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

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 have first clarified that the redistribution of 'return current' induced after the NBCD turn-off lowers the safety factor of magnetic shear reversal, qmin. Linear MHD stability analysis has also revealed that this lowering of safety factor leads to n = 1 kink-ballooning instability with the modes localized around the internal transport barrier (ITB) of higher qmin (~6) discharges. It was also pointed out that an increase in the bootstrap current under continuous NB heating can lead to ITB reconstruction and thus causes subsequent ? collapses.