Reduction of tokamak self-driven current by magnetic island perturbations

Magnetic island (MI) perturbations, likely unavoidable in long pulse discharges, may cause a reduction of plasma self-driven current in tokamaks. A novel effect on tokamak self-driven current revealed by global gyrokinetic simulations results from MI-induced electric potential islands, which have dominant mode numbers the same as that of the MI, whereas centered at both the inner and outer edge of the island. The non-resonant potential islands are shown to drive a current through an efficient nonlinear parallel acceleration of electrons. In large-aspect ratio (large-A) tokamak devices, this new effect can result in a significant global current reduction to the electron bootstrap current when the MI size is sufficient large, in addition to the local current loss across the island region due to the pressure profile flattening. It is shown that there exists a critical magnetic island width for large-A tokamaks beyond which the electron bootstrap current loss is global and increases rapidly with the island size. As such, this process may introduce a size limit for tolerable magnetic islands in large-A tokamak devices in the context of steady state operation. On the other hand, the current loss by MIs in low-A tokamaks (e.g., spherical tokamak) is minor. The reduction of the axisymmetric current by the MI scales with the square of island width. However, the loss of the current is mainly local in the island region, and the pace of the current loss as the increase of MI size is substantially slower compared to large-A tokamaks. In particular, the bootstrap current reduction MIs in STs is even smaller in the reactor-relevant high-β_p regime where NTM islands are more likely to develop. The connection of this newly revealed effect of MI on the current to some experimental observations will be also discussed.