Exploration of the parameter space of piecewise omnigenous stellarator magnetic fields

Abstract: Piecewise omnigenous fields are stellarator magnetic fields that are optimized with respect to radial neoclassical transport thanks to a second adiabatic invariant that is piecewisely constant on the flux-surface. They are qualitatively different from omnigenous fields (including quasi-isodynamic or quasisymmetric fields), for which the second adiabatic invariant is a flux-surface constant. Piecewise omnigenous fields thus open an alternative path towards stellarator reactors. In this work, piecewise omnigenous fields are characterized and parametrized in a systematic manner. This is a step towards including piecewise omnigenity as an explicit design criterion in stellarator optimization, and towards a systematic study of the properties of nearly piecewise omnigenous stellarator configurations.