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Numerical simulations of a magnetically shielded Hall effect thruster with a centrally mounted cathode are performed with an axisymmetric hybrid particle-in-cell/fluid code and are partially validated with experimental data. A full description of the plasma discharge inside the thruster chamber and in the near plume is presented and discussed, with the aim of highlighting those features most dependent on the magnetic configuration and the central cathode. Compared to traditional magnetic configurations, the acceleration region is mainly outside the thruster, whereas high plasma densities and low temperatures are found inside the thruster. Thus, magnetic shielding does not decrease plasma currents to the walls, but reduces significantly the energy fluxes, yielding low heat loads and practically no wall erosion. The injection of neutrals at the central cathode generates a secondary plasma plume that merges with the main one and facilitates much the drift of electrons toward the chamber. Once inside, the magnetic topology is efficient in channeling electron current away from lateral walls. Current and power balances are analyzed to assess performances in detail. [ABSTRACT FROM AUTHOR]

A key issue in the development of theory and models for plasma propulsion devices is to describe the instabilities and fluctuations of the devices. It has been widely recognized that many Hall effect thrusters (HETs) exhibit oscillations at frequencies in the range of ∼ 20 kHz. These ionization-related oscillations are generally referred to as Breathing Mode oscillations and have been the subject of considerable research. Here, for the first time, we report direct temporally resolved measurements of the ground state neutral density variation during the period of the oscillation. We used the laser-based Two-Photon Absorption Laser Induced Fluorescence (TALIF) technique to measure neutrals within the plume of a 1.5 kW HET operating on krypton (Kr). Our TALIF scheme employs a frequency-doubled, pulsed dye laser operating at ∼ 212 nm to probe ground state Kr atoms. A novel phase-binning approach is used to recover the time-dependent signal by assigning the timing of each collected TALIF signal (laser shot) relative to the phase of the discharge current. We find that the neutral density fluctuates quite strongly over the period of the oscillation, and that this fluctuation leads the current fluctuation as expected. [ABSTRACT FROM AUTHOR]

The work presents the results of an experimental study of the characteristics of the RF capacitive discharge placed in a magnetic field with a predominant radial component. The experiments are performed in the plasma source, which has the geometry of a Hall thruster. The integrated characteristics of the discharge, as well as the axial distribution of the probe ion current in the channel of the plasma source, are measured using three diagrams of the organization of the external discharge circuit: with the open for DC discharge circuit, as well as in the combination of a capacitive discharge with a direct current discharge. The main measurements are made in the argon in the flowrate range of 0.75–1.8 mg/s and the power of the generator 80–300 W at a frequencies of 2, 4, and 13.56 MHz, the induction of the magnetic field lying in the range of 100–300 G. Evaluation of the parameters of the plasma engine based on capacitive RF discharge when working in air, argon and krypton are made. [ABSTRACT FROM AUTHOR]

This paper presents a recently developed full kinetic particle simulation code package, which is a two-dimensional highly integrated and universal framework for low-temperature plasma simulation on both Cartesian and axisymmetric coordinate systems. This code package is named CFIRM, since it is designed based on the continuous Galerkin immersed-finite-element (IFE) particle-in-cell (PIC) model with the polynomial-preserving-recovery (PPR) technique and the Monte-Carlo-collision (MCC) method. Both the traditional and implicit PIC methods were implemented in the package. Incorporating the advantages of all these methods together, the CFIRM code can adopt explicit or implicit PIC schemes to track the motion trajectory of charged particles and deal with the collisions between plasma and neutral gas. Additionally, it can conveniently handle complex interface problems on structured grids. The CFRIM code has excellent versatility in low-temperature plasma simulation and can easily extend to various particle processing modules, such as the variable weights and adaptive particle management algorithms which were incorporated into this code to reduce the memory utilization rate. The implementation for the main algorithms and the overall simulation framework of the CFIRM code package are rigorously described in details. Several simulations of the benchmark cases are carried out to validate the reliability and accuracy of the CFIRM code. Moreover, two typical low-temperature plasma engineering problems are simulated, including a hall thruster and a capacitively coupled plasma reactor, which demonstrates the applicability of this code package. [ABSTRACT FROM AUTHOR]

