Common physics basis for the SPARC and COMPASS-U 3D coil designs*

Non-axisymmetric field coils have been designed to provide efficient error field compensation and suppress edge localized modes in two new high field tokamaks, SPARC and COMPASS-U. These designs utilize the Generalized Perturbed Equilibrium Code's (GPEC's) representation of the multimodal, non-axisymmetric plasma response to optimize the geometric coupling between 3D coil arrays and the desired core or edge plasma response. Error field compensation coils are designed to couple to the plasma-amplified kink that dominates the drive of core resonances. The maximum allowable error field is projected to these new machines using an empirical scaling supported by nonlinear MHD modeling. Asymmetric construction and assembly tolerances are then balanced against the corresponding kA-turns needed for compensation to levels below the allowable limit. The coupling to edge resonant magnetic perturbation (RMPs) is similarly optimized for edge localized mode (ELM) suppression, with kA-turn requirements set by thresholds projected using a combination of reduced theory and nonlinear modeling. Edge perturbations are further decoupled from the core response as much as possible using higher toroidal mode numbers as well as by careful tuning of the poloidal spectrum. These physics-driven coil designs thus provide confidence in our ability to realize previously demonstrated 3D field benefits in new high field reactor regimes.