  Disruptions are a serious issue in tokamaks. In a disruption, the thermal
energy is lost by means of an instability which could be a resistive wall
tearing mode (RWTM). During precursors to a disruption, the plasma edge region
cools, causing the current to contract. Model sequences of contracted current
equilibria are given, and their stability is calculated. A linear stability
study shows that there is a maximum value of edge $q_a \approx 3$ for RWTMs to
occur. This also implies a minimum rational surface radius normalized to plasma
radius from RWTMs to be unstable. Nonlinear simulations are performed using a
similar model sequence derived from an equilibrium reconstruction. There is a
striking difference in the results, depending on whether the wall is ideal or
resistive. With an ideal wall, the perturbations saturate at moderate
amplitude, causing a minor disruption without a thermal quench. With a
resistive wall, there is a major disruption with a thermal quench, if the edge
$q_a \le 3.$ There is a sharp transition in nonlinear behavior at $q_a = 3.$
This is consistent with the linear model and with experiments. If disruptions
are caused by RWTMs, then devices with highly conducting walls, such as the
International Tokamak Experimental Reactor (ITER) will experience much milder,
tolerable, disruptions than presently predicted.
