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 3 kW . Noninvasive diagnostics including 105 GHz 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 1013 cm−3 while Te remains essentially constant and low 1.7–2.0 eV . The downstream region exhibits profound neutral depletion maximum 92% line-averaged ionization , where Te rises linearly with increasing rf power up to 4.9 eV and ne remains constrained below 6.5 1012 cm−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. © 2008 American Institute of Physics. DOI: 10.1063/1.2950301