Resistive stability of a plasma with runaway electrons

In tokamak disruptions the Ohmic current is often replaced by a current of runaway electrons, which
is likely to be more peaked in the center of the discharge than the predisruption current. This raises
the question of the resistive stability of the postdisruption plasma, where the equilibrium current is
entirely carried by the runaway electrons while the cold 10 eV background plasma is relatively
resistive. It is found that the linear properties of the classical tearing mode are essentially determined
by the cold bulk plasma, and the growth rate is approximately the same as in a plasma without
runaways but with the same current profile. The nonlinear saturation amplitude is different,
however. In a symmetric plasma slab, the saturated island size is larger when the current is carried
by runaway electrons than in the Ohmic case, and undergoes a nonlinear bifurcation when the
stability index exceeds a critical value.