This work concerns the experimental study of the plasma emitted by a Hall effect thruster in the context of chaotic behavior. Its aim was to check whether low-dimensional deterministic chaos could explain the fluctuations observed in the current waveforms: discharge current and ion current measured by a planar probe with a guarding ring. The discharge voltage was used as a control parameter, and the dependence of the thruster efficiency on this parameter was studied. Several standard chaos diagnosis tools, such as the presence of attractors with non-integral dimensions, the specific shape of Poincaré cross sections, and the positive values of the largest Lyapunov exponent, provided a strong evidence for deterministic chaos in the tested current waveforms. This chaotic behavior occurred when the thruster was operating in the global mode of oscillations dominated by so-called breathing oscillations, adversely affecting the operation of the device. [ABSTRACT FROM AUTHOR]

The object of this study is the ST-25 Hall thruster with limited discharge power, no more than 200 W, designed by Flight Control LLC (Ukraine). The problem that was solved in the current paper was to determine the effect of residual gas pressure in vacuum chambers on the operating parameters of the Hall thruster. To solve this task, the operating parameters of the ST-25 thruster were determined, which was tested in three vacuum chambers with different sizes of residual pressure. As a result of the laboratory study into the operating parameters of the ST-25 thruster, the volt-ampere characteristics of the engine discharge at fixed values of the working gas (xenon) flow rate were obtained. The dependences of the engine thrust on the mass flow rate of the working gas at fixed values of the discharge voltage were derived. Based on the experimental data, the dependences of the specific impulse of the engine anode unit on the discharge voltage, as well as the dependence of efficiency of the engine anode unit on the discharge voltage were calculated. The studies showed that when the residual pressure in the vacuum chamber is reduced by 2–3 times, the operating parameters of the engine increase by 15–20 %. Such a reduction in residual pressure increases thrust by 25–40 %. Special feature of the results is the determination of threshold values of residual gas pressure in vacuum chambers during experimental studies, in which the operating parameters of the Hall thruster are similar to its operating parameters under space conditions. This work’s findings could be used in practice when conducting experimental studies of electric rocket engines, when it is necessary to estimate the operating parameters of Hall thrusters that will be obtained under actual space conditions. [ABSTRACT FROM AUTHOR]

Featured Application: The present study can be applied to the study of the influence of a Hall thruster plume on the spacecraft surface. This article aims to conduct an in-depth investigation into the environmental impact of Hall thruster plumes on spacecraft surface charging. The non-uniform plasma plume generated by Hall thrusters may trigger charging and discharging effects, making the assessment of surface charging risks crucial. Through numerical simulations using SPIS system, this study evaluates the surface charging characteristics of a complex spacecraft in orbit, simulating the effects of turning on and off the thrusters, as well as varying distances between the thrusters and the spacecraft. The simulation demonstrates that turning on the thrusters significantly affects spacecraft charging, reducing the potential difference between spacecraft surfaces from 3740 V to 19.2 V, effectively alleviating electrostatic discharge on the spacecraft surface. The closer the thruster is to the spacecraft, the more CEX ions are collected on the surface, influenced by the beam ions, resulting in a surface potential change of 1.3 V, with minor effects on surface potential but contributing to increased deposition contamination on the spacecraft surface. [ABSTRACT FROM AUTHOR]

We present the development of a portable Thomson scattering diagnostic system allowing simultaneous spatially and temporally resolved plasma property measurements for low density plasmas. The setup uses a compact pulsed Nd:YAG laser (532 nm) as the light source with suppression by two volume Bragg grating notch filters and dispersion with a single-stage spectrometer before measurement with an intensified camera. A key issue is the detailed light collection and how it impacts the sensitivity and elastic light suppression, for which we have investigated two optical configurations, one based on a 7 × 1 linear fiber bundle and the other based on a slit spatial-filter. We find that the configuration with the slit spatial-filter provides a higher sensitivity by a factor of ∼2 along with more uniform spatial response. We have developed a custom pulsed-plasma setup with a modulation at 20 kHz, representative of the Hall thruster breathing mode oscillation, to show the possibility of temporally resolved measurements for electric propulsion applications. We have successfully recorded the variations in electron number density and temperature with sub-mm spatial resolution and capturing ten temporal points over the 50 µs modulation period. The detection limit of electron density (with the spatial-filter configuration) is ∼1.6 × 1017 m−3, which is ∼1/10 of the plasma density in the acceleration channel of Hall thrusters. [ABSTRACT FROM AUTHOR]

