  Low-collisionality stellarator plasmas usually display a large negative
radial electric field that has been expected to cause accumulation of
impurities due to their high charge number. In this paper, two combined effects
that can potentially modify this scenario are discussed. First, it is shown
that, in low collisionality plasmas, the kinetic contribution of the electrons
to the radial electric field can make it negative but small, bringing the
plasma close to impurity temperature screening (i.e., to a situation in which
the ion temperature gradient is the main drive of impurity transport and causes
outward flux); in plasmas of very low collisionality, such as those of the
Large Helical Device displaying impurity hole, screening may actually occur.
Second, the component of the electric field that is tangent to the flux surface
(in other words, the variation of the electrostatic potential on the flux
surface), although smaller than the radial component, has recently been
suggested to be an additional relevant drive for radial impurity transport.
Here, it is explained that, especially when the radial electric field is small,
the tangential magnetic drift has to be kept in order to correctly compute the
tangential electric field, that can be larger than previously expected. This
can have a strong impact on impurity transport, as we illustrate by means of
simulations using the newly-developed code KNOSOS (KiNetic
Orbit-averaging-SOlver for Stellarators).
