Electrostatic probe apparatus for measurements in the near-anode region of Hall thrusters

Near-anode processes in Hall-current plasma thrusters are largely uncharacterized in the experimental literature. In order to perform measurements in the near-anode region, the high potential of the anode relative to ground, small spatial variations of plasma properties, and the complicated thruster geometry are just some of the features that must be taken into consideration. A diagnostic apparatus for measurements in the near-anode region of Hall thrusters, comprising biased and emissive electrostatic probes, a high-precision positioning system, and low-noise electronic circuitry, was developed and tested. Test data for this apparatus indicate that radially inserted probes negligibly perturb the discharge. Accurate near-anode measurements of the plasma density, electron temperature, and plasma potential performed with this diagnostic have allowed the first experimental identification of the electron-repelling anode sheath predicted theoretically in Hall thrusters