Physics Basis to Inform 3D Coil Design for ARC

Non-axisymmetric perturbed equilibrium calculations are used to assess the ARC power plant design sensitivity to intrinsic error fields as well as guide potential 3D coil designs for error field correction. Intrinsic error fields can be caused by small misalignments or asymmetries in the tokamak and potentially seed tearing modes leading to disruptions. An empirical scaling law is used to predict the critical n=1 error field leading to disruptions in ARC. The MHD code GPEC is used to identify the dominant mode for a kinetically constrained ARC equilibrium, i.e. the mode structure that causes the strongest plasma response amplifying the intrinsic error field further. Scans of various 3D coil geometries (distance, height, location, orientation) are performed for window-frame and single filament coils to assess the possibility of designing external error field correction coils that can correct for the predicted n=1 error field component that couples to the dominant mode. The PENTRC code is used together with GPEC to calculate the neoclassical toroidal viscosity torque induced by the 3D coils, which is then included in a simple model to consider its effect on the critical error field via changes in rotation. Furthermore, the coupling of the 3D coils to the pedestal region is also quantified to validate the capability of controlling edge-localized modes with resonant magnetic perturbations. These calculations will help inform ARC coil design and construction tolerance considerations as the design progresses.