Plasma Perturbations in High-Speed Probing of Hall Thruster Discharge Chambers: Quantification and Mitigation

Abstract

An experimental investigation is presented to quantify the effect of high-speed probing on the plasma parameters inside the discharge chamber of a 6-kW Hall thruster. Understanding the nature of these perturbations is of significant interest given the importance of accurate plasma measurements for characterizing thruster operation. An array of diagnostics including a high-speed camera and embedded wall probes is employed to examine in real time the changes in electron temperature and plasma potential induced by inserting a high-speed reciprocating Langmuir probe into the discharge chamber. It is found that the perturbations onset when the scanning probe is downstream of the electron temperature peak, and that along channel centerline, the perturbations are best characterized as a downstream shift of plasma parameters by 15-20% the length of the discharge chamber. A parametric study is performed to investigate techniques to mitigate the observed probe perturbations including varying probe speed, probe location, and operating conditions. It is found that the perturbations largely disappear when the thruster is operated at low power and low discharge voltage. The results of this mitigation study are discussed in the context of recommended methods for generating unperturbed measurements of the discharge chamber plasma.

Numerical simulations of the XR-5 Hall thruster for life assessment at different operating conditions

“…At the same time, numerical and experimental results are in closer agreement at low power conditions (over-estimation occurs by a factor of approximately 1.2 at low power conditions and 1.8 and high power conditions). This latter phenomenon can be a consequence of the plasma being less perturbed by the probe injection at lower power settings, as indicated in [19].…”

“…The shape of this profile has been modified as new experimental measurements have become available until reaching a general qualitative shape that can be applied to any thruster with no major modifications. It is shown in [22] that even though the specifics of the plasma properties inside the acceleration channel cannot be determined by the use of injected probes [19], changes in the anomalous collision profile inside the channel with the aim of bounding experimental uncertainties have little effect on the plasma solution in the plume.…”

“…However, once such measurements are in place (as in the case of the present work), physics-based codes like Hall2De become powerful tools in the thruster life qualification process. Adding to the complexity of this endeavor, it is now known that the injection of plasma probes upstream of the peak magnetic field location in Hall thrusters perturbs the plasma and, in consequence, the measurements [19]. These perturbations appear to become larger as power density increases.…”

“…The JPL in-house numerical code for the simulation of the plasma discharge in Hall thrusters, Hall2De [20][21], includes an implementation of the anomalous transport of electrons by means of an effective anomalous collision frequency that acts in lowering the Hall parameter [18]. Anomalous collision frequency profiles that produce S numerical solutions consistent with experimental measurements in other thrusters, such as the H6 [19,[21][22], have been previously identified and the lessons learned from those investigations on the variation of the anomalous collision frequency in the computational domain have been applied in simulations of the XR-5. In a previous paper [23], candidate anomalous collision frequency profiles were identified for eleven throttling conditions of the XR-5 ranging from 0.3 kW to 4.5 kW with discharge voltages in the range of 150 to 400 V. Due to the multiple choices available in the selection of a possible anomalous collision frequency profile and the limited number of experimental measurements at each operating condition, three guidelines were followed: the computed discharge current must agree with the experimental value, the predicted thrust must be within 15% of the measured thrust, and the electron temperature downstream of its peak along the acceleration channel centerline must be in close agreement with the experimental measurements inferred from injected emissive probes.…”

Triple Langmuir Probes Measurements of LaB6 Hollow Cathodes Plume

“…The moving system and the arm were the same used to measure the plasma parameters inside the SITAEL Hall thruster channel [39], whereas the Langmuir probe was modified according to the expected plasma parameters in the cathode plume. Following the approach described in Andreussi et al [39], the arm rapidly inserts and extracts the probe from the cathode plume, with a maximum residence time in the dense plasma region of less than 0.2 s. It is important to notice that, even though the residence time of the probe is less than 0.2 s, such measurements are intrusive, since the spatial gradients as well as part of the plasma dynamics may be affected by the probes [40]. As a matter of fact, all the fundamental plasma frequencies are higher than few Hz, and the plasma rapidly adapts itself to the presence of the probe.…”