Characterising fast ion losses in tokamaks with quasisymmetry metrics

Abstract

In this work, ARC and SPARC equilibria with randomly generated coil misalignments are analysed using the metric of quasisymmetry error, frequently used in stellarator theory as a proxy for fast ion confinement, and a strong correlation between this metric and ripple-induced alpha power loss is found. In discussions of magnetic confinement devices, it is often taken for granted that tokamaks, unlike stellarators, are devices with axisymmetric magnetic fields. However, any tokamak built from discrete toroidal field (TF) coils will have non-axisymmetric components of the magnetic field due to the effect of TF ripple, and these components only grow when inevitable assembly errors are taken into account. Physically constructed tokamaks therefore have magnetic fields which can best be classified as near axisymmetric, and which share many characteristics with quasi-axisymmetric stellarators, allowing both classes of devices to be analysed using similar methods.