On shear-Alfvén wave-induced energetic ion transport in optimised stellarators

In this work, we investigate prompt ion drift orbit losses caused by shear-Alfvén waves (SAWs) in quasi-symmetric and quasi-isodynamic (QI) stellarators optimised for equilibrium confinement of energetic particles (EPs). We use the ideal reduced magnetohydrodynamic (MHD) model for SAW perturbations and study their impact on the collisionless EP drift dynamics. We present a semi-analytical model for resonance between the passing EP and SAW, generalised to arbitrary quasi-symmetric configurations including the quasi-poloidal case relevant to QI equilibria. Analysis reveals that an increase in the number of field periods upper N Subscript normal f normal p N f p $N_{\mathrm{fp}}$ suppresses stochasticity in quasi-helical (QH) and quasi-isodynamic, but not quasi-axisymmetric (QA) stellarators. We show that wave-induced transitions between passing and trapped orbits cause significant losses in QA and QH, but not in QI configurations. For the considered equilibria at scales relevant to fusion power plants, we numerically determine the SAW amplitudes needed to induce prompt loss of fusion-born alpha particles. Using the weighted Birkhoff averaging technique, we confirm that the onset of prompt losses across all orbit classes occurs with the onset of stochasticity in ion motion. This motivates extending the stochasticity-onset criterion beyond passing orbits in future work.