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

A strong, current-free, electric double-layer with ePhi/kT(e)similar to3 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 T-e appears 25% greater than the downstream T-e, 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. (C) 2003 American Institute of Physics.