Numerical modelling of sawtooth crash using two-fluid equations

The nonlinear growth of m/n = 1/1 internal kink modes is studied numerically using twofluid equations. As already reported earlier (Gunter et al 2015 Plasma Phys. Control. Fusion 57 014017), short sawtooth crash times (< 100 mu s) are found for typical ASDEX Upgrade parameters, in agreement with experimental observations. These fast sawtooth crashes are associated with large parallel electric field perturbations, giving rise to the generation of supra-thermal electrons. Slow sawtooth crashes (similar to ms) are obtained only for a sufficiently small (1-q(0)) and a large local electron diamagnetic drift frequency at the q = 1 surface, where q(0) is the value of the safety factor q at the magnetic axis. Kink modes are shown to drive sheared plasma rotation, propagating from the q = 1 surface towards the magnetic axis during the nonlinear phase. After the sawtooth crash, the driven plasma rotation is in the co-(counter-) current direction inside (outside) the q = 1 surface, as observed in experiments.