Optical Method for Monitoring Ion Current Changes in a Low-Pressure Plasma Source With Intended Application in Electric Propulsion

We analyzed the operation and characteristics of a radio frequency-driven low-pressure argon gas-fed plasma source, the result being that the extracted ion current strongly correlates with the density of argon metastables. Such an insight provides a practical advantage for the future engineering of the described plasma source. The plasma source exploits a Birdcage resonator and operates on the principle of electron cyclotron resonance (ECR), with a static magnetic field generated by a pair of Helmholtz coils. The general performance of the source was determined by measuring the extracted ion current at different flux densities of the stationary magnetic field near the resonance condition. This ion current was correlated with the plasma parameters obtained through optical emission spectroscopy (OES). Using the branching fraction method (BFM), the lowest argon metastable density (1s5 in Paschen’s notation) was calculated. Both correlated quantities confirm that the source performs best at ECR.