Fast ion transport in quasisymmetric equilibria in the presence of a resonant Alfv\'{e}nic perturbation

TL;DR

This paper investigates how resonant Alfvén eigenmodes influence ion transport in quasisymmetric magnetic fields, revealing conditions under which transport is dominated by these modes or by QS-breaking effects.

Contribution

It provides a detailed analysis of AE-driven ion transport in quasisymmetric configurations, highlighting differences from tokamak behavior and the impact of QS deviations.

Findings

AE amplitudes consistent with experiments cause stiff transport above a threshold

Avoidance of phase-space island overlap in quasihelical configurations

Enhanced transport in low magnetic shear configurations due to wide phase-space islands

Abstract

Significant progress has been made in designing magnetic fields that provide excellent confinement of the guiding enter trajectories of alpha particles using quasisymmetry (QS). Given the reduction in this transport channel, we assess the impact of resonant Alfv\'{e}n eigenmodes (AEs) on the guiding center motion. The AE amplitudes are chosen to be consistent with experimental measurements and large-scale simulations. We evaluate the drift resonance condition, phase-space island width, and island overlap criterion for quasisymmetric configurations. Kinetic Poincar\'{e} plots elucidate features of the transport, including stiff transport above a critical perturbation amplitude. Our analysis highlights key departures from the AE-driven transport in tokamaks, such as the avoidance of phase-space island overlap in quasihelical configurations and the enhanced transport due to wide phase-space islands in low magnetic shear configurations. In configurations that are closer to QS, with QS deviations $\delta B/B_0 \lesssim 10^{-3}$, the transport is primarily driven by the AE, while configurations that are further from QS, $\delta B/B_0 \sim 10^{-2}$, experience significant transport due to the QS-breaking fields in addition to the AE.