  Achieving impurity and helium ash control is a crucial issue in the path
towards fusion-grade magnetic confinement devices, and this is particularly the
case of helical reactors, whose low-collisionality ion-root operation scenarios
usually display a negative radial electric field which is expected to cause
inwards impurity pinch. In these work we discuss, based on experimental
measurements and standard predictions of neoclassical theory, how plasmas of
very low ion collisionality, similar to those observed in the impurity hole of
the Large Helical Device, can be an exception to this general rule, and how a
negative radial electric field can coexist with an outward impurity flux. This
interpretation is supported by comparison with documented discharges available
in the International Stellarator-Heliotron Profile Database, and it can be
extrapolated to show that achievement of high ion temperature in the core of
helical devices is not fundamentally incompatible with low core impurity
content.
