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

Abstract

Summary form only given as follows. Measurements are performed on an argon helicon plasma with an axial magnetic field (with and without a magnetic nozzle) of up to 1 kG in the antenna source region and 1.5 kG at the nozzle peak. Discharges are produced for pressures between 0.15 and 5 mtorr with incident 13.56 MHz RF power levels of between 300 and 3000 W. The electron density is determined with microwave interferometer as the fill pressure and the magnetic field strength and geometry are varied. The plasma density and electron temperature are measured using a double Langmuir probe. 488 nm Ar II emission calibrated for the measured electron temperature is used to determine the axial plasma density profile. Collisional-radiative models for Ar II and Ar I are used to determine the electron temperature and neutral density respectively by comparing excited-level population densities obtained with experimental spectra with densities predicted computationally. Axial variations of plasma density, electron temperature, and neutral density are measured and discussed for a wide range of fill pressures and magnetic field strengths for the uniform and nozzle configurations. Effects of neutral depletion are observed, including an increase in the electron temperature and a maximum plasma density limit that is a function of fill pressure and the magnetic field. Observations of substantial localized plasma density reduction as the magnetic field and coupled RF power are increased are observed. Additionally, the axial ion velocity distribution function is measured using tunable diode laser-induced fluorescence (LIF) to determine the effect of neutral depletion on the axial ion flux.