Plasma Discharge Variance and Beamlet Trajectories of Alternative Propellants in ECR Gridded Ion Thruster

Evaluation of alternatives to traditional xenon propellant has become an increasingly important area of study for electric space propulsion. Myriad thruster configurations and ionization schemes have been considered with electron cyclotron resonance offering great potential due to its relative insensitivity to reactive atomic species such as oxygen. This study borrows from the design methodology of a 10-cm-class waveguide ECR gridded ion thruster as a test bed for alternative propellants. The thruster design process, simulation of magnetic and electric fields inside the waveguide and plasma discharge chamber are presented. Experimental evaluation of the downstream plasma properties are reported for argon, nitrogen, and air propellants at mass flowrates of 75, 100, 150, and 200 µg/s and absorbed microwave power ranging from approximately 5 to 20 W. The argon propellant saw a range of electron temperatures from 1.4 to 2.3 eV with corresponding number densities of 8.1x1013 to 2.8x1014 m-3. Similar electron temperature ranges for nitrogen and air observed were 2.1 to 4.1 eV and 1.8 to 3.4 eV, respectively. Number density ranges of 1.04x1013 to 2.4x1014 m-3 for nitrogen, and 6.0x1013 to 1.6x1014 m-3 for air are reported. Spatial variation of the plasma discharge is considered on the basis of propellant species, absorbed power, mass flowrate, and angle relative to the monopole antenna. Emitted light intensity distributions are compared with the magnetic field geometry inside the discharge chamber to elucidate the observed asymmetry. Additionally, ion trajectory modeling is undertaken using SIMION with development of the code presented with qualitative comparison to literature. A grid geometry case study is performed and results for xenon, argon, N2, and O2 are presented.