Development of a high-speed, reciprocating electrostatic probe system for Hall thruster interrogation

Abstract

The use of electrostatic probes to measure local plasma parameters inside the discharge chamber of a Hall thruster presents significant difficulties. The high-temperature, dense plasma, and Hall current in the accelerating channel heat the probe rapidly causing ablation of probe material, which perturbs thruster operation and reduces probe lifetime. Results are presented which show the extent of perturbation to discharge current, cathode potential, and thrust for the case where probe material is ablated. A simple thermal model of probe material heating is developed and ablation times for a typical probe configuration are presented. Using the results of the thermal model, a high-speed axial reciprocating probe ͑HARP͒ system was developed to enable probe survival and reduce thruster perturbations during interrogation of the discharge chamber of a Hall thruster. Results using the HARP system are presented showing a significant reduction in thruster perturbation. The results also indicate that a mechanism other than material ablation is contributing to perturbation of the thruster. Based on emissive probe data, the tungsten conductor appears to provide a low impedance path between magnetic field lines, enhancing electron transport to the anode.

Discharge Chamber Plasma Structure of a 30 cm NSTAR-type Ion Engine

“…The linear encoder provides positioning resolution to 5 µm. 16 A Pacific Scientific SC950 digital, brushless servo drive controls the motor. The entire table is enclosed in a stainless steel shroud with a graphite outer skin.…”

“…As such, careful attention is paid to minimizing stray capacitance in the circuit design including the use of batteries to supply the bias voltage. [16][17][18] A potentiometer attached to the battery output sets the electrode bias voltage.…”

Effects of Segmented Electrode in Hall Current Plasma Thrusters

“…9 In addition to these perturbations, uncertainties of the emissive probe measurements include a voltage drop across the filament produced by the dc heating power supply (~10 V) and a double sheath formed between the hot floating probe and plasma (∆ϕ ≈ T e ).…”

Diagnostic Setup for Characterization of Near-Anode Processes in Hall Thrusters

“…However, introducing movable probes axially into the acceleration region, with T e ~ 20 eV, can cause significant perturbations to the Hall thruster discharge. 8,9,10 This might cause an inaccuracy of up to several volts in the measured plasma potential, which is of the order of the expected potential change over the entire near-anode region.…”