Numerical study of runaway current impact on sawtooth oscillations in tokamaks

This study investigates the influence of runaway current in runaway plasmas on the dynamics of sawtooth oscillations and resultant loss of runaway electrons (RE) using the 3D magnetohydrodynamic (MHD) code M3D-C^1 (Jardin \it et al 2012 \it J. Comput. Sci. Discovery \bf 6 014002). Using an HL-2A-like equilibrium, we confirm that in the linear phase, the impact of REs on resistive internal kink instabilities is consistent with previous research. In the nonlinear phase, as the runaway current fully replaces the plasmas current, we observe a significant suppression of sawtooth oscillations, with the first sawtooth cycle occurring earlier compared to the case without runaway current. Following the first sawtooth collapse, plasma current density, runaway current density, and safety factor (q) flatten within the q=1 surface, albeit displaying fine structures. Subsequently, the growing high torodial (n) and poloidal (m) mode number modes disrupt the magnetic surfaces, leading to the loss of REs outside the q=1 surface, while minimally affecting the majority of REs well-confined within it. Thus, in the current model, the physical processes associated with the presence of sawtooth oscillations do not effectively dissipate runaway current, as REs are assumed to be collisionless. In addition, the final profile of runaway current density exhibits increased steepening near the q=1 surface in contrast to the initial profile, displaying a distinctive corrugated inhomogeneity influenced by the growing fluctuation of the n=0 component. Finally, detailed convergence tests are conducted to validate the numerical simulations.