The theoretical specific impulse of Hall thrusters using a krypton propellant is much higher than that of those using xenon. However, the actual benefits of this improvement are limited by the difficulty in achieving krypton atom ionization, and its efficiency is much lower than that of xenon. In this study, the anode specific impulse and anode efficiency of Hall thrusters using krypton propellant were improved by adding a small quantity of xenon. A dual-channel supply of xenon and krypton was achieved by optimizing the discharge channel structure based on the design of the aft-loaded magnetic field. The effects of the xenon and krypton supply method and xenon fraction on the discharge characteristics of the Hall thruster were investigated by conducting discharge experiments using the mixed propellant. The results indicated that injecting a small quantity of xenon significantly improved the ionization of krypton, improved the ionization and acceleration efficiency of the thruster, and fully utilized the high specific impulse characteristics of krypton. Furthermore, the addition of a small quantity of xenon also significantly increased the upper limit of the discharge voltage for stable operation using krypton from 500 V. At the maximum tested xenon fraction, the maximum discharge voltage was loaded to 900 V. At a discharge voltage of 800 V, the optimal anode efficiency and anode specific impulse increased by 4.5 % and 534 s, respectively, compared with those for pure xenon propellant (with an anode efficiency of 54 % and anode specific impulse of 3034 s). The results of this study are of great significance for the design of high-efficiency, high-specific-impulse, and low-cost Hall thrusters. • A dual-channel gas supply structure is proposed, which can supply two kinds of gases into the discharge channel individually. • Thruster performance is optimal when krypton is supplied downstream and auxiliary xenon is supplied upstream. • High-voltage operating ability under krypton is greatly improved with the assistance of a small quantity of xenon. [ABSTRACT FROM AUTHOR]

Traditionally, space missions to outer planets have relied on large amounts of chemical propellant or flybys to achieve their objectives. This large mass consumption may be significantly reduced using electric propulsion (EP), since it allows for larger specific impulses. However, this improvement is counteracted by the lower thrust per power levels achievable with EP engines compared to chemical thrusters. Here we discuss how to find a balance for a purely solar EP (SEP) uncrewed mission to Mars. We present system engineering and mission analyses focused on the orbital mechanics. The result is the conceptual design of a realistic mission to Mars, in which any target orbit around Mars can be reached using solely present SEP technology. In particular, for a 2000 kg spacecraft propelled with a Hall effect thruster (HET), the interplanetary transfer can be performed in 363 days with less than 400 kg of fuel consumption, provided that the vehicle leaves the Earth orbit with a suitable specific energy that is attainable using current launchers. Subsequently, 300 kg of propellant are sufficient to allow for the planetary capture, plane change, and circularisation manoeuvres, finally inserting the spacecraft into a polar orbit of height between 300 km and 1000 km above the surface of Mars. • The requirements of a Solar Electric Propulsion mission to Mars are analysed. • Optimisation methods for trajectory design are developed. • A 363-day interplanetary transfer can be performed with minimal mass consumption. • Efficient capture, plane change and circularisation manoeuvres around Mars are found. • A Solar Electric Propulsion mission to Mars is feasible with present technology. [ABSTRACT FROM AUTHOR]

The erosion of the accelerating chamber walls is one of the main factors limiting the operational life of Hall effect thrusters (HETs), and it is mainly related to the sputtering of ceramic walls due to the impacting energetic ion particles. The erosion phenomenon is investigated by means of a numerical model that couples the plasma model HYPICFLU2, used for evaluating the local distributions of ion energies and incidence angles, and a sputtering model specific for the xenon–Borosil pair, which is the most used in HETs application. The sputtering yield model is based on the measurements by Ranjan et al. that are improved with a linear factor to include wall temperature effect, recently studied by Parida et al. The experimental eroded profiles of SPT100 walls are selected as benchmark. The results show that there is a decrease in erosion speed with time, in accordance with experimental measurements, but the model underestimates, by about 50–60%, the erosion at the channel exit, which suggests a stronger dependence of sputter yield on surface temperature. Thus, the need for new experimental measurements of sputtering in the range of impact energy, angle, and wall temperature, respectively, of 10–250 eV, 0–85°, 30–600 °C, arises. [ABSTRACT FROM AUTHOR]

Hall thrusters are plasma-based devices that have established themselves as one of the most attractive and mature electric propulsion technologies for space applications. These devices often operate in a regime characterized by low frequency, large amplitude oscillations of the discharge current, which is commonly referred to as the 'breathing mode'. The intensity of these oscillations depends on the thruster's design and operating conditions and can reach values of the order of the average discharge current, posing issues for the thruster's performance and for coupling with the driving electronics. A 0D model of the thruster discharge was developed to investigate the core physical mechanisms leading to the onset and sustenance of the breathing mode. The model was found to be capable of reproducing oscillations with characteristics in line with those observed in the breathing mode. In this work, we extend the use of the 0D model to investigate the effect of the magnetic field intensity and of different propellants on the system stability. [ABSTRACT FROM AUTHOR]

