Modelling of (2,1) NTM Threshold in JET

High performance, advanced scenario discharges with qmin above unity are often limited by the n = 1 MHD mode in JET. Although the resistive nature of the mode is not clear in its initial phase, it evolves to a large island on q = 2, with significant confinement degradation (~15% in H89 in our example). We investigate here the possibility that this mode is a (2,1) Neoclassical Tearing Mode (NTM) by computing the critical island width at which such mode would be unstable, using the non linear MHD code XTOR where the relevant bootstrap current physics is accounted for, as well as perpendicular and parallel heat transport. Two different situations observed in the JET Tokamak are studied. In the first case, where the normalized kinetic pressure is at bN ~ 2.5, slightly above the computed no-wall limit (with MISHKA), n = 1 MHD activity consists of short bursts that do not impact the performances, while in the second experimental case, with bN ~ 2.8, still above the computed no-wall limit, bursts of n = 1 mode are followed by a transition to a long-lived mode with significant confinement degradation (figure 1). Quasi-linear calculations with XTOR show that the (2,1) NTM threshold is decreasing when approaching the observed mode triggering time, the main reason being the increase of the local magnetic shear at q = 2 as the plasma current diffuses. Although sensitive to the input equilibrium, these results are consistent with a limitation of these discharges by (2,1) NTM, assuming a seeding process of constant amplitude.
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