Numerical MHD stability studies: toroidal rotation, viscosity, resistive walls and current holes

The linear magnetohydrodynamic (MHD) stability of ideal and resistive, axisymmetric toroidal equilibria is investigated with respect to various physical effects, such as differential toroidal rotation, viscosity, ideal and resistive external walls and current holes. For this purpose, the CASTOR code has been comprehensively extended. Static equilibria, equilibria with toroidal flow and equilibria with current holes serve as input to this code called CASTOR_FLOW code. ASDEX Upgrade type equilibria with toroidal flow are computed up to a toroidal Mach number of Mta = 0.5, and compared with the static solution. Using these equilibria, the stabilizing effect of differential toroidal rotation on double tearing modes (DTMs) is investigated. The studies show that the computation of equilibria with flow is necessary for toroidally rotating plasma with Mta ≥ 0.2. The stability of DTMs is also studied for equilibria with current holes. Further, the stabilizing effect of a resistive wall on an external ideal kink mode is investigated.