Measurements on an argon helicon plasma with variable magnetic nozzle

Abstract

Summary form only given. The possible existence of a plasma double layer arising at the field gradient of a magnetic nozzle is investigated in a high-density (1013 cm-3), 13.56 MHz radiofrequency (RF) argon helicon plasma with high fractional ionization (>90%). A half-turn double helix antenna couples RF power to the low pressure (mtorr) gas in a 10-cm-diameter, 2-m-long cylindrical Pyrex chamber. The gas flows continuously, fed into the chamber upstream from the turbo pump. Several electromagnets produce a static axial magnetic field, and can be repositioned to produce a wide range of field profiles. Magnetic field strengths range from up to 1 kG in the antenna region to up to 1.5 kG at the peak of the downstream magnetic nozzle. Laser-induced fluorescence (LIF) is used to measure the ion energy distribution function to determine drift velocity and temperature along the experimental axis. The tunable diode laser is centered at 668 nm, and the wavelength is monitored by iodine cell absorption, which provides the fine resolution (0.0001 nm) that is required by the LIF diagnostic. Plasma potential measurements are made via a Langmuir probe to determine the potential profile in the plasma in the presence of the magnetic nozzle and to determine if a double layer forms. Power scaling of up to 3 kW of input RF power is also performed to determine the effect on double layer formation. Additionally, RF field measurements are made via B-dot probe and correlated with Ar II optical emission to characterize the helicon wave propagation under a variety of magnetic nozzle configurations