

Radiative shock experiments at the National Ignition Facility use a hohlraum to launch a shock into 20 mg cm$^{-2}$ plastic foam. These experiments use x-ray Thomson scattering and streaked, self-emission measurements to observe the shock velocity and electron temperature as a function of time. The analysis of the self-emission data uses the thick-thin model of a radiative shock to extract the temperature using the measured velocity and making some assumptions about the upstream conditions. The scattering measurements largely agree with the self-emission results providing further confidence in the analysis.

The work presented in this thesis introduces and characterizes a new experimental design for the Omega laser facility to measure photoionization fronts in the laboratory, which has never been done before. An additional design for a new photoionization front experiment on the Z-machine shows that a complementary experiment is possible on this facility. The differences between the velocity of a photoionization front and Marshak-like waves are explored in greater detail and an analysis of the curvature of each type of heat front shows this could be a measurable feature to distinguish a photoionization front from a Marshak wave. Finally, radiative shock experiments on the National Ignition Facility use x-ray Thomson scattering and self-emission data with a novel data analysis technique to make multiple electron temperature measurements for the first time.