Plasma Collection Width Measurements in a 10-cm Ring Cusp Discharge Chamber

Effective loss area is a key parameter in determining ion thruster discharge performance and stability. A 20 cm partial conic ion thruster discharge chamber was designed and constructed to investigate plasma collection at the anode in small thrusters. Discharge plasma conditions and discharge performance was characterized. Effective loss area was assessed by measuring plasma current density profiles across magnetic cusps as well as measuring the widths of the dark “burn lines” that formed along the centers of the magnets. Current density profiles were obtained at different cusp probe bias voltages to contrast the relative current contributions from plasma electrons, primary electrons, and ions. Loss widths appear to scale with the hybrid radius both at the anode and at various heights above the anode. Additionally, it was found that current entering the cusp region was significantly attenuated as it is traveled toward the anode surface. This finding is consistent with the collection loss cone associated with the magnetic cusp. Also observed were significant asymmetries in current density profiles acquired across the magnetic cusps when the discharge was operated with zero gas flow. At these virtually plasma-free conditions, current profiles over cusps suggest primary electron current density is spatially extended on the side of the cusp facing the filament cathode.