Ion acceleration in plasmas emerging from a helicon-heated magnetic-mirror device

Using laser-induced fluorescence, measurements have been made of metastable argon-ion, Ar +* (3d 4 F 7/2 ), velocity distributions on the major axis of an axisymmetric magnetic-mirror device whose plasma is sustained by helicon wave absorption. Within the mirror, these ions have sub-eV temperature and, at most, a subthermal axial drift. In the region outside the mirror coils, conditions are found where these ions have a field-parallel velocity above the acoustic speed, to an axial energy of ∼30 eV , while the field-parallel ion temperature remains low. The supersonic Ar +* (3d 4 F 7/2 ) are accelerated to one-third of their final energy within a short region in the plasma column, ⩽1 cm , and continue to accelerate over the next 5 cm. Neutral-gas density strongly affects the supersonic Ar +* (3d 4 F 7/2 ) density.