Studies of anode sheath phenomena in Hall thrusters.

摘要

Several issues associated with plasma-wall interactions in Hall thrusters are considered, with the primary focus being experimental and theoretical studies of the plasma-anode sheath. The main goal is to investigate how the sheath voltage drop depends on the thruster operating conditions. Unlike in classic glow discharges, anode sheath phenomena were not studied in Hall thrusters. This investigation gives an insight into the fundamental physics of Hall discharges and has important practical implications for the device.; A diagnostic apparatus, comprising biased and emissive electrostatic probes, a high-precision positioning system, and low-noise electronic circuitry was developed and used for measurements in the near-anode region of a Hall thruster. A dielectric coating that appears on the anode surface during thruster operation was used as a natural way of decreasing the anode collecting surface area. A quasi-1D model was constructed to describe the behavior of the plasma in the quasineutral acceleration region and non-quasineutral near-anode region at different thruster operating conditions, namely, the discharge voltage and the propellant mass flow rate.; Accurate, non-disturbing near-anode measurements of the plasma density, electron temperature, and plasma potential demonstrated that the anode fall changes from positive to negative upon increase of the anode collecting surface area. The numerical solutions obtained with the electron temperature that grows with the discharge voltage demonstrate an increase of the magnitude of the negative anode fall with the increase of the applied voltage, in agreement with the experimental results. Probe measurements in a Hall thruster with three different magnetic field configurations show that an anode fall at the clean anode can be changed from negative to positive by using a magnetic field that changes polarity somewhere along the channel.; The main physical conclusions are: one, the anode sheath in Hall thrusters can be electron-repelling or electron-attracting depending on the anode collecting surface area and the magnetic field configuration; and, two, the anode sheath formation in Hall thrusters differs essentially from that in the other gas discharge devices, like a glow discharge or a hollow anode, because the Hall thruster utilizes long electron residence times to ionize rather than high neutral pressures.