Analytical Discharge Performance Model for rf and Kaufman Ion Thrusters

A comprehensive 0-D model of the plasma discharge in magnetic ring-cusp ion thrusters was previously developed. The model utilizes conservation of particles into and out of the ion thruster and conservation of energy into the discharge and out of the plasma, and provided the first closed solution for predicting ion thruster discharge chamber performance. The 0-D discharge model was benchmarked against NSTAR and NEXIS ion thrusters, and successfully predicts the discharge loss as a function of mass utilization efficiency for a variety of other ion thrusters including NEXT and XIPS. The analytical model has been modified to handle other discharge chamber configurations and the different magnetic confinement physics found in rf ion thrusters and Kaufman thrusters. In the case of rf ion thrusters, there is no applied magnetic field to provide confinement of the electrons and ions, but the AC magnetic field induced by the inductively coupled rf coil affects the particle confinement and transport, and is important in determining the discharge loss and thruster performance. Kaufman thrusters utilize an applied, relatively strong, divergent magnetic field shielding the anode, and rely on cross-field diffusion of the electrons to the anode to establish an efficient discharge. The electron current to the anode/wall in this case is modeled using a diffusion equation with a Bohm diffusion coefficient. The modifications to the model required to address the physics of rf and Kaufman thrusters will be discussed and results showing the predicted ion thruster performance and stability presented.