Low-shear three-dimensional equilibria and vacuum magnetic fields with flux surfaces

TL;DR

This paper explores the construction and limitations of low-shear three-dimensional vacuum magnetic fields with flux surfaces, emphasizing the role of symmetry and perturbation methods in stellarator design.

Contribution

It demonstrates the possibility of constructing such fields order by order under stellarator symmetry and investigates conditions for neighboring flux surfaces in 3D equilibria.

Findings

Stellarator symmetry enables order-by-order construction of vacuum fields.

Low shear vacuum fields with flux surfaces are harder to find than force-free fields.

Existence of vacuum magnetic fields analogous to tokamak snakes is shown.

Abstract

Stellarators are generically small current and low plasma beta ($\beta= p/B^2\ll1$) devices. Often the construction of vacuum magnetic fields with good magnetic surfaces is the starting point for an equilibrium calculation. Although in cases with some continuous spatial symmetry flux functions can always be found for vacuum magnetic fields, an analogous function does not, in general, exist in three dimensions. This work examines several simple equilibria and vacuum magnetic field problems with the intent of demonstrating the possibilities and limitations in the construction of such states. Starting with a simple vacuum magnetic field with closed field lines in a topological torus (toroidal shell with a flat metric), we obtain a self-consistent formal perturbation series using the amplitude of the nonsymmetric vacuum fields as a small parameter. We show that systems possessing stellarator symmetry allow the construction order by order. We further indicate the significance of stellarator symmetry in the amplitude expansion of the full ideal magnetohydrodynamics (MHD) problem as well. We then investigate the conditions that guarantee neighboring flux surfaces given the data on one surface, by expanding in the distance from that surface. We show that it is much more difficult to find low shear vacuum fields with surfaces than force-free fields or ideal MHD equilibrium. Finally, we demonstrate the existence of a class of vacuum magnetic fields, analogous to `snakes' observed in tokamaks, which can be expanded to all orders.