Higher order theory of quasi-isodynamicity near the magnetic axis of stellarators

Abstract: The condition of quasi-isodynamicity is derived to second order in the distance from the magnetic axis. We do so using a formulation of omnigenity that explicitly requires the balance between the radial particle drifts at opposite bounce points of a magnetic well. This is a physically intuitive alternative to the integrated condition involving distances between bounce points, used in previous works. We investigate the appearance of topological defects in the magnetic field strength (``puddles''). A hallmark of quasi-isodynamic fields, the curved contour of minimum field strength, is found to be inextricably linked to these defects. Our results pave the way to constructing solutions that satisfy omnigenity to a higher degree of precision, and also to simultaneously consider other physical properties, like shaping and stability.