Active charge exchange spectroscopy of helium ions for momentum transport studies in the HSX stellarator

Neoclassical particle loss rates and supra-thermal electron losses driven by electron cyclotron resonance heating have been shown to be the primary flow drive mechanisms in HSX [1]. Changes to the electron heating mode as well as the magnetic topology between the optimal quasi-helically symmetric configuration and a broken symmetry configuration allow for variation in flow drive to be studied. An overhaul of the plasma facing wall of the Helically Symmetric eXperiment (HSX) has significantly reduced impurity content of the plasma, causing the 529 nm C⁺⁵ line previously used for Charge Exchange Recombination Spectroscopy (CHERS) measurements to drop below detection levels, motivating an adaptation of the system to use of the 468 nm He⁺¹ emission line. Helium glow discharge cleaning, as well as plasma operation with helium as main fueling gas is used in HSX so that the He⁺¹ emission can be observed in every discharge. While the switch to the He⁺¹ emission line creates challenges due to the higher electron impact stimulated emission, the CHERS measurements additionally benefit in that fully ionized helium is present throughout a larger plasma volume than fully ionized carbon, allowing improved ion temperature and flow measurements towards the edge of the plasma.

[1] Y. Yamamoto et al., Nuclear Fusion 62, 064004 (2022)