Suppression of resistive ballooning mode by energetic ions in quasi-axisymmetric stellarator plasmas

Abstract The interaction between energetic particles (EPs) and resistive ballooning modes (RBMs) in quasi-axisymmetric stellarator (QAS) plasmas is investigated via MHD hybrid simulations using the MEGA code for the Chinese First Quasi-axisymmetric Stellarator (CFQS) configuration. Simulations show RBMs ( m= 9/n = −4 ) are stabilized by EPs at low beta values ( \beta_ℎ < 0.8%), which is consistent with theoretical predictions. However, as \beta_ℎ exceeds 0.8%, the RBM is suppressed and a new EP-driven reversed shear Alfvén eigenmode (RSAE)-like instability ( m= 4/ n= −2) becomes dominant, with significantly increased frequency and growth rate. A resistivity ( \eta) scan at high \beta_ℎ (1.5%) reveals a transition: a mode exhibits RSAE characteristics at low , while transitioning to a resistivity-driven ballooning mode at high \eta. This suggests the EP-driven instability observed in initial high-resistivity simulations originates from the RSAE branch but its structure is strongly modified, displaying pronounced ballooning features. This work highlights the dual role of EPs and the crucial influence of resistivity on instabilities in QAS, providing insights for high-beta operation.