Fast particle trajectories, quadratic flux-minimizing maps, and integrability in quasiaxisymmetric and quasihelical stellarators

Energetic particles can be damaging to device walls in stellarator reactors, posing a threat to device longevity. These particles can be lost from stellarator equilibria via mechanisms like diffusive and convective transport. Following fast particle trajectories and understanding the relationship between loss mechanisms and field geometry can further guide improvements in stellarator optimization. We investigate the motion of trapped and passing fast particles in quasiaxisymmetric and quasihelical stellarator equilibria through the application of guiding-center trajectory mappings for trapped and passing particles in phase-space. We apply quadratic flux-minimization methods to produce periodic pseudo-orbits that resolve the nearby integrable Hamiltonian system in the presence of island overlap and phase-space chaos. We demonstrate application of the maps for both trapped and passing particles in quasiaxisymmetric and quasihelical equilibria in vacuum and non-zero β regimes. Analytic results for trapped particle trajectories in the near-axis model are compared to numerical results, along with theoretically predicted and numerically determined characteristic frequencies.