Quasilinear particle transport from gyrokinetic instabilities in general magnetic geometry

TL;DR

This paper derives a general quasilinear theory for particle fluxes caused by gyrokinetic instabilities in arbitrary magnetic geometries, revealing how flux directions depend on magnetic curvature, mode structure, and impurity charge.

Contribution

It provides a novel, geometry-independent analytical framework for understanding particle transport in magnetized plasmas, including implications for stellarator design and impurity behavior.

Findings

Particle flux direction depends on magnetic curvature and temperature gradients.

In quasi-isodynamic stellarators, flux is outward unless temperature gradients are large.

Magnetic fluctuations from ion-temperature-gradient instabilities have a small effect.

Abstract

The quasilinear particle flux arising from gyrokinetic instabilities is calculated in the electrostatic and collisionless approximation, keeping the geometry of the magnetic field arbitrary. In particular, the flux of electrons and heavy impurity ions is studied in the limit where the former move quickly, and the latter slowly, along the field compared with the mode frequency. Conclusions are drawn about how the particle fluxes of these species depend on the magnetic-field geometry, mode structure and frequency of the instability. Under some conditions, such as everywhere favourable or unfavourable magnetic curvature and modest temperature gradients, it is possible to make general statements about the fluxes independently of the details of the instability. In quasi-isodynamic stellarators with favourable bounce-averaged curvature for most particles, the particle flux is always outward if the temperature gradient is not too large, suggesting that it might be difficult to fuel such devices with gas puffing from the wall. In devices with predominantly unfavourable magnetic curvature, the particle flux can be inward, resulting in spontaneous density peaking in the centre of the plasma. In the limit of highly charged impurities, ordinary diffusion (proportional to the density gradient) dominates over other transport channels and the diffusion coefficient becomes independent of mass and charge. An estimate for the level of transport caused by magnetic-field fluctuations arising from ion-temperature-gradient instabilities is also given and is shown to be small compared with the electrostatic component.