Generalized Grad-Shafranov equation for non-axisymmetric MHD equilibria

TL;DR

This paper introduces a generalized Grad-Shafranov equation that extends the classical model to non-axisymmetric MHD equilibria, revealing hidden symmetries and reformulating the 3D equilibrium problem.

Contribution

It uncovers a hidden symmetry leading to a generalized Grad-Shafranov equation applicable to non-axisymmetric equilibria, expanding the classical understanding.

Findings

Generalized equation accounts for non-axisymmetric equilibria with pressure gradients.

Solutions satisfy force balance averaged over a hidden symmetry.

Reformulates 3D equilibrium problem as finding an optimal volume-preserving symmetry.

Abstract

The structure of static MHD equilibria that admit continuous families of Euclidean symmetries is well understood. Such field configurations are governed by the classical Grad-Shafranov equation, which is a single elliptic PDE in two space dimensions. By revealing a hidden symmetry, we show that in fact all smooth solutions of the equilibrium equations with non-vanishing pressure gradients away from the magnetic axis satisfy a generalization of the Grad-Shafranov equation. In contrast to solutions of the classical Grad-Shafranov equation, solutions of he generalized equation are not automatically equilibria, but instead only satisfy force balance averaged over the one-parameter hidden symmetry. We then explain how the generalized Grad-Shafranov equation can be used to reformulate the problem of finding exact three-dimensional smooth solutions of the equilibrium equations as finding an optimal volume-preserving symmetry.