Trapped-particle precession and modes in quasi-symmetric stellarators and tokamaks: a near-axis perspective

TL;DR

This paper analytically investigates trapped-particle precession and modes in near-axis quasi-symmetric stellarators and tokamaks, revealing how geometry influences particle behavior and turbulence stability.

Contribution

It provides an analytic framework for trapped-particle drift in near-axis magnetic fields, linking geometry, stability, and turbulence in stellarators and tokamaks.

Findings

Maximum-J property is unattainable but approaches with increased plasma beta and shaping.

Analytic calculation of trapped electron energy in asymptotic regimes.

Available energy relates to MHD stability, indicating potential stability-turbulence links.

Abstract

This paper presents the calculation of the bounce-averaged drift of trapped particles in a near-axis framework for axisymmetric and quasisymmetric magnetic fields that possess up-down and stellarator symmetry respectively. This analytic consideration provides important insight on the dependence of the bounce-averaged drift on the geometry and stability properties of the field. In particular, we show that, although the maximum-$\mathcal{J}$ property is unattainable in quasisymmetric stellarators, one may approach it through increased plasma $\beta$ and triangular shaping, albeit going through a reduced precession scenario with potentially higher particle losses. The description of trapped particles allows us to calculate the available energy of trapped electrons analytically in two asymptotic regimes, providing insight into the behaviour of this measure of turbulence. It is shown that the available energy is intimately related to MHD-stability, providing a potential synergy between this measure of gyrokinetic turbulence and MHD-stability.