The vacuum interface of magnetically injected switch plasmas: Computational aspects

Summary form only given. A plasma opening switch is used on PBFA-II to increase the voltage and power delivered to the ion diode by compressing the rise time of the accelerator current pulse. Two switches under consideration use a poloidal field in addition to the toroidal field generated by the accelerator pulse. To improve the accuracy of switch simulations, it was necessary to perform simulations of the plasma injection process using the full resistive MHD (magnetohydrodynamic) model in MACH2. These simulations present substantial numerical difficulties. The vacuum field is determined by solving the resistive diffusion equation implicitly at large resistivity, and hence high Courant number, in the vacuum regions. In the plasma region, the flow is of low Alfven number, thus implying that a reasonably accurate vacuum field solution is required. The computational treatment of the vacuum-plasma interface is crucial, since the diffusion coefficient changes by several orders of magnitude there. The problems caused by these computational issues, and solutions to them in the context of MACH2´s numerical algorithms have been considered.