Energetic particle optimization of quasi-axisymmetric stellarator equilibria

An important goal of stellarator optimization is to achieve good confinement of energetic particles such as, in the case of a reactor, alphas created by deuterium-tritium fusion. In this work, a fixed-boundary stellarator equilibrium was re-optimized for energetic particle confinement via a two-step process: first, by minimizing deviations from quasi-axisymmetry (QA) on a single flux surface near the mid-radius, and secondly by maintaining this improved QA while minimizing the analytical quantity gamma(C), which represents the angle between magnetic flux surfaces and contours of J(||), the second adiabatic invariant. This was performed multiple times, resulting in a group of equilibria with significantly reduced energetic particle losses, as evaluated by Monte Carlo simulations of alpha particles in scaled-up versions of the equilibria. This is the first time that energetic particle losses in a QA stellarator have successfully been reduced by optimizing gamma(C). The relationship between energetic particle losses and metrics such as QA error (EqaEqa ) and gamma(C )in this set of equilibria were examined via statistical methods and a nearly linear relationship between volume-averaged gamma(C )and prompt particle losses was found.

Automated summary

The goal of this work was to optimize stellarator equilibrium for good confinement of energetic particles. This was achieved by minimizing deviations from quasi-axisymmetry and reducing the angle between magnetic flux surfaces and contours of the second adiabatic invariant. Multiple optimizations resulted in equilibria with significantly reduced energetic particle losses, as evaluated by Monte Carlo simulations.