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

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.

References in corpus (4)

• SciPy 1.0--Fundamental Algorithms for Scientific Computing in Python
• Computation of multi-region relaxed magnetohydrodynamic equilibria
• Properties of a new quasi-axisymmetric configuration
• Constructing stellarators with quasisymmetry to high order

Cited by in corpus (5)

• Magnetic fields with precise quasisymmetry for plasma confinement
• A single-field-period quasi-isodynamic stellarator
• Direct stellarator coil optimization for nested magnetic surfaces with precise quasi-symmetry
• Stellarator optimization for nested magnetic surfaces at finite $\beta$ and toroidal current
• Adjoint methods for quasisymmetry of vacuum fields on a surface