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A method is demonstrated to optimize a stellarator's geometry to eliminatemagnetic 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 magnetohydrodynamicstability, it is convenient to use a magnetic equilibrium representation thatassures the existence of magnetic surfaces. However, this representation hidesthe possible presence of magnetic islands, which are typically undesirable. Toinclude both surface-based objectives and island widths in a singleoptimization, two fixed-boundary equilibrium calculations are run at eachiteration of the optimization: one that enforces the existence of magneticsurfaces (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 theoptimization. An example is presented in which, particularly on the surfacewhere quasisymmetry was targeted, quasisymmetry is achieved more accuratelythan in previously published examples.

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