Non Linear MHD Modelling of NTMs in JET Advanced Scenarios
Advanced Tokamak Scenarios, with qmin > 1, often encounter MHD limits due to Kink or Resistive Wall Modes (RWM), or to Neoclassical Tearing Modes (NTM) that degrade performances or evolve to disruption. NTM on q = 2 is of particular concern in this respect, and we have investigated its non linear threshold on JET Advanced Scenarios using the non linear MHD code XTOR . We show that in the experiment considered, the triggering of a (2,1) NTM is consistent with a critical island width that decays with time during the discharge, and results from a weakening of curvature stabilization as magnetic shear increases at the resonance. Plasma rotation, which has been evidenced as a stabilizing effect in several experimental devices, is also investigated. We show that plasma flow increases only moderately (about 5%) the critical island width in the experimental range. Conversely, in the low momentum torque limit (ITER), the threshold for (2,1) NTM excitation increases significantly (about 20%) with the magnetic Prandtl number Prm = m0n/h between the perpendicular collisional (Prm~ 2) and the toroidal turbulent (Prm ~1100) values.