Non-local interchange modes in quasi-symmetric stellarators using NIMSTELL

Pressure-driven instabilities are crucial in determining the operational beta limits of stellarators. While stability of localized interchange and ballooning can be estimated using flux-surface local criteria, predicting radially extended pressure-driven modes is not as straightforward. Here, we study such instabilities in a quasi-helically symmetric configuration using the NIMSTELL code. When the rotational transform is bounded between two rational numbers, a common practice in stellarator optimization, it can admit a non-monotonic profile, resulting in a region of low global shear. The almost-rational rotational transform in this region facilitates nearly resonant interchange modes, which are fast growing and radially extended. This is a potentially disruptive phenomenon and must be avoided. The margin by which the extrema in the rotational transform must differ from a rational number to render a configuration stable depends on the beta. Linear calculations in a range of beta values and preliminary nonlinear calculations are presented. In some cases, the instability grows faster than the infinite-n ballooning mode, suggesting that relying only on ballooning criteria may not be sufficient to ensure MHD stability.