Resistive instabilities in reversed shear discharges and wall stabilization on JT-60U

Resistive instabilities and wall stabilization of ideal kink modes with low toroidal mode number n are investigated in JT-60U reversed shear discharges. Localized burst-like MHD activity with n ⩽3 is observed in the negative shear region with large pressure gradient near the internal transport barrier; this activity is identified as the resistive interchange mode. The burst-like MHD activity is benign with respect to the internal transport barrier and no clear degradation of plasma stored energy by the burst-like MHD activity is observed. The resistive interchange mode, however, results in a major collapse through non-linear coupling with a tearing mode in the positive shear region. A resistive wall mode (RWM) followed by a major collapse is observed in wall stabilized high β discharges exceeding the ideal stability limit with the wall at infinity (β N >β N no - wall ). No clear continuous slowing down of the plasma toroidal rotation is observed before the growth of an RWM.