Speaker Institution : University of Maryland, Baltimore County
Title : Numerical Investigation of Spheromak Formation Efficiency
Abstract : Nonlinear numerical computation with the NIMROD code is used to investigate the effect of the gun current profile on bounded spheromak plasma formation and sustainment with coaxial helicity injection (CHI). The goal of this research is to maximize the rate of injected helicity and improve confinement by determining and controlling the CHI gun current parameters that will optimize spheromak performance. Our approach for this study is to expand beyond the achievable operational regimes of previous experiments in order to find candidate modes of operation for future experimental studies. In particular, we study the effect of differing ramp rates of gun current (dIg/dt), including rates much greater than those achieved experimentally on the Sustained Spheromak Physics eXperiment (SSPX). Due to plasma inductance, fast ramp rates for the gun current will produce larger bias voltages across the injector gap, and therefore, increase the rate of magnetic helicity injection. This will increase the helicity available to be transferred to closed magnetic surfaces during reconnection and self-organization, resulting in a spheromak with a greater degree of twist in the closed torus and improved confinement. While this research initially studies spheromak plasmas in the SSPX flux conserver geometry, it is not a direct continuation of previous SSPX simulation campaigns. Eventually, we plan to study the spheromak performance with different flux conserver designs, including a planar coaxial helicity injection gun.