A Model of Tokamak Locked Mode Disruptions

Locked modes are precursors to major disruptions. During locked modes, the temperature decreases in the plasma edge region. This causes the current to contract. A model is given to analyze the MHD stability of contracted current equilibria. If there is sufficient current contraction, resistive wall tearing modes are destabilized. This requires that the q = 2 surface be sufficiently close to the wall. The threshold conditions obtained in the model are consistent with experimental observations of the conditions for a thermal quench in a disruption.