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Stability of Reversed Shear Discharges in JT-60U

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  • JT-60Uにおける負磁気シア放電の安定性
  • JT 60U ニ オケル フジキ シア ホウデン ノ アンテイセイ
  • JT-60Uにおける負磁気シア放電の安定性

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Abstract

Magnetohydrodynamic(MHD)stability has been studied in discharges with negative central magnetic shear, i.e., reversed shear, configuration in JT-60 U. The disruptive upper limit of the normalized beta, β_N=β(I_p/aΒ)^<-1>[%mT/MA], is β_N≲2 at q_<min>~2(q_<min> : the minimum safety factor)in discharges with the L-mode edge and is close to the stability limit against an ideal n=1(n : toroidal mode number)kink mode with the free boundary condition. Resistive MHD instabilities give rise to major collapses at the lower β_N regime than the ideal stability limit. Linear stability analysis with a two-dimensional marginal stability analysis code, MARG2D, showed that a double tearing mode can be unstable at the q=3 surface in the experimental situation, and the numerical result is not in contradiction to the experimental one. Major collapses often occur when the safety factor at the plasma suface is close to integer values, suggesting that an external kink mode is related to the causal mechanism for major collapses. The numerical analysis revealed that a stable external kink mode with the free boundary condition can destabilize resistive MHD modes in the reversed shear configuration.

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