Helically symmetric equilibria without net longitudinal current and their linear MHD stability are studied with the HASE quasi-analytical equilibrium code and the HERA 2D spectral code. The modes investigated comprise low-m-number internal, external, high-m-number modes obtained with the large-k-number option of the HERA code and localized (k → ∞) ballooning modes. Stable straight Heliotron equilibria with beta up to <β> = 0.01 and stable straight Heliac equilibria with beta up to <β> = 0.29 are obtained. The correspondence between instability as obtained from the ballooning-mode equation in part of the plasma volume and from finite-k analysis is discussed.
Beta limits for the BIG DEE