Energetic particle optimization of quasi-axisymmetric stellarator equilibria

An important goal of stellarator optimization is increasing confinement of energetic particles, such as those born from fusion reactions in a reactor. In this work, the fixed-boundary stellarator equilibrium LI383 was optimized for energetic particle confinement via a two-step process using the stellarator optimization code suite STELLOPT. In the first step, the equilibrium was optimized for high quasi-axisymmetry (QA) on a single flux surface near the mid-radius, and in the second, a minimization of the analytical quantity Γ_C was performed while maintaining the improved quasi-axisymmetry. This process was performed multiple times on the same initial equilibrium, resulting in a group of equilibria with significantly improved energetic particle confinement, as demonstrated by Monte Carlo simulations of fusion alphas in scaled-up versions of the optimized equilibria. This is the first successful attempt to improve energetic particle confinement in a QA stellarator by optimizing Γ_C. Finally, a statistical analysis was performed, examining the relationship between energetic particle losses and analytical metrics such as QA error and Γ_C. In particular, Γ_C was strongly correlated with losses, with a nearly linear relationship between volume-averaged Γ_C and particle losses after 1 ms.