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This paper provides perspectives on recent progress in the understanding of the physics of devices where the external magnetic field is applied perpendicularly to the discharge current. This configuration generates a strong electric field, which acts to accelerates ions. The many applications of this set up include generation of thrust for spacecraft propulsion and the separation of species in plasma mass separation devices. These ExB plasmas are subject to plasma-wall interaction effects as well as various micro and macro instabilities, and in many devices, we observe the emergence of anomalous transport. This perspective presents the current understanding of the physics of these phenomena, state-of-the-art computational results, identifies critical questions, and suggests directions for future research, Comment: 59 pages, 25 Figures, submitted to Physics of Plasmas

We present results from numerical simulations of the cooling-core cluster A2199 produced by the two-dimensional (2-D) resistive magnetohydrodynamics (MHD) code MACH2. In our simulations we explore the effect of anisotropic thermal conduction on the energy balance of the system. The results from idealized cases in 2-D axisymmetric geometry underscore the importance of the initial plasma density in ICM simulations, especially the near-core values since the radiation cooling rate is proportional to ${n_e}^2$. Heat conduction is found to be non-effective in preventing catastrophic cooling in this cluster. In addition we performed 2-D planar MHD simulations starting from initial conditions deliberately violating both thermal balance and hydrostatic equilibrium in the ICM, to assess contributions of the convective terms in the energy balance of the system against anisotropic thermal conduction. We find that in this case work done by the pressure on the plasma can dominate the early evolution of the internal energy over anisotropic thermal conduction in the presence of subsonic flows, thereby reducing the impact of the magnetic field. Deviations from hydrostatic equilibrium near the cluster core may be associated with transient activity of a central active galactic nucleus and/or remnant dynamical activity in the ICM and warrant further study in three dimensions., Comment: 16 pages, 13 figures, accepted for publication in MNRAS

Recent analytical and numerical investigations have shown that instabilities in the lower hybrid frequency range can grow in the plume region downstream of the front magnet pole pieces of a magnetically shielded Hall thruster. Growth of the Modified Two-Stream (MTSI) and Lower Hybrid Drift (LHDI) instabilities was found with wave propagation in the r-z (or in-) plane and azimuthal (or out-of-plane) directions, respectively. In the analysis of the LHDI electron collisions were found to be insufficient to quench the instability in a region contained by the thruster ion beam and hollow cathode plume. The implication of the findings is that ions may be heated anomalously by these instabilities, in both the out-of-plane and in-plane directions, thereby altering the velocity field and enhancing the energy of those ions that strike nearby thruster surfaces. In this article we summarize the solution of the dispersion relation for each of these instabilities and provide additional results that can be compared directly with data from future plasma diagnostics, including with direct measurements of the wave dispersion.

The performance of an axisymmetric multi-fluid Hall thruster code that incorporates a self-consistent, data-driven closure model for the anomalous electron transport is investigated. Five different operating conditions of the H9 magnetically shielded Hall thruster are simulated with the Jet Propulsion Laboratory’s Hall2De. In order to capture the inherent uncertainty associated with the closure model, O(100) simulations are run for each condition, each of using a coefficient set sampled randomly from a probability distribution. The results of these simulations provide probabilistic predictions of thruster performance quantities including thrust, and discharge current, as well as several component efficiencies and centerline plasma properties. The model is found to yield converged solutions at all conditions, with large 10 kHzrange oscillations and performance trends with voltage and flow rate similar to experiment. The model under-predicts the thrust by 15-25% and over-predicts the discharge current by 20% on average compared to experiments at the same discharge voltage and mass flow rate. This performance discrepancy is due to lower beam utilization, mass utilization, and divergence efficiency than experiment, resulting from high Hall parameters in the acceleration region, which lead to a protracted ion acceleration region. The physical processes underlying this result are discussed in the context of future data-driven modeling efforts.

NASA’s Psyche mission will launch in 2022 and begin a 3.6-year cruise to the metallic asteroid Psyche, where it will examine this unique body. The baseline spacecraft design is a hybrid of JPL’s deep-space heritage subsystems with commercial partner Maxar’s electric propulsion, power, and structure subsystems. All primary propulsion will be done with SPT-140 thrusters, which will be the first use of Hall thrusters for a NASA mission. The electric propulsion subsystem and its implementation for the Psyche mission are described here. Major testing activities have included the successful completion of subsystem integrated testing with the design modifications required for Psyche, and a series of low-power thrust repeatability tests that were performed in support of navigation analyses. Thruster performance models have been further validated with new SPT-140 flight data, and new analyses of thruster swirl torque have been performed that result in much higher values than previously estimated. Analysis of recent Maxar flight data has also provided a new understanding of in-flight propellant usage uncertainties. Subsystem integration and test activities are now underway and the status and plans are discussed.

