Ideal MHD stability of the mega-ampere spherical tokamak

In this work three techniques that refine the magnetic reconstruction of the mega-ampere spherical tokamak (MAST) equilibrium are detailed: kinetic reconstruction, in which the thermal pressure profile is fitted to thermal electron and ion data; bootstrap (BS) reconstruction, in which the edge current profile is modified to be self-consistent with the BS fraction (in the limit that edge current is BS dominated); and fast-particle reconstruction, in which an effective fast-ion pressure component is added, representing ions driven by charge exchange of the thermal ions with injected neutrals. Kinetic reconstructions for some high performance shots suggest normalized beta, β_n up to 4.95 and BS fractions up to 30%, with internal inductance l_i ≈ 1 and pressure peaking factor p(0)/(p] ≈ 2.2. Full-orbit simulations suggest that up to 25% of the total stored energy in these high performance discharges is in the fast-ion population: addition of effective fast-particle pressures boosts β_n to 5.56. Ideal MHD pressure driven stability thresholds of n = 1, 2 and oc displacements are examined for kinetic and BS reconstructions of four high β_n MAST discharges. Based on kinetic reconstructions it is found that the no-wall instability threshold to external n = 1 displacements is β_n ≈ 5-6, and the with-wall β_n limit 10% higher than the no-wall limit. In comparison, the n = 1 no-wall instability threshold based on BS reconstructions is slightly below (≈95%) that determined using kinetic reconstructions. Comparison to the MAST database suggests that MAST is approaching a regime in which passive stabilization is required to prevent ideal disruptions at higher β_n. Finally, vertical stability of an earlier set of MAST discharges is also examined, an estimate of the MAST effective wall for n = 0 modes provided, and the wall radius for marginal stability parameterized as a function of l_i and K. Together, these provide a measure of proximity to marginal stability.