Interactions between energetic ions (EIs) and kinetic ballooning modes (KBMs) are inevitable in fusion reactors characterized by high-β plasmas with a large population of alpha particles (, where ). In this work, the effects of EIs on KBM stability are investigated using first-principles gyrokinetic simulations, which demonstrate the roles of magnetohydrodynamic (MHD) ballooning-interchange drive, wave–particle resonance and orbital effects in several parameter regimes of practical interest. In particular, it is found that EI–KBM interactions are mostly determined by three dimensionless parameters: the EI–thermal electron temperature ratio , the KBM perpendicular wave vector normalized by EI orbit width and the EI pressure ratio βh, which are associated with resonance condition, finite orbit width (FOW) screening and EI drive strength for Alfvénic modes. For typical orderings of and , becomes crucial for relevant physical processes. (i) In the short-wavelength regime of , the response of EIs to KBM electromagnetic fluctuations is greatly reduced due to strong FOW screening, which leads to a weakly stabilizing effect on KBMs via thermal ion dilution, despite the large βh. (ii) In the long-wavelength regime of , passing EIs can non-perturbatively destabilize KBMs through transit motion resonance, attributed to the fact that large poloidal and toroidal frequencies mostly cancel each other and satisfy locally around rational surfaces (p is an integer), while the net response of trapped EIs is near zero due to the mismatch of resonance conditions. For minor ion species characterized by 1, such as helium ash, FOW screening is modest with and the drive strength is perturbative with , which drive KBMs through MHD ballooning-interchange and wave–particle resonance similar to thermal ions. These findings are helpful for understanding the effects of alpha particles and helium ash on KBM stability and plasma confinement in future fusion reactors.
Low n electromagnetic modes in spherical tokamaks