The two-dimensional, fluid-based Hall thruster code Hall2De is adapted to simulate a low temperature propulsive magnetic nozzle. Electron cyclotron resonance (ECR) thrusters developed at the Office National d’Etudes et de Recherches Aérospatiales (ONERA) in France and the University of Michigan are investigated. For each device, the simulated domain is defined from the nozzle throat to a location 10 thruster radii downstream. A numerical mesh is defined based on the measured magnetic field, and the governing fluid equations for singlycharge ions, electrons, and neutrals are solved. The boundary conditions for the throat of the magnetic nozzle are based on measured values of electron temperature and ion velocity. Multiple boundary conditions for the electron temperature in the downstream regions of the domain are considered. The simulated ion velocity and potential agrees well with experiment, while the simulated divergence efficiencies and thrusts are 25% lower and 45-55% higher than experiment. When a Neumann boundary condition for electron temperature is applied to the far-plume boundaries, the nozzle was seen to be purely isothermal. This suggests that the thermal conductivity along field lines in real devices is much lower than classical simulations predict, and anomalous resistivity may be needed to reproduce the temperature distribution observed in experiment. The larger beam divergence observed in our simulation may also be related to the lack of anomalous resistivity.

The mechanism that led to the erosion of the inner front pole cover in wear tests of the Hall-effect Rocket with Magnetic Shielding (HERMeS) at 300 V and 12.5 kW has been elusive. Several classical mechanisms have been considered yet none has explained conclusively the source of the discrepancy between wear measurements and simulations. In this article we consider a new mechanism. The electrostatic dispersion relation for an unbounded, homogeneous plasma in a fixed magnetic field is solved numerically in the near-plume region of this thruster. The plasma conditions have been obtained from Hall2De (r-z) multifluid-PIC simulations. We find that the modified two-stream instability can be excited in most regions of the domain, with the fastest growth approaching the fluid-limit value of ωLH/2 near the magnetic poles where ωpe/ωce~1. In this region the relative drift between magnetized electrons and beam ions perpendicular to the magnetic field can exceed 3uTi, while that between counter-streaming beam and cathode ions can exceed 4uTi. The latter suggests that the ion-ion cross-field lower hybrid instability may also be active here. Unlike the classical ion-acoustic instability, lower hybrid instabilities can heat ions to temperatures that are comparable to the electron temperature. Such anomalous heating would enhance the average energy of these ions and, in turn, the erosion of the nearby surfaces they sputter. An idealized model that scales the anomalous ion heating with ωLH has been incorporated in Hall2De and the simulations now yield significantly improved agreement with the measured wear rates and IVDFs, as reported in our companion paper.

Wear measurements of the inner pole cover in HERMeS have shown as much as ten times higher erosion rates than past numerical simulations predicted. Also, measurements of ion velocity distribution functions (IVDF) suggest that ion heating occurs near the chamfered regions of the acceleration channel and around the inner pole surface. In our companion paper we present theoretical evidence that the plasma conditions in this region can excite instabilities in the lower hybrid frequency. Here, we provide further evidence of their presence through comparisons between numerical simulations and measurements. After including in the 2-D axisymmetric hydrodynamic-PIC code Hall2De an idealized model of the ion temperature based on a scaling with the lower hybrid frequency, we find the agreement between the computed and measured IVDF is significantly improved. In addition, the computed erosion rates at the inner pole surface are now in close agreement with wear test measurements.

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The Hall Effect Rocket with Magnetic Shielding (HERMeS) is a 12.5 kW Hall thrusterelectric propulsion string that has been in development by NASA Glenn Research Center(GRC) and NASA JPL since 2012. Due to the magnetically shielded design, service life-limiting erosion of the boron nitride discharge has been virtually eliminated. The innerfront pole cover (IFPC) has now been identied as the component dening erosion-basedservice life. Optical emission spectroscopy (OES) is used as an in-situ diagnostic to measurerelative erosion trends during operation of the HERMeS thruster during a series of shortduration wear tests. Erosion trends obtained from the OES data will be compared totraditional erosion data measured with a non-contact prolometer.

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The Psyche mission will employ Stationary Plasma Thrusters (SPT-140) for primary propulsion in space and will be the first to use Hall thrusters beyond lunar orbit. One of the longest standing risks in the implementation of this EP technology on missions like Psyche is our limited understanding of the effects of ground test facilities on the performance and wear of the thruster.

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The Hall Effect Rocket with Magnetic Shielding (HERMeS) is a 12.5 kW Hall thruster electric propulsion string that has been in development by NASA Glenn Research Center(GRC) and NASA JPL since 2012. Due to the magnetically shielded design, service life-limiting erosion of the boron nitride discharge has been virtually eliminated. The inner front pole cover (IFPC) has now been identified as the component defining erosion-based service life. Optical emission spectroscopy (OES) is used as an in-situ diagnostic to measure relative erosion trends during operation of the HERMeS thruster during a series of short duration wear tests. Erosion trends obtained from the OES data will be compared to traditional erosion data measured with a non-contact profilometer.

Numerical simulations have been conducted to assess the erosion rates at the pole covers of the magnetically shielded miniature (MaSMi) Hall thruster. MaSMi is part of the Ascendant Sub-kW Transcelestial Electric Propulsion System (ASTRAEUS), whose design requirements demand a xenon throughput of 100 kg, thus enabling deep space missions using SmallSats. The results of our simulations show that the proposed thickness of the graphite pole covers will not be completely eroded at end of life. We find that the erosion rates are larger near the corners of the covers, which are directly exposed to the plasma in the acceleration channel.

