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Plasmas with volume-averaged beta equal to 3% have been produced in beam-heated ISX-B discharges. Previous modelling of such ISX-B plasmas, at somewhat lower beta and with limited detail as regards plasma shape, has shown that high-n ballooning instabilities should be excited, although none is observed. The authors now use data that allow them to specify free-boundary equilibria, which are influenced by the ISX-B poloidal coil system and iron core, and compare these equilibria with criteria for ideal magnetohydrodynamic (MHD) stability to both low- and high-n number ballooning and kink modes. Both analytic estimates and codes are used in the comparison. It is found that the equilibria studied are predicted to be unstable to both low- and high-n modes, in contrast to the experimental results. Several candidates for resolving these differences are discussed.
Munro et al. (Sat,) studied this question.