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Achieving impurity and helium ash control is a crucial issue in the pathtowards fusion-grade magnetic confinement devices, and this is particularly thecase of helical reactors, whose low-collisionality ion-root operation scenariosusually display a negative radial electric field which is expected to causeinwards impurity pinch. In these work we discuss, based on experimentalmeasurements and standard predictions of neoclassical theory, how plasmas ofvery low ion collisionality, similar to those observed in the impurity hole ofthe Large Helical Device, can be an exception to this general rule, and how anegative radial electric field can coexist with an outward impurity flux. Thisinterpretation is supported by comparison with documented discharges availablein the International Stellarator-Heliotron Profile Database, and it can beextrapolated to show that achievement of high ion temperature in the core ofhelical devices is not fundamentally incompatible with low core impuritycontent.

Moderation of neoclassical impurity accumulation in high temperature plasmas of helical devices