Onset of high-n ballooning modes during tokamak sawtooth crashes

A new phenomenon has been found during the nonlinear stage of the tokamak sawtooth crash in relatively high β plasmas. The m/n=1/1 magnetic island evolution gives rise to convection of the pressure inside the q=1 radius and builds up steep pressure gradient across the island separatrix, and thereby trigger ballooning instabilities below the threshold at the equilibrium. Effects of the ballooning modes on the magnetic reconnection process during the sawtooth crash are discussed.