Exploring the effect of geometry on ballooning stability in optimized stellarators

Ballooning instabilities are pressure-driven magnetohydrodynamic (MHD) instabilities in which regions of poor curvature in the magnetic field allow a confined plasma to "balloon" outward at high plasma pressure. As such, ballooning modes can play a key role in determining β-limits in stellarators. Previous work suggests that components of the local magnetic shear which break symmetry in quasi-symmetric stellarators can further destabilize MHD ballooning modes at high β [1].  In this work, we leverage the scalability of the StellaratorOptimization.jl package on high-performance computing environments to perform parameter scans of plasma geometry characteristics, to investigate the relationship between plasma geometry and ballooning stability in quasi-symmetric stellarators. Ballooning stability is assessed using a version of the COBRAVMEC code, that was rewritten and refactored in Julia. We explore the trade-off between quasi-symmetry and ballooning stability as optimization targets, as a function of plasma geometry. 

1. C. C. Hegna and S. R. Hudson, Phys. Rev. Lett. 87, 035001 (2001).