Quadratic Flux-Minimizing Maps for Characterization of Energetic Particle Transport Mechanisms in Quasihelical and Quasiaxisymmetric Stellarators
Amelia Chambliss, Columbia University
Even if the magnetic field in a stellarator is integrable, phase-space integrability is not guaranteed, as energetic particles can be subject to different resonances than the thermal bulk. Both trapped and passing particle trajectories can experience convective losses, caused by wide phase-space island formation, and diffusive losses caused by phase-space island overlap. Using guiding-center trajectory mappings for each particle class, we numerically determine trapped particle precession frequency and passing particle effective helicity to determine sensitivity to resonances. In the presence of resonances, we apply a quadratic flux-minimization method to characterize phase-space island width and overlap for energetic particles. For trapped particles in QH, low-shear toroidal precession frequency profiles near zero result in wide island formation. While QA toroidal precession frequencies do not cross through zero, we observe that island overlap is more likely for QA since higher precession frequency shear results in the crossing of more low-order resonances. Resonance crossings and variation of phase-space island widths across pitch angles is also considered. This work offers new analysis of energetic particle physics in QA and QH that can be later adapted into optimization metrics for improved confinement of energetic particles during the design stage.