Analysis of slow formation of plasmas in a coaxial double theta pinch

Abstract

Plasma formation in a coaxial double theta pinch device is analyzed. This device, called the Coaxial Slow Source (CSS), has demonstrated the formation of annular field reversed configurations on diffusive timescales. Three analytical models are developed and used to: characterize the coupling between the magnetic configuration and the driving circuit, predict a temperature limit due to impurity radiation, and reveal an instability due to localized radiation cooling. A comprehensive model incorporating the interactions of neutral and ionized particles is used to simulate the CSS experiment on a computer. This two-dimensional MHD two-fluid code called scCOAX is adapted from an earlier version used by Hahn (1). Simulations are compared to experimental data from the CSS to test the models, and quantify the physical processes involved.The slow forming plasma is strongly influenced by the surrounding neutral fill gas due to: the energy lost by ionization of these neutrals, and enhanced heat conduction from charge exchange collisions between neutrals and ions. The friction between the neutral and ionized fluids is important in modeling the formation dynamics. Impurities also play a role in the power balance of such plasmas by strongly emitting line radiation. Radiation from impurities appear unimportant unless the plasma contacts the vacuum chamber walls.