X-Ray Measurements of the Time-Dependent Electron Energy Distribution in the PFRC-2 Fusion-Research-Plasma Device

Abstract

This thesis is based on work done with the Princeton Field-Reversed Configuration 2 (PFRC-2), a plasma device located at the Princeton Plasma Physics Laboratory (PPPL). This thesis focuses on the examination of x-ray spectra detected via an AmpTek Fast Silicon Drift Detector (SDD). This x-ray emission comes via bremsstrahlung and line radiation from electrons inside the plasma chamber and yields information about electron energy distribution (EED), density, and effective volume of the plasma observed by the detector. The goal is to analyze x-ray emission spectra to inform us about the EED and plasma density responsible for the x-ray emission. An important finding is that the x-ray emission can grow in brightness with increasing average energy in a characteristic time of order 10 ms, far longer than the observed instability time scale (0.1 ms) and estimated energy confinement time scale, also < 0.1 ms. The growth rate is comparable to the classical penetration time, although we have observed it in some cases to be of an order even greater than this time. When the plasma rotates, the x-rays are observed to come in bursts at twice the frequency of the plasma rotation. The detector was operated at the low energy extreme of its published energy range. We report on the efforts needed to make these measurements reliable and informative. Finally, we observe spectra and comment on their behavior as a function of multiple key independent variables. We find x-ray energy to increase with power and exhibit no conclusive correlation between average energy and the presence of a stability-inducing gas puff in the plasma.