The erosion of the accelerating chamber walls is one of the main factors limiting the operational life of Hall effect thrusters (HETs), and it is mainly related to the sputtering of ceramic walls due to the impacting energetic ion particles. The erosion phenomenon is investigated by means of a numerical model that couples the plasma model HYPICFLU2, used for evaluating the local distributions of ion energies and incidence angles, and a sputtering model specific for the xenon–Borosil pair, which is the most used in HETs application. The sputtering yield model is based on the measurements by Ranjan et al. that are improved with a linear factor to include wall temperature effect, recently studied by Parida et al. The experimental eroded profiles of SPT100 walls are selected as benchmark. The results show that there is a decrease in erosion speed with time, in accordance with experimental measurements, but the model underestimates, by about 50–60%, the erosion at the channel exit, which suggests a stronger dependence of sputter yield on surface temperature. Thus, the need for new experimental measurements of sputtering in the range of impact energy, angle, and wall temperature, respectively, of 10–250 eV, 0–85°, 30–600 °C, arises.

Plasma structure and electron cross field in the z – θ plane of a Hall thruster E × B plasma under an azimuthally inhomogeneous magnetic field are studied by both experimental and numerical approaches. The work is intended to identify a primary role of electron dynamics on the structure formation by manipulating only the strongly magnetized electrons. The plasma potential distribution shows an axial–azimuthal variation; a low magnetic field region results in spatial potential saturation further downstream. The plasma density structure shows a 1D-like azimuthal variation with less axial deformation. A dense region is observed near the location of ∇ B > --> 0 , where electrons are expected to undergo the ∇ B and curvature drift toward the anode where neutrals are introduced. The potential structure is in close correlation to the Hall parameter distribution, indicating that electron dynamics plays a primary role in plasma structure formation, and via multiple consecutive stepwise physical steps, it eventually affects the density structure formation. In the z – θ space, the cross-field transport by E × B and diamagnetic drifts dominantly determines the electron flow and increases the overall axial electron mobility due to the azimuthal inhomogeneity. It is shown that most of the current is carried by the largest structure, but as the macroscopic structure fades out downstream, small structures grow and share the current. By considering the conservation laws, we show that a relation between azimuthal distributions of physical properties is formed to conserve the axial flux by a balance of specific forces, a balance between the resistive force and the magnetic force in the near-anode region and a balance between the electric/pressure force and the magnetic force in the acceleration and plume region, which differs from the Boltzmann relation satisfied in the radial dimension. Based on this principle, with a simplified test case having a uniform plasma density distribution, we show an analytic relation between azimuthal distributions of the magnetic field and the plasma potential and confirm the relation by a 2D hybrid simulation. [ABSTRACT FROM AUTHOR]

The magnetic field is the most important element in designing a Hall thruster and improving thruster performance because it directly influences the behavior of electrons in the discharge channel. In this work, magnetic field tailoring, parallel magnetic fields to the thruster channel wall, has been attempted in a cylindrical Hall thruster, and the resultant ion beam properties are studied. The magnetic field tailored cylindrical Hall thruster demonstrated much higher ion current and propellant efficiencies than the conventional cylindrical Hall thruster, with an identical mass flow rate. A large fraction of multiply charged ions (>65%) was observed and reduced beam emission was demonstrated near the channel wall. Further, the channel wall is solely coated without erosion even at the end of the channel. Hence, tailoring of the magnetic field in cylindrical Hall thrusters could significantly enhance the potential of Hall thrusters in space applications owing to their higher propellant efficiency and reduced wall interaction. [ABSTRACT FROM AUTHOR]

Plasma oscillations below 100 kHz in a high-power, magnetically shielded Hall-effect thruster are characterized experimentally. A time-resolved laser-induced fluorescence diagnostic was used to measure the evolution of the axial ion velocity distribution along the discharge channel centerline. A method was developed to correct for artificial broadening of the distribution due to finite spatial resolution, enabling accurate ion temperature measurements in the acceleration region. Time-dependent ion heating behavior is revealed, which varies with axial location. Electrostatic, collisionless particle-pushing calculations were employed to simulate the effects of a shifting acceleration front on ion motion. It is found that ions exchange energy with these oscillations, which accounts for a portion of the ion velocity fluctuations observed in the thruster plume. Phasing relationships between ion dynamics and discharge current oscillations are discussed in the context of prior Hall thruster experiments. [ABSTRACT FROM AUTHOR]

