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 (D-T) 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 quasi-axisymmetry while minimizing the analytical quantity ΓC , 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 ΓC . The relationship between energetic particle losses and metrics such as QA error (Eqa) and ΓC in this set of equilibria were examined via statistical methods and a nearly linear relationship between volume-averaged ΓC and prompt particle losses was found. Data for all figures in "Energetic particle optimization of quasi-axisymmetric stellarator equilibria" by LeViness et al., accepted by Nuclear Fusion 2022.

仿星器优化的核心目标之一，是实现对高能粒子的优异约束——在聚变反应堆场景中，此类高能粒子特指氘-氚（D-T）聚变产生的α粒子（alpha particles）。本研究采用两步流程，对某固定边界仿星器平衡态开展高能粒子约束性能的再优化：第一步，在靠近中半径的单磁面上最小化准轴对称（quasi-axisymmetry，QA）偏差；第二步，在保留优化后准轴对称特性的前提下，最小化解析量Γ_C——该量表征磁面与第二绝热不变量J||的等值线之间的夹角。上述优化流程经多次执行后，得到了一组平衡态。通过对缩放后的平衡态开展α粒子蒙特卡洛（Monte Carlo）模拟评估，该组平衡态的高能粒子损失显著降低。本研究首次通过优化Γ_C，成功降低了准轴对称仿星器中的高能粒子损失。本研究通过统计方法，分析了该组平衡态中高能粒子损失与准轴对称误差（E_qa）、Γ_C等指标的关联关系，发现体积平均Γ_C与瞬态粒子损失之间近乎呈线性相关。本数据集涵盖LeViness等人于2022年被《核聚变》（Nuclear Fusion）录用的论文《准轴对称仿星器平衡态的高能粒子优化》中所有配图的对应数据。