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The control of impurity accumulation is one of the main challenges for futurestellarator fusion reactors. The standard argument to explain this accumulationrelies on the, in principle, large inward pinch in the neoclassical impurityflux caused by the typically negative radial electric field in stellarators.This simplified interpretation was proven to be flawed by Helander et al.[Phys. Rev. Lett. 118, 155002 (2017)], who showed that in a relevant regime(low-collisionality main ions and collisional impurities) the radial electricfield does not drive impurity transport. In that reference, the effect of thecomponent of the electric field that is tangent to the magnetic surface was notincluded. In this Letter, an analytical calculation of the neoclassical radialimpurity flux incorporating such effect is given, showing that it can be verystrong for highly charged impurities and that, once it is taken into account,the dependence of the impurity flux on the radial electric field reappears.Realistic examples are provided in which the inclusion of the tangentialelectric field leads to impurity expulsion.

Stellarator impurity flux driven by electric fields tangent to magnetic surfaces