1Kinetic Calculations of the NTM Polarisation Current: Reduction for Small Island Widths and Sign Reversal Near the Diamagnetic Frequency

Abstract The polarisation current associated with a Neoclassical Tearing Mode is studied by means of drift kinetic δ f simulations. This current has been invoked as a possible explanation for both the observed threshold for the minimum island size that can grow unstable and the scaling of the plasma pressure at the mode onset with the normalised gyroradius. The numerical approach presented in this paper does not require assumptions on the island size or the island rotation frequency, which are in contrast necessary in the analytic theory. The calculations are performed in toroidal geometry in the presence of a helical perturbation. In the case of an island width comparable to the ion banana width (typical for the early phase of a NTM) it turns out that the polarisation current decays linearly with decreasing island width. Moreover it is found that the sign of the polarisation current can flip for rotation frequencies close to the diamagnetic frequency. The kinetic effects mentioned above are not included in the present theory and must be considered in order to determine both sign and size of the polarisation-current contribution to the NTM evolution. 1