Low-frequency (5–30 kHz) discharge current oscillations characterize the operation of Hall thrusters and represent a valuable metric not only to monitor the thruster behavior, but also to optimize the thruster performance. Two types of low-frequency oscillation modes are commonly observed: a global "breathing" mode, associated with the propellant ionization, and a local mode, typically characterized by a lower oscillation amplitude and the appearance of azimuthal spokes. The main characteristics of discharge oscillations and the transition between the two oscillation modes vary greatly with the thruster geometry and the operating condition. In this work, we present the results of an experimental campaign carried out on a 20 kW-class thruster prototype, SITAEL's HT20k, with an exchangeable discharge channel and a magnetic circuit. Three different channel sizes were tested over a wide range of operating conditions and magnetic fields. For each operating point, a high frequency measurement of the discharge current was performed, recording the main characteristics of the oscillations. The data collected were then processed to derive the influence coefficients of each thruster parameter on the discharge current characteristics, as well as their dispersion. Finally, this allowed us to formulate general, data-driven scaling laws for the discharge current salient features, such as oscillation amplitude and dominant frequency. The gathered insight sheds light on the physical processes involved in the thruster discharge. At the same time, the possibility to model with simple functional laws the main oscillatory mode of Hall thrusters offers a unique aid to the optimization of thruster design and the evaluation of thruster performance during life. [ABSTRACT FROM AUTHOR]

Turbulence in hollow cathodes used for space propulsion is believed to play an important role in anomalous electron transport and ion heating. In this work, the implementation of coherent Thomson scattering to identify and characterize MHz-frequency ion acoustic turbulence and kHz-frequency oscillations in the plume of a hollow cathode is achieved. In the presence of a background magnetic field of a Hall thruster, a number of unstable modes are observed. A directive ion acoustic mode propagating predominantly within a restricted angle around the magnetic field is found, exhibiting an energy scaling with wavenumber k of the form k − 5.2 ± 0.58 , which differs from the classic Kadomtsev k − 3 scaling for unmagnetized conditions. Bi-directional ion acoustic mode fluctuations propagating over a range of angles with respect to the magnetic field have been measured, possibly signifying the existence of a large-amplitude plasma wave, similar to the Buneman instability. Finally, electron density fluctuations in the kHz-frequency range, a possible consequence of drift-driven instabilities in the plane perpendicular to the magnetic field, have also been identified. These results not only are an indication of the diversity of wave types that exist in hollow cathode plumes but also point to the key role played by the presence of, and the configuration of, the magnetic field in their appearance. [ABSTRACT FROM AUTHOR]

We present a combined electrostatic and magnetostatic plasma diagnostic tool used to measure current fractions of ion species in a Hall-effect thruster plume. Hall thrusters produce thrust by accelerating singly and multiply charged ions to high velocity. Singly charged ions fall through potential differences from their point of creation to the location of measurement, which varies as a function of time and position resulting in a relatively wide distribution of energy. This process is further complicated by both direct and multi-step creation of multiply charged ions and by ion-neutral collisions. The result is that Wien velocity filter (ExB) spectra typically have overlapping peaks. Analysis of these spectra requires assumptions about the velocity distribution function (VDF) that are hard to verify and can result in the incorrect species current distribution used to calculate thruster performance efficiencies. We present how adding an energy filter in line with an ExB probe can result in a more accurate measurement of the species current fraction while also providing information about the ion energy distribution of each species. The combined diagnostic was used to measure the species current distribution in a 1.5 kW Hall thruster operating on krypton. The diagnostic setup and data analysis of the probe measurements are presented and compared against results utilizing established VDF-fit and integration techniques applied to standard spectra measured by a standalone ExB probe. [ABSTRACT FROM AUTHOR]

A sub-30 W Hall effect thruster known as the Narrow Channel Hall Thruster (NCHT) was recently developed at the Aerospace Plasma Laboratory, Technion. In this work, the near field plume region of the NCHT is analyzed using emissive and planar Langmuir probes. The NCHT discharge is also analyzed using a particle-in-cell simulation, capable of providing 2D spatial distributions of the plasma parameters inside the channel itself. Both experimental and simulation results confirm that most of the potential drop ∼60% occurs outside the channel. The results also show that ion current loss to the walls and anode is substantial, similar in value of the ion beam current itself. [ABSTRACT FROM AUTHOR]

