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(EX7/2) Resistive Instabilities in Reversed Shear Discharges and Wall Stabilization on JT-60U

S. Takeji¹⁾, S. Tokuda¹⁾, T. Fujita¹⁾, T. Suzuki¹⁾, A. Isayama¹⁾, S. Ide¹⁾, Y. Ishii¹⁾, Y. Kamada¹⁾, Y. Koide¹⁾, T. Matsumoto¹⁾, T. Oikawa¹⁾, T. Ozeki¹⁾, Y. Sakamoto¹⁾, the JT-60U Team^1)
 
1) Japan Atomic Energy Research Institute, Ibaraki, Japan

Abstract.  Resistive instabilities and wall stabilization of ideal low toroidal mode number, n, kink modes are investigated in JT-60U 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, $\beta_{\mathrm{N}}^{}$, of about unity or even lower. The resistive interchange modes appear as intermittent burst-like magnetohydrody-namic (MHD) activities and higher n $\leq$ 3 modes are observed occasionally in higher $\beta_{\mathrm{N}}^{}$ 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 owing to a coupling with tearing modes at the outer mode rational surface over the minimum safety factor. Stability analysis revealed that stability parameter of tearing modes, $\Delta{^\prime}$, at the outer mode rational surface is affected by the free-boundary condition. The result is consistent with the experimental evidence that major collapse tends to occur when plasma edge safety factor, q^*, is near integer values. 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.

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