Modelling of stochastic magnetic perturbation by RWMEF coils on NSTX

A 3-D field line integration code, TRIP3D, has been modified to model stochastic magnetic perturbation produced by a resistive wall mode, error field (RWMEF) coil in the NSTX tokamak with very low aspect ratio. Multiple field lines with a uniform poloidal angle interval on each flux surface are automatically traced for the first time to follow the lines with large elongation plasmas. Each RWMEF coil can be configured to produce perturbation fields with dominant toroidal mode numbers of n ≤ 1 or 3. In this study, it is found that the strongest stochastic layer is produced by the n ≤ 3 configuration rather than n ≤ 1 for the same coil current. Two NSTX divertor discharges, a lower single null and a double null have been modelled with different RWMEF-coil currents and toroidal modes. RWMEF currents of 2 kAt are sufficient to produce a strong stochastic field and significantly perturb the plasma boundary due to weak toroidal field in the spherical tokamak. The edge electron thermal diffusivity due to stochastic magnetic field is estimated to be 1 m² s^-1 with a 2 kAt current, which is comparable to that in DIII-D with an 8 kAt C-coil current. Currents of this magnitude, when used in the DIII-D I-coil configured for n ≤ 3 perturbations suppress large edge localized modes (ELMs) and thus may have an impact on ELMs in NSTX. The result has been verified by the initial experiments.