Investigating the Applicability of the Mercier Criterion to Stellarator Optimization

Future fusion reactors will rely on magnetic configurations that can maintain stable high-beta plasmas. Stellarators can uniquely exploit 3D shaping to explore a vast parameter space, allowing for optimization of a magnetic configuration for desirable physics metrics. Both the ideal and resistive Mercier criteria can be used in stellarator optimization to produce finite-beta plasma configurations that are theoretically stable to radially-localized interchange modes. However, stellarator experiments have exceeded these linear stability boundaries and crossed into Mercier unstable regimes with minimal performance degradation, making it unclear how effective the Mercier criterion is at predicting detrimental magnetohydrodynamic (MHD) modes. To investigate this issue, we construct a family of stellarators with a range of ideal and resistive Mercier criterion values using the simultaneous perturbation stochastic approximation method, starting from a 2.5% beta quasi-axisymmetric configuration that was shown to be MHD unstable. Linear growth rates computed with the extended‑MHD code M3D‑C1 are used to validate the linear stability boundaries predicted by these reduced models and to identify threshold values of the Mercier criterion for use in optimization.