ARC Physics Basis - MHD [magnetohydrodynamics]

ARC is designed to produce 400MW of net electricity and prove the commercial feasibility of a fusion power plant. In order to achieve this goal ARC has to operate with optimal core performance in a stationary scenario that minimizes wear on the first wall and divertor. This requires avoiding or mitigating magnetohydrodynamic (MHD) instabilities which have the potential to not only degrade the plasma core but also lead to deleterious transient heat loads on plasma facing components. Therefore, this work aims at characterizing the MHD stability of the high performance ARC scenario and inform the design of error field correction coils. Firstly, simulations of vertical displacement events show that an in-vessel coil is not needed and instead the poloidal shaping coils can be used to control vertical stability. These simulations also inform the demands on the corresponding coil power supplies. Stability analysis of the ideal kink mode with or without a conducting wall and kinetic effects suggests that the ARC baseline scenario operates deeply in the stable region. Using RDCON, tearing modes at the m/n = 2/1 and 3/2 surfaces are shown to be linearly stable, and including thermal transport effects in the rational surfaces lead to further stabilization. However, other transient plasma instabilities can seed neoclassical tearing modes (NTMs). The marginally stable width of NTMs in ARC strongly depends on the internal inductance and can fall below 0.1% of the normalized poloidal flux. Furthermore, an empirical cross-machine model of the n = 1 error field leading to a disruption predicts a critical error field larger than SPARC but smaller than ITER. 3D coils can be designed with GPEC based on a simple model that calculates the maximum correctable error field that is limited by the neoclassical toroidal viscosity torque. Broad scans of different coil geometries identify a set of 2 rows of off-midplane coils to be a suitable solution. It is also determined that such a set of 3D coils is capable of correcting n = 2 error fields to some degree and creating strong enough n = 2 or n = 3 edge resonant perturbation fields for the suppression of edgelocalized modes at reasonable coil currents. The final design of the first ARC will be further informed by results from SPARC.