Ideal MHD stability of the mega-ampere spherical tokamak

Abstract

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, βn up to 4.95 and BS fractions up to 30%, with internal inductance li ≈ 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 βn 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 βn MAST discharges. Based on kinetic reconstructions it is found that the no-wall instability threshold to external n = 1 displacements is βn ≈ 5-6, and the with-wall βn 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 βn. 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 li and K. Together, these provide a measure of proximity to marginal stability.