Resistive Wall Tearing Mode Disruptions

This paper deals with resistive wall tearing mode (RWTM) disruptions. RWTMs are closely related to resistive wall modes (RWMs). The nonlinear behavior of these modes is strongly dependent on the resistive wall outside the plasma. A conducting wall is highly mitigating for RWTM disruptions. The consequence for ITER, which has a highly conducting wall, is that the thermal quench (TQ) time could be much longer than previously conjectured. Active feedback stabilization is another possible way to mitigate or prevent RWTM disruptions. Simulations of disruptions are reviewed for DIII-D and MST. MST has a longer resistive wall time than ITER, and disruptions are not observed experimentally when MST is operated as a standard tokamak. Simulations indicate that the RWTM disruption time scale is longer than the experimental shot time. In general, edge cooling by tearing mode island overlap or by impurity radiation causes contraction of the current profile, which destabilizes RWTMs. The equilibria studied here have a q = 2 rational surface close to the edge of the plasma, and low edge current density. A sequence of low edge current model equilibria hasmajor disruptions only for a resistive, not ideal, wall, and approximately edge q < 3. This is consistent with typical regimes of tokamak disruption avoidance, suggesting that typical tokamak disruptions could be RWTMs.