Study of the efficiency of molecular gases in an ECR-based plasma cathode for current extraction *

Electron cyclotron resonance (ECR) sources are capable of high ionization efficiencies, which is why these sources have been investigated in the past for plasma cathode applications where the plasma generated is used to source electrons for a secondary application. Such devices have been featured on gridded ion thruster missions as a neutralizer.  Much of the plasma cathode literature has focused on the use of inert gases. On the other hand, operation on molecular gases affords one with greater flexibility, reduced propellant management complexity, and higher fuel density. In this work we investigate the operation of an ECR plasma cathode on three molecular gases: dry air, N₂ and CO₂.  Because of the myriad of energy loss pathways, discharge efficiency with such gases can be expected to be lower than operation with noble gases.  Here we present discharge losses, measured as the ratio of total input power to extracted current as a function of discharge current and flow rate.   Emission spectra was collected in the orifice to assess molecular speciation.  Plasma conditions just outside the orifice were measured using a Langmuir probe.  Energy partitioning between ECR discharge power, power losses due to inelastic molecular energy sinks, and plume production is also estimated.