Resistive stability of a plasma with runaway electrons

In tokamak disruptions the Ohmic current is often replaced by a current of runaway electrons, whichis likely to be more peaked in the center of the discharge than the predisruption current. This raisesthe question of the resistive stability of the postdisruption plasma, where the equilibrium current isentirely carried by the runaway electrons while the cold 10 eV background plasma is relativelyresistive. It is found that the linear properties of the classical tearing mode are essentially determinedby the cold bulk plasma, and the growth rate is approximately the same as in a plasma withoutrunaways 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 carriedby runaway electrons than in the Ohmic case, and undergoes a nonlinear bifurcation when thestability index exceeds a critical value.