Energetic particle loss mechanisms in reactor-scale equilibria close to quasisymmetry

TL;DR

This paper investigates collisionless alpha particle transport in 3D stellarator equilibria, classifying loss mechanisms and comparing their impact across different quasi-symmetric configurations to inform optimization strategies.

Contribution

It provides a detailed classification of trapping states and transport mechanisms in quasihelical and quasiaxisymmetric equilibria, highlighting differences in loss behaviors and potential optimization pathways.

Findings

Quasihelical configurations show reduced prompt losses due to natural protection against ripple-trapping.

Loss mechanisms include banana drift convection, ripple transport, and diffusive banana tip motion.

Transitions between trapping classes can occur over long timescales, affecting overall particle confinement.

Abstract

Collisionless physics primarily determines the transport of fusion-born alpha particles in 3D equilibria. Several transport mechanisms have been implicated in stellarator configurations, including stochastic diffusion due to class transitions, ripple trapping, and banana drift-convective orbits. Given the guiding center dynamics in a set of six quasihelical and quasiaxisymmetric equilibria, we perform a classification of trapping states and transport mechanisms. In addition to banana drift convection and ripple transport, we observe substantial non-conservation of the parallel adiabatic invariant which can cause losses through diffusive banana tip motion. Furthermore, many lost trajectories undergo transitions between trapping classes on longer time scales, either with periodic or irregular behavior. We discuss possible optimization strategies for each of the relevant transport mechanisms. We perform a comparison between fast ion losses and metrics for the prevalence of mechanisms such as banana-drift convection [1], transitioning orbits, and wide orbit widths. Quasihelical configurations are found to have natural protection against ripple-trapping and diffusive banana tip motion leading to a reduction in prompt losses.