The reduction of neoclassical energy transport in stellarators has traditionally focussed on optimizing magnetic fields for small values of 'effective helical ripple' — , the geometric factor associated with electron transport—and relying on the radial electric field, , needed to maintain ambipolarity in the plasma, to simultaneously diminish ion energy losses to a tolerable level. As one must generally expect , such a strategy has a drawback for reactor operation, however, as negative values of 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 , where the are the neoclassical particle diffusion coefficients of the bulk-plasma electrons () and ions (). 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 rather than increasing , implying the need for more than just minimization of 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 values which arises in simulations of conventionally optimized reactor candidates. This range is found to be considerable, and includes examples with 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 in such cases can also lead to the appearance of (a so-called 'electron root') within the core of plasmas having central densities as large as m−3. Having 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 which accompany an electron root (where 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.
This paper explores ways to improve the exhaust of helium ash in stellarator fusion reactors by manipulating the neoclassical transport of bulk ions and impurities. It shows that increasing the ratio of electron to ion diffusion coefficients can counteract impurity retention, and that this can be achieved by decreasing the ion diffusion coefficient rather than just minimizing the electron diffusion.