Operation of an Annular Helicon Plasma Source

The plasma parameters of a nominally 1.5 kW annular helicon plasma source are measured and its feasibility as an ionization stage of a two-stage Hall thruster is investigated. All experiments are performed in a 4 by 7 m stainless steel vacuum chamber at pressures below 3.1 x 10 ―5 torr for argon and 5.6 x 10 ―5 torr for xenon. Ion number density, electron temperature, and electron energy distribution function measurements are taken at several axial and radial locations inside the device at each operating condition with a ratio-frequency-compensated Langmuir probe. The annular helicon plasma source is characterized over a range of applied radio frequencies (2―14 MHz), magnetic field strengths (0―400 G), and radio frequency forward-power settings (100―1400 W) for both argon and xenon propellants. The peak ion number density measured in the annular helicon is 2.6 x 10 17 m ―3 for argon and 2.4 x 10 17 m ―3 for xenon at 1000 W of radio frequency power. The annular helicon electron energy distribution function peak and shape vary with the radio frequency, from a minimum of 3.7 eV at 13 MHz to a maximum of 15.0 eV at 11 MHz for argon propellant over the preceding operating conditions.