  The control of impurity accumulation is one of the main challenges for future
stellarator fusion reactors. The standard argument to explain this accumulation
relies on the, in principle, large inward pinch in the neoclassical impurity
flux 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 electric
field does not drive impurity transport. In that reference, the effect of the
component of the electric field that is tangent to the magnetic surface was not
included. In this Letter, an analytical calculation of the neoclassical radial
impurity flux incorporating such effect is given, showing that it can be very
strong 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 tangential
electric field leads to impurity expulsion.
