  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.
