  Disruptions are a serious problem in tokamaks, in which thermal and magnetic
energy confinement is lost. This paper uses data from the DIII-D experiment,
theory, and simulations to demonstrate that resistive wall tearing modes (RWTM)
produce the thermal quench (TQ) in a typical locked mode shot. Analysis of the
linear RWTM dispersion relation shows the parameter dependence of the growth
rate, particularly on the resistive wall time. Linear simulations of the locked
mode equilibrium show that it is unstable with a resistive wall, and stable
with an ideally conducting wall. Nonlinear simulations demonstrate that the
RWTM grows to sufficient amplitude to cause a complete thermal quench. The RWTM
growth time is proportional to the thermal quench time. The nonlinearly
saturated RWTM magnetic perturbation amplitude agrees with experimental
measurements. The onset condition is that the q = 2 rational surface is
sufficiently close to the resistive wall. Collectively, this identifies the
RWTM as the cause of the TQ. In ITER, RWTMs will produce long TQ times compared
to present-day experiments. ITER disruptions may be significantly more benign
than previously predicted.
