Effect of Rotation on the Modelled NTM Threshold in JET Advanced Scenarios

Neoclassical Tearing Modes (NTM) are part of the major MHD instability that should be avoided in a fusion reactor, for causing confinement degradation or disruption. It consists of a magnetic island that is metastable, i.e. it is linearly stable and grows to a large size when fed by a primary mode. This metastable nature explains why both the issue of the non linear threshold, and of the coupling to the primary mode that provides the seed, are crucial. In recent years, the threshold in performance for exciting a NTM has been shown to increase significantly with plasma rotation and flow shear, and the role of flow shear on the intrinsic stability rather than on the primary mode has been pointed out. On the other hand, theoretical works do not provide a clear explanation for this observation, and generally predict that flow shear has a weak effect or is destabilizing for magnetic islands in sub-Alfvénic flows. A stabilizing effect of flow shear has however been found using MHD models retaining perturbations parallel to the magnetic field (dB||, dV|| 0), either above a threshold in the magnetic Prandlt number Prm = m0n / h in cylindrical geometry, or without viscosity in toroidal geometry due to toroidal curvature and mode coupling. The picture arising from experimental studies is that velocity shear acts in a similar way as magnetic shear on the D¢ stability parameter, thanks to a coupling between the plasma flow and the resistive layer at high Prm. In the present work, we investigate this issue, which is crucial for extrapolating to ITER, by computing the non linear threshold of the (2,1) NTM for a typical JET Advanced Tokamak discharge using the full MHD code XTOR, where, in addition to geometrical effects, anisotropic heat transport and bootstrap current perturbation are described.
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