Direct optimization of ion transport in a W7-X-like reactor case*

We directly optimize stellarator neoclassical ion transport while holding 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 ε_eff, the geometric factor that characterizes the amount of radial transport due to particles in the 1/ν regime. At reactor-relevant collisionalities, electrons are typically in the 1/ν regime and ions are typically in the √ν regime. Traditional optimizations thus minimize electron transport and rely on the radial electric field (E_r) 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 our optimizations, we increase the ratio of the thermal transport coefficients L^e₁₁/Lⁱ₁₁, which previous work has shown can create impurity screening and an outward-pointing E_r. Both effects are very beneficial for impurity expulsion. We obtain self-consistent density, temperature, and E_r profiles at reactor-relevant conditions for optimized equilibria. These equilibria are expected to enjoy significantly improved impurity transport properties.