Direct optimization of neoclassical ion transport in stellarator reactors

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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"><mml:mrow><mml:msub><mml:mi>ϵ</mml:mi><mml:mrow><mml:mi>eff</mml:mi></mml:mrow></mml:msub></mml:mrow></mml:math> , the geometric factor that characterizes the amount of radial transport due to particles in the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"><mml:mrow><mml:mn>1</mml:mn><mml:mrow><mml:mo>/</mml:mo></mml:mrow><mml:mi>ν</mml:mi></mml:mrow></mml:math> regime. Under expected reactor-relevant conditions, core electrons will be in the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"><mml:mrow><mml:mn>1</mml:mn><mml:mrow><mml:mo>/</mml:mo></mml:mrow><mml:mi>ν</mml:mi></mml:mrow></mml:math> regime and core fuel ions will be in the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"><mml:mrow><mml:msqrt><mml:mi>ν</mml:mi></mml:msqrt></mml:mrow></mml:math> 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 this work, we increase the ratio of the thermal transport coefficients <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"><mml:mrow><mml:msubsup><mml:mi>L</mml:mi><mml:mrow><mml:mn>11</mml:mn></mml:mrow><mml:mrow><mml:mi>e</mml:mi></mml:mrow></mml:msubsup><mml:mrow><mml:mo>/</mml:mo></mml:mrow><mml:msubsup><mml:mi>L</mml:mi><mml:mrow><mml:mn>11</mml:mn></mml:mrow><mml:mrow><mml:mi>i</mml:mi></mml:mrow></mml:msubsup></mml:mrow></mml:math> , 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.