Flow damping in stellarators close to quasisymmetry

TL;DR

This paper reviews criteria for how close stellarators are to quasisymmetry, crucial for minimizing flow damping and turbulent transport, and introduces new results in a specific collisionality regime affecting particle transport.

Contribution

It synthesizes recent formal criteria for quasisymmetry proximity and presents new findings on particle transport in a collisionality regime with ripple wells.

Findings

Particle flux scales with deviation parameter $eta$, not $eta^{3/2}$.

Ripple wells are not always the main source of transport.

New collisionality regime analysis extends understanding of particle trapping effects.

Abstract

Quasisymmetric stellarators are a type of optimized stellarators for which flows are undamped to lowest order in an expansion in the normalized Larmor radius. However, perfect quasisymmetry is impossible. Since large flows may be desirable as a means to reduce turbulent transport, it is important to know when a stellarator can be considered to be sufficiently close to quasisymmetry. The answer to this question depends strongly on the size of the spatial gradients of the deviation from quasisymmetry and on the collisionality regime. Recently, formal criteria for closeness to quasisymmetry have been derived in a variety of situations. In particular, the case of deviations with large gradients was solved in the $1/\nu$ regime. Denoting by $\alpha$ a parameter that gives the size of the deviation from quasisymmetry, it was proven that particle fluxes do not scale with $\alpha^{3/2}$, as typically claimed, but with $\alpha$. It was also shown that ripple wells are not necessarily the main cause of transport. This paper reviews those works and presents a new result in another collisionality regime, in which particles trapped in ripple wells are collisional and the rest are collisionless.