Heat conduction in an irregular magnetic field: Part II. Heat transport as a measure of the effective non-integrable volume

TL;DR

This paper introduces a metric to quantify the effective volume of non-integrability in magnetic fields, which influences heat transport in magnetized plasmas, validated through model fields and applicable to stellarator design.

Contribution

It proposes a new metric for the effective volume of non-integrability based on anisotropic heat diffusion, validated with analytic and numerical models.

Findings

The metric aligns with the effective volume of non-integrability as perpendicular diffusion approaches zero.

Magnetic field structures significantly impact heat transport, with chaotic regions facilitating pressure gradients.

Mode spectrum influences heat transport in chaotic magnetic fields.

Abstract

Given the large anisotropy of transport processes in magnetized plasmas, the magnetic field structure can strongly impact heat diffusion: magnetic surfaces and cantori form barriers to transport while chaotic layers and island structures can degrade confinement. When a small but non-zero amount of perpendicular diffusion is included, the structure of the magnetic field becomes less important, allowing pressure gradients to be supported across chaotic regions and island chains. We introduce a metric for the effective volume over which the local parallel diffusion dominates based on the solution to the anisotropic heat diffusion equation. To validate this metric, we consider model fields with a single island chain and a strongly chaotic layer for which analytic predictions of the relative parallel and perpendicular transport can be made. We also analyze critically chaotic fields produced from different sets of perturbations, highlighting the impact of the mode number spectrum on the heat transport. Our results indicate that this metric coincides with the effective volume of non-integrability in the limit $\kappa_{\perp} \rightarrow 0$. We propose that this metric be used to assess the impact of non-integrability on the heat transport in stellarator equilibria.