Observations of neutral depletion and plasma acceleration in a flowing high-power argon helicon plasma

Neutral depletion effects are observed in a steady-state flowing argon helicon plasma with a magnetic nozzle for high rf input powers (up to 3kW). Noninvasive diagnostics including 105GHz microwave interferometry and optical spectroscopy with collisional-radiative modeling are used to measure the electron density (ne), electron temperature (Te), and neutral density (nn). A region of weak neutral depletion is observed upstream of the antenna where increasing rf power leads to increased electron density (up to ne=1.6×1013cm−3) while Te remains essentially constant and low (1.7–2.0eV). The downstream region exhibits profound neutral depletion (maximum 92% line-averaged ionization), where Te rises linearly with increasing rf power (up to 4.9eV) and ne remains constrained (below 6.5×1012cm−3). Flux considerations indicate accelerated plasma flow (Mach 0.24) through the antenna region due to an axial pressure gradient with reduced collisional drag from neutral depletion.