Violating Suydam criterion produces feeble instabilities

Abstract

Eigenmode analysis of a magnetic shear localized ideal interchange mode indicates a marginal stability condition consistent with the Suydam (and Mercier) criterion. However, the linear eigenmode analysis shows that it is not a very restrictive condition because the mode growth rate is exponentially small just above the marginal stability condition and the modes are easily stabilized by finite ion Larmor radius (FLR) effects. This suggests that for practical applications to toroidal confinement experiments, the Suydam–Mercier criterion does not introduce a restrictive condition for operational limits. The criterion for robust growth appears to be almost a factor of two greater than the Suydam criterion. It is also shown that even the presence of usual nonideal effects (FLR, electron diamagnetic flow and resistivity) does not alter this higher criterion for robust growth.