Current-free double-layer formation in a high-density helicon discharge

TLDR

A rapid potential drop at the vessel neck occurs only below about 0.5 mTorr, and rf‑heated electrons supply the power that sustains the double‑layer and accelerate ions into the diffusion chamber. The double‑layer forms self‑consistently within the plasma without any external current, distinguishing this experiment from previous studies. A current‑free double‑layer with eΦ/kTe≈3 and a thickness of less than 50 Debye lengths was observed, together with a 25 % higher upstream electron temperature and a downstream density hole.

Abstract

A strong, current-free, electric double-layer with eΦ/kTe∼3 and a thickness of less than 50 debye lengths has been experimentally observed in an expanding, high-density helicon sustained rf (13.56-MHz) discharge. The rapid potential decrease is associated with the “neck” of the vacuum vessel, where the glass source tube joins the aluminum diffusion chamber, and is only observed when the argon gas pressure is less than about 0.5 mTorr. The upstream electron temperature Te appears 25% greater than the downstream Te, and there is a density hole on the downstream edge. This experiment differs from others in that the potentials are self-consistently generated by the plasma itself, and there is no current flowing through an external circuit. The plasma electrons are heated by the rf fields in the source, provide the power to maintain the double-layer, and hence accelerate ions created in the source out into the diffusion chamber.