Kinetic effects on the currents determining the stability of a magnetic island in tokamaks

Abstract

The role of the bootstrap and polarization currents for the stability of ă neoclassical tearing modes is investigated employing both a drift ă kinetic and a gyrokinetic approach. The adiabatic response of the ions ă around the island separatrix implies, for island widths below or around ă the ion thermal banana width, density flattening for islands rotating at ă the ion diamagnetic frequency, while for islands rotating at the ă electron diamagnetic frequency the density is unperturbed and the only ă contribution to the neoclassical drive arises from electron temperature ă flattening. As for the polarization current, the full inclusion of ă finite orbit width effects in the calculation of the potential ă developing in a rotating island leads to a smoothing of the ă discontinuous derivatives exhibited by the analytic potential on which ă the polarization term used in the modeling is based. This leads to a ă reduction of the polarization-current contribution with respect to the ă analytic estimate, in line with other studies. Other contributions to ă the perpendicular ion current, related to the response of the particles ă around the island separatrix, are found to compete or even dominate the ă polarization-current term for realistic island rotation frequencies.