Intermittent collapse after NBCD turn-off in JT-60U fully non-inductive reversed shear discharges

Takei, Nahoko; Nakamura, Yukiharu; Ushigome, Masahiro*; Suzuki, Takahiro; Aiba, Nobuyuki; Takechi, Manabu; Tobita, Kenji; Takase, Yuichi*; Fukuyama, Atsushi*; Jardin, S. C.*

Non-disruptive -collapses with a regular intermittency have been observed after a forced turn-off of neutral beam current drive (NBCD) in JT-60U fully non-inductive, reversed shear (RS) discharges. Self-consistent transport simulations with improved core confinement and linear MHD stability analysis have first clarified that redistribution of return current induced after the NBCD turn-off lowers the safety factor of magnetic shear reversal, leading to the n =1 kink-ballooning instability with localized modes around internal transport barrier (ITB). It was also pointed out that an increase of the bootstrap current under continuous NB heating can lead to ITB reconstruction and thus causes subsequent beta-collapses.