  A potential threat to the performance of magnetically confined fusion plasmas
is the problem of impurity accumulation, which causes the concentration of
highly charged impurity ions to rise uncontrollably in the center of the plasma
and spoil the energy confinement by excessive radiation. It has long been
thought that the collisional transport of impurities in stellarators always
leads to such accumulation (if the electric field points inwards, which is
usually the case), whereas tokamaks, being axisymmetric, can benefit from
"temperature screening", i.e., an outward flux of impurities driven by the
temperature gradient. Here it is shown, using analytical techniques supported
by results from a new numerical code, that such screening can arise in
stellarator plasmas too, and indeed does so in one of the most relevant
operating regimes, where the impurities are highly collisional whilst the bulk
plasma is in any of the low-collisionality regimes.
