Resistive wall mode and fishbone mode in ITER steady state scenario: roles of fusion-born alphas and plasma flow

Abstract

Drift-kinetic effects of fusion-born alpha particles on the n=1 (n is the toroidal mode number) resistive wall mode (RWM) is numerically investigated for a recent design of the ITER 10 MA steady state plasma scenario, utilizing a magneto-hydrodynamic (MHD)-kinetic hybrid toroidal model. While the fluid theory predicts unstable RWM as the normalized plasma pressure βN exceeds the no-wall Troyon limit and with the mode growth rate monotonically increasing with βN, inclusion of the drift-kinetic contribution of trapped alphas qualitatively modifies the behavior by stabilizing the mode at high βN. In fact, a complete stabilization of the n=1 RWM up to the ideal-wall Troyon limit is found. On the other hand, another unstable branch - the alpha-driven n=1 fishbone mode (FB) – is identified in the high-βN regime, with the mode frequency matching that of the toroidal precession frequency of trapped alphas. Fast plasma toroidal flow however helps mitigate the FB instability. Kinetic stabilization of the RWM and flow stabilization of the (alpha-triggered) FB result in an enhancement of βN from the design value of 3.22 to 3.52 for the ITER scenario considered, while still maintaining stable plasma operation against the aforementioned MHD instabilities.