Simulations of symmetric halo currents

It is proposed that halo current magnitude during vertical disruptions can be significantly affected by a number of strongly interacting processes, involving the generation of neutrals, their ionization and recombination, open-surface parallel losses, Ohmic heating due to the large back-emfs, the generation of impurities, and mixed-circuit effects. To investigate the balance of such processes we present and interpret results from one-dimensional multi-fluid transport simulations. The simulations show that, as expected, core plasma current is mainly lost due to the reduction of the core cross-sectional area rather than the thermal quench, and that large values of the safety factor tend to produce low poloidal halo current. In addition, it is seen that increasing the resistance of the external halo path suppresses halo current, as do impurities, naturally released from the divertor during the termination event. High values of the safety factor insulate the halo plasma from parallel transport effects, increasing electrical conductance and toroidal halo current. For mega ampere spherical tokamak device parameters, values of the poloidal halo current are of the order of 15% of the original plasma current. (author)