Resistive Instabilities in Reversed Shear Discharges and Wall Stabilization on JT-60U
S. Takeji, S. Tokuda, T. Fujita, T. Suzuki, A. Isayama, S. Ide, Y. Ishii, Y. Kamada, Y. Koide, T. Matsumoto, T. Oikawa, T. Ozeki, Y. Sakamoto, The JT-60 Team
Abstract.
Resistive instabilities and wall stabilization of ideal low toroidal mode number, n, kink modes are investigated in JT-60U reversed shear discharges aiming at improvement of stability of reversed shear discharges. Resistive interchange modes with n=1 are found to appear in reversed shear discharges with large pressure gradient at the normalized beta, bN, of about unity. The resistive interchange modes appear as intermittent burst-like magnetohydrodynamic (MHD) activities and higher n/FONT>3 modes are observed occasionally in higher bN regime. No clear degradation of the plasma stored energy is observed by the resistive interchange modes themselves. It is also found that resistive interchange modes can lead to major collapse through a mode coupling with tearing modes at the outer mode rational surface near the minimum safety factor. Stability analysis revealed that stability parameter of tearing modes D' at the outer mode rational surfaces is affected by free-boundary modes, supporting an experimental evidence that major collapse tends to occur when plasma edge safety factor q* is just below integer. Stabilization of ideal low n kink modes by the JT-60U wall is demonstrated. Magnetohydrodynamic perturbations that are attributed to resistive wall modes are observed followed by major collapse in wall-stabilized discharges.