Energetic particle optimization of quasi-axisymmetric stellarator equilibria

• Published in: Nuclear fusion Vol. 63; no. 1
• Main Authors: LeViness, Alexandra, Schmitt, John C., Lazerson, Samuel A., Bader, Aaron, Faber, Benjamin J., Hammond, Kenneth C., Gates, David A.
• Format: Journal Article
• Language: English
• Published: IAEA IOP Publishing 12.12.2022
• ISSN: 0029-5515; 1741-4326

Abstract 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 Γ C <inline-graphic href='nfaca4e3ieqn1.gif' type='simple'/> , which represents the angle between magnetic flux surfaces and contours of J | | <inline-graphic href='nfaca4e3ieqn2.gif' type='simple'/> , 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 Γ C <inline-graphic href='nfaca4e3ieqn3.gif' type='simple'/> . The relationship between energetic particle losses and metrics such as QA error ( E q a <inline-graphic href='nfaca4e3ieqn4.gif' type='simple'/> ) and Γ C <inline-graphic href='nfaca4e3ieqn5.gif' type='simple'/> in this set of equilibria were examined via statistical methods and a nearly linear relationship between volume-averaged Γ C <inline-graphic href='nfaca4e3ieqn6.gif' type='simple'/> and prompt particle losses was found.