Two-fluid effects on resistive wall mode stability in ITER

A systematic numerical investigation is carried out to examine the two-fluid effects on the stability of the n=1 (n is the toroidal mode number) resistive wall mode (RWM) in an ITER 10 MA/5.3 T/Q = 5 steady-state scenario. Results are compared with the single-fluid prediction while varying (i) the plasma fluid flow, (ii) the plasma pressure, and (iii) the strength of the parallel sound wave damping (PSWD) mimicking the ion-Landau kinetic resonant effect. Inclusion of two-fluid effects opens a stability window for the RWM in the slow plasma regime (with the flow speed well below 1% of the toroidal Alfvén speed) in ITER. This stability window is robust against variation of the plasma pressure (within the RWM regime) when combined with strong PSWD. A second stability window for the mode is also identified at faster plasma (fluid) flow according to the two-fluid model, similar to that obtained with the single-fluid model. Partial cancellation of the fluid flow by the thermal ion diamagnetic flow, within the two-fluid model, however results in a larger rotation threshold (in the fast-flow regime) for the mode stabilization than the single-fluid prediction. A two-fluid model with instantaneous thermal equilibration, on the other hand, finds comparable rotation threshold to the single-fluid counterpart in the fast-flow regime.