Assessing nonlinear MHD stability in optimized quasisymmetric stellarators

We report on the application of the M3D-C1 extended MHD code to assess the nonlinear stability properties of a recently developed 2-field-period quasi-axisymmetric stellarator configuration, optimized at 2.5% plasma beta for good energetic particle confinement and self-consistent bootstrap current [1]. Overall, the configuration was found to be unstable to fast-growing magnetohydrodynamic (MHD) modes with moderate to high poloidal and toroidal mode numbers. Strong toroidal mode coupling, arising due to the absence of axisymmetry, leads to rapid destabilization of other modes in the N=2 mode family (modes with even toroidal mode number). Nonlinearly, these modes were found to generate radially elongated perturbations of the pressure profile and led to rapid destruction of nearly all flux surfaces present in the initial equilibrium.

While there is experimental evidence to suggest that non-axisymmetric plasmas may have improved nonlinear MHD stability, this first-of-a-kind M3D-C1 analysis demonstrates that MHD stability must remain a critical consideration when designing stellarator plasmas, especially for high-performance applications such as fusion pilot plants. This motivates the on-going development of new tools and reduced models that will allow for nonlinear MHD stability to be incorporated into stellarator optimization.

[1] Landreman et al., Physics of Plasmas 29, 082501 (2022).