Direct optimization of neoclassical ion transport in stellarator reactors

TL;DR

This paper presents a novel optimization approach for stellarator reactors that enhances impurity expulsion by controlling ion and electron neoclassical transport, leading to improved impurity management and ion confinement.

Contribution

It introduces a direct optimization of ion transport while maintaining moderate electron transport, resulting in favorable impurity expulsion and improved confinement in stellarator reactors.

Findings

Optimized equilibrium shows improved impurity transport properties.

Increasing the ratio of electron to ion transport coefficients creates outward electric fields.

Self-consistent profiles demonstrate reactor-relevant conditions with enhanced impurity expulsion.

Abstract

We directly optimize stellarator neoclassical ion transport while holding neoclassical electron transport at a moderate level, creating a scenario favorable for impurity expulsion and retaining good ion confinement. Traditional neoclassical stellarator optimization has focused on minimizing $\epsilon_\mathrm{eff}$, the geometric factor that characterizes the amount of radial transport due to particles in the $1/\nu$ regime. Under expected reactor-relevant conditions, core electrons will be in the $1/\nu$ regime and core fuel ions will be in the $\sqrt{\nu}$ regime. Traditional optimizations thus minimize electron transport and rely on the radial electric field $\left(E_r\right)$ that develops to confine the ions. This often results in an inward-pointing $E_r$ that drives high-$Z$ impurities into the core, which may be troublesome in future reactors. In this work, we increase the ratio of the thermal transport coefficients $L_{1 1}^{e}/L_{1 1}^{i}$, which previous research has shown can create an outward-pointing $E_r$. This effect is very beneficial for impurity expulsion. We obtain self-consistent density, temperature, and $E_r$ profiles at reactor-relevant conditions for an optimized equilibrium. This equilibrium is expected to enjoy significantly improved impurity transport properties.