Assessing the impact of vessel eddy currents on SPARC equilibrium reconstructions*

Accurate determination of the plasma shape, position, and divertor strike points is required to ensure heat and particle loads to in-vessel components are within design limits during operation of SPARC. However, with time varying magnetic coil currents, as well as changing plasma current and shape, the eddy currents induced through various conducting structures and vacuum vessel walls can make equilibrium reconstructions of the plasma geometry challenging. These currents can be especially large during critical periods for plasma control, including plasma start-up and strike point sweeps. To assess the impact of these eddy currents on equilibrium reconstruction quality, a suite of codes including, TSC, FreeGS, ThinCurr, and MEQ were used to determine eddy currents in the conducting structures for different operational scenarios. The newest code, ThinCurr, calculates the full 3D structure of the eddy currents in a more realistic conducting structure model. This allows for comparison with the 2D eddy current structures calculated in the TSC and MEQ codes, as well as possible refinement of the 2D models. The synthetic equilibria from these codes and the corresponding eddy currents were used as inputs to the EFIT equilibrium reconstruction code. The reconstruction quality in certain key geometric parameters was found to degrade when eddy currents were not considered by EFIT, however the level of degradation was dependent on the eddy current distribution and amplitude.