A spectroscopic neutral density diagnostic and exploration of the role of metastable states in fusion and astrophysical plasmas

Abstract

Understanding the transition region between fully ionized and neutrally dominated plasmas is important to the study of the magnetosphere of the earth, the corona/chromosphere transition regions of the sun, and detached divertors in fusion devices. Regions with partial ionization are typically low temperature plasmas, for which there is a lack of spectroscopic tools generalizable to a wide variety of plasma parameters and sizes. This work presents an Argon spectroscopic neutral density diagnostic that relies on atomic modeling. Argon is used due to its prevalence in Low Temperature Plasma environments. This work also investigates the role of time dependent metastable states in atomic modeling. The Compact Toroidal Hybrid (CTH) device at Auburn University is used as a testbed for the neutral density diagnostic, both in benchmarking and use. For this work the CTH device operates at an electron temperature of 1-10 eV and 1 x 10^10 to 1 x 10^12 cm^-3. The ALEXIS device at Auburn University is used to investigate the role of time dependent metastable states in atomic modeling. In this project, the ALEXIS device operates at an electron temperature of 1-10 eV and 1 x 10^9 to 1 x 10^10 cm^-3. The assumptions in the neutral Ar collisional-radiative modeling for CTH and ALEXIS are confirmed using spectroscopic observations. The neutral argon density diagnostic on CTH is benchmarked across a range of plasma parameters. The neutral density diagnostic on CTH is then used to measure the plasma resistivity. The neutral density diagnostic, and the approach to dealing with metastable states, is expected to make the diagnostic applicable to a range of low temperature plasma devices. Results will be shown from using the neutral diagnostic on CTH to measure resistivity.