Equilibrium Reconstructions, Stability Calculations, and Disruption Event Characterization of Plasmas in the MAST and MAST Upgrade Spherical Tokamaks

The MAST-U spherical tokamak experiment, an upgrade of the MAST device, is in its first physics campaign. Disruptions in MAST-U, as well as in the database of MAST discharges, are studied to characterize the events that cause them. Loss of vertical stability control was not found to be common in MAST. MAST discharges were able to somewhat exceed the Greenwald density limit, with many density limit disruptions occurring in the current rampdown. Reconstructions of plasma equilibria using kinetic profiles are necessary for stability and disruption analysis. The VALEN code was used to determine effective resistances of the MAST and MAST-U conducting structures to provide estimated currents in those structures for EFIT reconstruction and to separate the plasma current measurement from Rogowski coil sets that enclose those currents. Ideal stability calculations of MAST plasmas, assisted also with machine learning techniques, indicate that the no-wall beta limit can be reliably determined. Projections of stability for MAST-U show that a larger stability gap between the no-wall and with-wall limits should be expected in MAST-U than in MAST, due to passive stabilization plates. Experiments using resonant field amplification to characterize MAST-U stability are planned.