Neutral depletion and plasma flows in a pulsed high power helicon plasma

Abstract

Neutral depletion effects are studied in a flowing, low pressure argon helicon discharge in a 10 cm diameter, 1.5 m long helicon source operating at 13.56 MHz with static magnetic fields up to 1 kG in both flat and nozzle configurations. Argon gas flow rates of 1 to 20 sccm correspond to the collisionless helicon wave regime at pressures of 10−5 Torr to the collisional regime at a few mTorr as one moves from the downstream turbo-pump region (z = 45 cm from the downstream antenna edge) to the upstream gas feed region (z = −50 cm) in the flowing system. RF power up to 10 kW is fed to an 18 cm long, half-turn, double helix antenna with 5 ms pulse period. By programmed tuning of the RF frequency (between 12 MHz and 15 MHz), the reflected power remains low (≪3%) as the plasma load presented to the helicon wave antenna changes during the during the RF pulse. Microwave interferometry (105 GHz), collisional radiative spectroscopic codes and diamagnetic loops are used to measure electron density and temperature [1]. An initial transient high density peak (≫1014/cc) is observed followed by the development of a neutral-depleted steady state leading to plasma acceleration due to the reduced neutral drag. Axial variations in the plasma density (≫1012/cc), electron temperatures (2–30 eV) and neutral depletion are observed along with the substantial plasma acceleration. Observations of neutral depletion, plasma acceleration and the mechanisms responsible will be presented.