Stellarator Techniques to Optimize 3D Mirrors for Improved MHD Stability and Reduced Particle Drift

Axisymmetric mirrors are susceptible to interchange instability because curvature and the pressure gradient are misaligned in the low-field region. Non-axisymmetric fields can stabilize the mode by modifying the field line curvature and imposing a magnetic well. However, the experimental implementation via Ioffe bars or baseball coils introduces radial loss through increased particle drift effects, not unlike early stellarators.

Recently, stellarator optimization has made immense improvements in coil optimization, 3D MHD equilibrium solvers, and the experimental realization of quasi-omnigeneous and quasi-symmetric fields. It is now possible to design stellarator coils that not only meet engineering constraints, but also accurately reproduce magnetic fields that have been optimized for MHD stability, minimal neoclassical transport, and even suppressed plasma turbulence. These tools enable the exploration of new 3D solution spaces that can be extended to non-axisymmetric mirrors.

We apply the stellarator optimization code SIMSOPT to design multiple coil sets with improved accuracy for the Straight Field Line Mirror proposed by Mosienko and Ågren. We analyze the resulting configurations by calculating particle drift orbits and the second adiabatic invariant J. We present initial work on adapting stellarator equilibrium code DESC to model open field line geometries, to enable the study of stability and finite beta effects.