Effect of resonant helical field on plasma current density gradient and neoclassical tearing mode



Q. Yu, S. Günter and K. Lackner

Max-Planck-Institut für Plasmaphysik, 85748 Garching, Germany

 

The effect of externally applied resonant helical field, i.e., resonant magnetic perturbations (RMPs), on the local equilibrium plasma current density profile has been studied theoretically. It is found that a moderate RMP below its penetration threshold, via non-linear mode coupling, induces an effective parallel electric field around its rational surface that can significantly change the local flux-surface-averaged current density gradient. At a given RMP amplitude, the modification of the current density profile increases with increasing electron temperature, and it significantly depends on the bi-normal electron fluid velocity at the resonant surface. The effect of this modification on the magnetic island growth has been studied numerically basing on two-fluid equations and using the large tokamak aspect ratio approximation. There are two new findings:

(a) Onset of neoclassical tearing modes (NTMs): Using externally applied static RMPs as triggers, a larger RMP is found to be required to excite the NTM’s onset with increasing the bootstrap current density, when the bi-normal electron fluid velocity is in the ion drift direction. This is contrary to the conventional understanding that the bootstrap current perturbation is always destabilizing for NTMs’ onset. For the electron fluid velocity in the electron drift direction, however, the opposite results are found.

(b) Suppression of small magnetic island growth by RMPs: The growth of small m/n=2/1 magnetic island (m/n being the poloidal/toroidal mode numbers), driven by an unfavorable plasma current density profile and bootstrap current perturbation, is found to be stabilized by moderate static 4/2 or 6/3 RMPs if the local electron fluid velocity is in the ion drift direction or sufficiently large in the electron drift direction. These results reveal that a weakly three-dimensional equilibrium, containing a moderate 4/2 RMP and the associated shielding current, can be more stable against the 2/1 mode, which often causes tokamak plasma major disruptions.