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 ( n e ) , electron temperature ( T e ) , and neutral density ( n n ) . A region of weak neutral depletion is observed upstream of the antenna where increasing rf power leads to increased electron density (up to n e = 1.6 × 10 13 cm − 3 ) while T e remains essentially constant and low ( 1.7 – 2.0 eV ) . The downstream region exhibits profound neutral depletion (maximum 92% line-averaged ionization), where T e rises linearly with increasing rf power (up to 4.9 eV ) and n e remains constrained (below 6.5 × 10 12 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.