Plume Structure and Ion Acceleration of a Helicon Plasma Source

Helicon plasma sources are capable of efficiently ionizing propellants and have been considered for application in electric propulsion. The literature suggests that the ion acceleration mechanism is a current-free double layer. Previous work shows that single-stage helicon thrusters can produce thrust in the range of 1-6 mN, but it is unknown whether the thrust contribution is due to direct ion acceleration versus thermal expansion. The ion energy and current density profiles in the plume of a helicon plasma source are measured across a range of operating conditions: 343-600 W RF power at 13.56 MHz, 50-350 G, and 1.5-mg/s Ar at a pressure of 1.6 × 10^-5 torr-Ar. The plasma potential, electron temperature, and ion number density are also measured inside the discharge chamber and in the plume up to 60 cm downstream of the exit plane and 45-cm radially outward from the device axis. Ions are found to have energies in the range of 20-40 V, with total beam currents in the range of 7-47 mA. The plume has an average divergence half angle of 82°, either evenly distributed across all angles or focused at large angles to the centerline. From these measurements, it is found that the estimated thrust due to ion acceleration is far less than what has been directly measured on the same device in previous work.