Assessment of Pole Erosion in a Magnetically Shielded Hall Thruster

Abstract

Numerical simulations of a 6-kW laboratory Hall thruster called H6 have been performed to quantify the erosion rate at the inner pole. The assessments have been made in two versions of the thruster, namely the unshielded (H6US) and magnetically shielded (H6MS) configurations. The simulations have been performed with the 2-D axisymmetric code Hall2De which employs a new multi-fluid ion algorithm to capture the presence of lowenergy ions in the vicinity of the poles. It is found that the maximum computed erosion rate at the inner pole of the H6MS exceeds the measured rate of back-sputtered deposits by ~4.5 times. This explains only part of the surface roughening that was observed after a 150-h wear test, which covered most of the pole area exposed to the plasma. For the majority of the pole surface the computed erosion rates are found to be below the back-sputter rate and comparable to those in the H6US which exhibited little to no sputtering in previous tests. Possible explanations for the discrepancy are discussed and a mitigation technique is proposed.ince their inception over several decades ago the application of Hall thrusters has been constrained to near-Earth missions largely due to their limited throughput capability. The source of this limitation has been erosion of the acceleration channel walls, which can lead to exposure of critical magnetic circuit components to the high-energy ion beam and, eventually, to engine failure. A new technique has been demonstrated to eliminate channel erosion in Hall thrusters [1-3] based on findings from physics-based numerical simulations with a magnetic-field-aligned-mesh (MFAM) code called Hall2De [4] and results from a Qualification Life Test (QLT) of the BPT-4000 [5]. The BPT-4000 is a commercial Hall thruster designed and built by Aerojet-Rocketdyne that reached a zero-erosion state upon conclusion of the QLT [6]. Termed "magnetic shielding" [5] the technique enforces the equipotentialization of specific magnetic field lines called "grazing lines" which, in turn, protect the channel walls from ion bombardment. The demonstration of the first principles of magnetic shielding took place at the Jet Propulsion Laboratory (JPL) in 2010-2012. During this 2-yr, proof-of-principle effort a 6-kW laboratory Hall thruster called H6 [7], was modified from its original configuration -called hereinafter the unshielded configuration or H6US -based on the guidance of numerical simulations with the Hall2De code. The magnetically shielded configuration, or H6MS, was then wear tested for 150 h at 300 V, 6 kW (20 A) and the erosion rates of the acceleration channel were found to be 2-3 orders of magnitude lower than those in the H6US. These measured rates were in agreement with the numerical simulation predictions. More recently, numerical simulations of the H6MS were performed for the discharge voltage range of 400-700 V at constant power (6 kW) and at 800 V, 9 kW [8]. The simulations were followed by performance and wear testing at the 800-V, 9-kW operating condition [9]. The combinat...

Particle Simulation of High Specific Impulse Operation of Low-Erosion Magnetic Layer Type Hall Thrusters

“…The introduction of the numerical methods are made in section 2, and the section 3 presents the simulation result of overall thrust performance, wall erosion inside the discharge channel, and the wall erosion on the front wall surface. The influence of the size of the computational domain and the possible cause of the predicted front wall erosion, which was not fully captured in previous study, 11) is discussed.…”

“…It is notable that the predicted front wall erosion is the phenomenon observed by the noble magnetic shielding thruster. 6,11) It is to be noted though that the front-wall material of the experimentally reported thruster was not BN as the design of the simulated thruster. However, the influence of the wall material on the plasma was considered to be minor.…”

“…It is notable that this near-front-wall potential structure is not following the magnetic field lines, because otherwise the ions will be repelled from the front wall as shown in previous study. 11) The qualitative reason of this deviation is clear as pointed out by Morozov 7 that when magnetic field lines cross over with wall, the presence of ionattracting pre-sheath, which is necessary for the maintenance of wall sheath, let the equipotential lines deviate from the magnetic field lines. Thus, it was implied that capturing the pre-sheath structure along the magnetic field lines is essential for the prediction of front wall erosion of "Magnetic Shielding" like low-erosion Hall thrusters.…”

Long-Life Technology for Space Flight Hall Thrusters

“…Nevertheless, there are still two primary problems that need to be addressed: (1) the large excitation power consumption and the relatively low thruster efficiency. As a result, an additional component for heat dissipation is required, especially for low-power HTs; (2) Ioannis et al [33][34][35] first discovered that the pole erosion of the magnetic shield of Hall thrusters is a by-product of magnetic shielding.…”

Near-Surface Plasma Characterization of the 12.5-kW NASA TDU1 Hall Thruster

“…Probe #1 was placed on the channel surface coincident with the thruster exit plane in order to investigate plasma properties that may be responsible for magnetic pole erosion in this region for magnetically shielded Hall thrusters. [22][23][24] Probes #6-8 were placed further upstream to ensure plasma properties were consistent with magnetic shielding throughout the channel, as well as to investigate the possibility of high plasma densities upstream of the chamfer that were indicated in previous plasma simulations and thruster testing. Due to uncertainties in the probe collection area, trends in plasma density could not be determined with reasonable certainty, and are therefore not reported here.…”