Reduction of neoclassical bulk-ion transport to avoid helium-ash retention in stellarator reactors

The reduction of neoclassical energy transport in stellarators has traditionally focused on optimizing magnetic fields for small values of `effective helical ripple' --- εeff, the geometric factor associated with electron 1/ν transport --- and relying on the radial electric field, E_r, needed to maintain ambipolarity in the plasma, to simultaneously diminish ion energy losses to a tolerable level. As one must generally expect E_r < 0, such a strategy has a drawback for reactor operation, however, as negative values of E_r tend to hinder the exhaust of helium ash, and this will become intolerable if it results in excessive fuel dilution. Theoretically, one can show that the neoclassical transport of low-Z impurities depends critically on the ratio L₁₁^e/L₁₁^i, where the L₁₁^σ are the neoclassical particle diffusion coefficients of the bulk-plasma electrons (σ=e) and ions (σ=i). Increasing the value of this ratio is shown here to counteract impurity retention, but maintaining good confinement of the bulk species requires this to be achieved by decreasing L₁₁^i rather than increasing L₁₁ᵉ, implying the need for more than just minimization of εeff in reactor design. To assess the prospects of such an endeavor, a predictive 1-D transport code is used here to determine the range of L₁₁^e/L₁₁^i values which arises in simulations of conventionally optimized reactor candidates. This range is found to be considerable, and includes examples with L₁₁^e/L₁₁^i values large enough to provide neoclassical temperature screening of the helium ash and even to flip the sign of the neoclassical convective velocity from inward- to outward-directed. More intriguingly, the strong reduction of L₁₁^i in such cases can also lead to the appearance of E_r > 0 (a so-called `electron root') within the core of plasmas having central densities as large as 2 x 10²⁰$~m$⁻³. Having E_r > 0 in the plasma core produces the ideal situation in which all thermodynamic forces aid in the exhaust of helium ash, although this benefit is tempered by the small values of L₁₁^z which accompany an electron root (where σ=z denotes helium ash). Means are discussed for improving the results presented here, so that avoiding helium-ash retention can be explicitly targeted in future reactor optimizations.