Ion velocity distribution function measurements in a helium helicon plasma

Summary form only given. The scarcity of strong absorption lines in accessible tuning ranges along with plasma saturation due to low ion population densities makes laser absorption spectroscopy of helium ions in plasma notoriously difficult. Helicon plasmas, with their characteristically high ion densities are a good candidate for helium ion spectroscopy experiments. Initial measurements of Doppler broadened ion velocity distribution functions (ivdf) involve injecting a tunable infrared diode laser, tuned to 1012.36 nm and chopped roughly at 1 kHz, along the axis of a 1.5 m long helicon plasma and a 4 m long expansion chamber. The single pass absorption as a function of laser wavelength is measured with a bandpass filtered photodiode. We will present measurements of the ivdf in helium helicon plasma as a function of RF power, source magnetic field, and neutral pressure. The calculated ion temperatures obtained from the ivdf indicate that the helium ions are at roughly room temperature, ~0.03 eV, Measurements of the ivdf of argon ions in argon doped helium plasmas will also be presented. As the partial pressure of argon is reduced to less than a few percent, the discharge switches from a dominantly argon plasma to a dominantly helium plasma. Assuming thermodynamic equilibrium, the argon ion temperature can then be used as a measure of the helium ion temperature