Reconnecting instabilities in JT-60SA during current ramp-up with off-axis N-NB injection

Abstract

We report results of global resistive magnetohydrodynamic (MHD) simulations, which predict that wide-spread reconnecting instabilities may cause an internal disruption when 5 MW off-axis negative-ion-based neutral beam (N-NB) injection is used during the current ramp-up phase of a JT-60SA plasma. The simulations include fast ions as passive test particles and were initialized with plasma profiles obtained from an integrated transport simulation. The beam-driven current produces a nonmonotonic safety factor profile with three q = 2 rational surfaces. The resulting double or multiple kink-tearing instabilities redistribute both the bulk and fast particles in the inner half of the plasma. While the radial profiles appear to be flattened, the actual field topology and particle distributions are found to saturate in helical states. Moreover, because magnetic drifts are still large during the current ramp-up phase, there is a large difference between the topology of the magnetic field and 500 keV deuteron orbits. Fast ion phase space islands may thus appear and overlap at different times and in different regions than their magnetic counterparts.