Stellarator optimization for good magnetic surfaces at the same time as quasisymmetry

TL;DR

This paper presents a novel optimization method for stellarators that simultaneously eliminates magnetic islands and enhances quasisymmetry, improving magnetic surface quality and physics properties.

Contribution

It introduces a combined equilibrium approach using VMEC and SPEC to optimize stellarator geometry for surface integrity and quasisymmetry in a single process.

Findings

Successful elimination of magnetic islands during optimization.

Enhanced quasisymmetry on targeted surfaces.

Improved magnetic surface quality compared to previous methods.

Abstract

A method is demonstrated to optimize a stellarator's geometry to eliminate magnetic islands and achieve other desired physics properties at the same time. For many physics quantities that have been used in stellarator optimization, including quasisymmetry, neoclassical transport, and magnetohydrodynamic stability, it is convenient to use a magnetic equilibrium representation that assures the existence of magnetic surfaces. However, this representation hides the possible presence of magnetic islands, which are typically undesirable. To include both surface-based objectives and island widths in a single optimization, two fixed-boundary equilibrium calculations are run at each iteration of the optimization: one that enforces the existence of magnetic surfaces (VMEC [S. P. Hirshman and J. C. Whitson, Phys. Fluids 26, 3553 (1983)]), and one that does not (SPEC [S. R. Hudson, et al, Phys. Plasmas 19, 112502 (2012)]). By penalizing the island residues in the objective function, the two magnetic field representations are brought into agreement during the optimization. An example is presented in which, particularly on the surface where quasisymmetry was targeted, quasisymmetry is achieved more accurately than in previously published examples.