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

Abstract

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.

Cited by in corpus (7)

• Impurity transport and bulk ion flow in a mixed collisionality stellarator plasma
• Robust stellarator optimization via flat mirror magnetic fields
• Study on impurity hole plasmas by global neoclassical simulation
• Impurity temperature screening in stellarators close to quasisymmetry
• Electron root optimisation for stellarator reactor designs
• Direct optimization of neoclassical ion transport in stellarator reactors
• Effects of collisions on impurity transport driven by electrostatic modes