Hollow cathodes are used in plasma contactor and electric propulsion devices to provide electrons for sustaining plasma discharges. Based on previous cathode life tests where erosion was observed on hollow cathode assembly components, it is desirable to understand the plasma flow field downstream of hollow cathodes. Plasma flow field measurements are presented herein for hollow cathode generated plasmas using both local and remotely located plasma diagnostics. Two cathode discharges are presented: 1) an open, no magnetic field configuration and 2) a setup simulating an ion thruster discharge chamber. In the open cathode configuration, large amplitude plasma potential oscillations, ranging from 20 V to 85 V within a 34 V discharge, were observed using a floating emissive probe. These oscillations were observed over a dc potential profile that contained a clear potential hill structure. A remotely located electrostatic analyzer (ESA) was used to measure the energy of ions produced within the plasma, and energies were detected that met and in some cases exceeded the peak plasma potentials detected by the emissive probe. In the prototype NSTAR-like discharge chamber configuration, plasma potentials from the emissive probe agreed with ion energies recorded by the remotely located ESA. A correlation model was used to compare the local and remote measurements in both cathode discharges. Nomenclature
Simulations of the erosion processes for two proposed sets of ion thruster grids for the NEXT project are presented. Structural failure and electron backstreaming due to accel grid erosion are discussed as two possible failure mechanisms of these grid sets. The TAG grid set is shown to outperform the NSTAR grid set both in terms of margin against electron backstreaming and accel grid mass loss at the primary operating condition studied. An investigation into the possibility of reducing the accel grid voltage magnitude for the TAG grid set showed improved propellant throughput capability. Results of erosion simulations predicting propellant throughput capability for the TAG grid set are presented for a range of NEXT operating conditions.
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The ffx code was used to investigate the lifetime and propellant throughput capability of the High Power Electric Propulsion (HiPEP) thruster ion optics as part of NASA's Project Prometheus. Erosion predictions are presented as a function of beamlet current, accel grid voltage, and propellant utilization efficiency. These predictions were then compared to the lifetime goals of the thruster, and for nominal operating conditions, the ffx code indicates that the HiPEP thruster will have propellant throughput capability of 100 kg/kW and lifetimes in excess of 135 khrs. In addition to lifetime assessment, a detailed study was completed where grid design parameters were varied in a systematic manner to determine their effects on beamlet current limitations and electron backstreaming margins. The parameters that were varied included the net and total accelerating voltage, grid spacing, aperture center-to-center distance, accel grid thickness, screen grid thickness, discharge voltage, and accel grid aperture diameter.
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