Numerical Investigation of the Formation and Translation of a Field Reversed Configuration with Neutrals and External Circuits Effects

Abstract

A Field Reversed Configuration (FRC) is a high-$\beta$ closed field line magnetic confinement topology, categorized as a compact toroid. Effects of plasma-neutral interactions in FRC formation and translation processes are studied using numerical simulations performed using the Cygnus code in axisymmetric two-dimensional geometries. A plasma-neutral interaction model is implemented in the Hall magnetohydrodynamics (MHD) code, Cygnus, that supports full coupling of external circuits driven magnetic fields to plasma fluid dynamics. Electron impact ionization, radiative recombination, and resonant charge exchange reactions are included in the plasma-neutral interactions. Results from simulations are compared to experimental measurements from the Venti Formation experiment at Helion Energy. After tuning anomalous transport coefficients to match experimental results, reasonable agreements to the experimental data are obtained. However, the level of agreements obtained by the code suggests that there are additional physics that are important in the FRC formation processes such as molecular neutral effects. In order to understand the importance of molecular neutral effects in FRC formations, an extension to the plasma-neutral fluid model that includes effects of diatomic deuterium species is developed. The model retains the simplicity of the original single species plasma-neutral model but allows for species generation and destruction for the following species: $D$, $D_2$, $D^+$, $D_2^+$, $D_3^+$, and $e$. Electron impact reactions including ionization, dissociative ionization, dissociation, dissociative excitation, dissociative recombination, and radiative recombination are implemented in the model. Heavy particle impact reactions including both resonant and non-resonant charge exchange and plasma-chemical reactions are implemented. A derivation of the molecular model including the required reaction collision terms is given. The molecular model is used to perform simulations of experimental conditions from the Venti Formation experiment using the same anomalous transport settings as the single species plasma-neutral model. Results from the molecular model is compared to both experimental and single species plasma-neutral simulation results to determine the importance of molecular neutral effects. Results from the molecular model show similar level of agreements to the experiment as the single species plasma-neutral model. However, it shows improved agreements to the experimentally measured single chord averaged plasma density in the early preionization phase. The simulation results show non-negligible diatomic deuterium population at the end of the preionization process, suggesting the importance of the diatomic deuterium species during the field reversal in the FRC formation processes.