Equilibrium and stability of high-Beta plasmas in Wendelstein 7-AS

Quasi-stationary, MHD-quiescent discharges with volume-averaged β-values up to 3.5% were sustained in the W7-AS for more than 100 energy confinement times. A stability limit was not observed. The achieved β appears to be limited by confinement, but is sensitive to the magnetic configuration, including the rotational transform and perturbations from the divertor control coils. The plasma equilibrium is reconstructed, fitting the magnetic diagnostic measurements and the Thomson-scattering pressure profile. Analysis of the free-boundary equilibria by PIES indicates that the β-limit and its variation may be due to deterioration of the flux surfaces and generation of magnetic stochasticity. Low-frequency n =1 and 2 MHD activity is often observed at intermediate β-values, but does not impede access to higher-β. Linear ideal-MHD free-boundary stability calculations indicate that the plasma should be unstable for 1% < β < 2.5%, in agreement with the experiment. The linear stability threshold severely underestimates the achievable β-values. During the high-β phase, strong MHD activity occurs only when the edge rotational transform equals 0.5 or 0.6. (author)