NASA's Hall Effect Rocket with Magnetic Shielding (HERMeS) 12.5kW Technology Demonstration Unit-1 (TDU-1) has been the subject of extensive technology maturation in preparation for flight system development. The TDU-1 thruster implements a magnetically shielded field topology and has demonstrated the elimination of the discharge channel erosion as a life limiting mechanism. Extensive wear testing the TDU Hall thrusters has identified the thruster front pole covers as the next life limiting component. This effort aims to explore and investigate alternate magnetic field topologies to assess whether reductions in the front pole cover erosion can be attained while still maintaining very low erosion rates on the discharge channel walls. Four candidate magnetic field topologies that reduce the effectiveness of the shielding along the discharge channel walls with the intent to also reduce the erosion rates along the front pole covers were designed. Three of the four candidate magnetic field topologies (B1, B2, and B4) have been manufactured and were subjected to an extensive test campaign that includes laser induced fluorescence (LIF), performance, stability, wear, plume, and thermal characterization. In Phase I, LIF measurements along the discharge chamber centerline found that upstream retraction of the thruster's peak magnetic field does result in an upstream shift of the acceleration zone but the magnitude of the shift does not correspond one-to-one to the shift in the location of the peak radial magnetic field magnitude. Phase II test segment results found at a normalized thruster magnetic field setting of 1, the thruster performance was very similar for all configurations. Discharge current waveforms ripple data indicated that configurations Bo, B1, and B2 has similar oscillatory profiles with the B2 configuration transitioning to higher oscillatory mode at 400V instead of the 450V observed for configurations B0 and B1. Configuration B4 ripple data indicates that the thruster was operating in a very oscillatory mode above 350V. Inner front pole cover erosion rates for configuration B1 were approximately 65% relative to B0 and the erosion rates for configuration B2 were 40% relative to B0. Analysis of the performance, stability, and wear test results of configurations B0, B1, B2, and B4 indicates that configuration B2 may present an alternative option for B0 because it mostly maintained the same performance and stability as configuration B0 but with 40% of the inner front pole cover erosion rate.

The Hall Effect Rocket with Magnetic Shielding (HERMeS) is a 12.5 kW Hall thruster co-developed by NASA Glenn Research Center and the Jet Propulsion Laboratory. HERMeS incorporates magnetic shielding to eliminate discharge channel erosion in order to reach its design lifetime of 50 kh at specific impulses up to 3000 s. The capabilities of the HERMeS thruster technology transferred to Aerojet Rocketdyne under the Advanced Electric Propulsion System (AEPS) program are described. HERMeS hardware testing is now focused at reducing risk and supporting the qualification of the AEPS thruster. These includes a series of progressively longer wear tests, plasma characterization and modeling supporting life qualification, magnetic field optimization, and environmental testing. Initial results from AEPS thruster testing show operation consistent with the HERMeS thrusters and steady-state operation has been achieved at 600 V, 12.5 kW.

Four SPT-140 Hall thrusters are the main components of the electric propulsion system for the Psyche mission. We employ the 2-D (r-z) code Hall2De to conduct numerical simulations of the SPT-140 with the goal of estimating erosion of the acceleration channel rings for the Psyche mission profile. The simulations are informed by direct measurements of the plasma conditions in the acceleration channel that were obtained using laser-induced fluorescence. We validate our simulation results with additional plasma measurements, wear test erosion rates, and performance data as a function of background pressure. We then make use of a series of numerical simulations to compute the progressive erosion of the channel walls, using as input parameters a profile for the Psyche mission that includes operation at 4.5, 2.5 and 1.0 kW with duration of all segments increased by a 50% margin. We find that the channel walls are not completely eroded at the end of the mission (plus 50% margin). We also observe a progressive decrease in thrust as the channel walls erode that is consistent with the wear test and flight data, with a rate of ~4.5 mN/kh for the first 2,500 h and constant thrust after that. The reasons for these thrust trends are explained.

The Mars 2020 Rover will have the capability to collect and cache samples for potential Mars sample return. Specifically, the sample caching system (SCS) is designed for coring Mars samples and acquiring regolith samples as well as handling, sealing and caching on Mars. As the potential first Martian samples that could be returned to Earth, assuring low levels of terrestrial contamination is of the utmost concern. In developing the SCS, the project prioritizes limiting sample contamination in organic, inorganic and biological areas. The focus of this paper is on the strategies being implemented to clean the assemble the sampling hardware to meet and maintain stringent contamination requirements.

The Mars 2020 Rover mission will have the capability to collect and cache samples for potential Mars sample return. Specifically, the sample caching system (SCS) is designed for coring Mars samples and acquiring regolith samples as well as handling, sealing and caching on Mars. As the potential first Martian samples that could be returned to Earth, assuring low levels of terrestrial contamination is of the utmost concern. In developing the SCS, the project prioritizes limiting sample contamination in organic, inorganic and biological areas. The focus of this paper is on the strategies being implemented to limit terrestrial organic and inorganic contamination in the samples.

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