To achieve high-thrust-density operation, we propose electrostatic–magnetic hybrid ion acceleration in which the empirical thrust density limit of the electrostatic acceleration is surpassed without violent plasma oscillation by combing the collisional momentum transfer mechanism, which is the ion acceleration mechanism of the electromagnetic acceleration. To achieve hybrid ion acceleration, we experimentally obtained two design criteria: one near anode propellant injection and another at the on-axis hollow cathode location. The thrust characteristics of three thrusters composed of a slowly diverging magnetic field between an on-axis hollow cathode and a coaxially set ring anode were examined. By injecting xenon propellant along the anode inner surface, the electron impact ionization process was enhanced, and generated ions are electrostatically accelerated through the radial-inward potential gradient perpendicular to the axial magnetic lines of force. The hybrid ion acceleration characteristics were obtained only if these two criteria were satisfied and the obtained thrust was consistent with the thrust formula derived for steady-state, quasi-neutral plasma flows. In addition to the criteria, strengthening the magnetic field and enhancing the propellant mass flux were effective for improving thrust density without deteriorating thrust efficiency. Among the experimental conditions in this study, the maximum thrust density was 70 N/m2 with an anode specific impulse of 1200 s, which cannot be achieved in a purely electrostatic thruster with thrust density 6.3 times than that of a typical Hall thruster. [ABSTRACT FROM AUTHOR]

Information on the ionization of propellant gases is invaluable for Hall thruster plasma studies because the ion beam is the source of thrust generation. This study proposes a new method that identifies the ionization location of Xe+ ions in Hall thruster plasmas by combining the laser-induced fluorescence spectroscopy and floating emissive probe measurements in annular and cylindrical Hall thrusters. In an annular Hall thruster, this method indicated that ionization became the largest at a few millimeters upstream of the maximum radial magnetic field, whose result agreed with a one-dimensional hybrid fluid/particle-in-cell (PIC) simulation result. On the other hand, the ionization region in a cylindrical Hall thruster was located deeper inside the discharge cavity than that of the annular thruster. In addition, a large upstream shift of the ionization region was observed with the decreasing fraction of the radial magnetic field in the cylindrical Hall thruster. [ABSTRACT FROM AUTHOR]

We report on the results of an experimental campaign to measure time-varying velocity distributions in the near-field of a low power Hall thruster. We employ a sample-hold technique, enhanced by parallelizing the measurement hardware into several signal processing channels that vastly increases the data acquisition rate. The measurements are applied to study flow field dynamics in a commercial BHT-600 Hall thruster undergoing unforced breathing mode oscillations in the 44–49 kHz range. A very detailed experimental picture of the near-field emerges from these studies. The results indicate that velocity fluctuations lessen further downstream of the exit plane. Along the thruster axis where there is a general appearance of a central jet, there is evidence of a low velocity ion population in between the periodic bursts of high velocity ions, indicative of local ionization of neutrals outside of the thruster. One possible source of this residual ionization may be background chamber gas, which is not unexpected with the limited pumping capacity of ground test facilities. [ABSTRACT FROM AUTHOR]

Hall effect thrusters operating at power levels in excess of several hundreds of kilowatts have been identified as enabling technologies for applications such as lunar tugs, large satellite orbital transfer vehicles, and solar system exploration. These large thrusters introduce significant testing challenges due to the propellant flow rate exceeding the pumping speed available in most laboratories. Even with proposed upgrades in mind, the likelihood that multiple vacuum facilities will exist in the near future to allow long duration testing of high-power Hall thrusters operating at power levels in excess of 100 kW remains extremely low. In this article, we numerically explore the feasibility of testing Hall thrusters in a quasi-steady mode defined by pulsing the mass flow rate between a nominal and a low value. Our simulations indicate that sub-second durations available before the chamber reaches critical pressure are sufficiently long to achieve the steady-state current and flow field distributions, allowing us to characterize thruster performance and the near plume region. [ABSTRACT FROM AUTHOR]

Erosion characteristics on the cover of the inner front pole in a 12.5 kW Hall thruster were measured over a wide range of operating conditions in tests of 6–14 h duration using an accelerated test method and a very sensitive, radioactive tracer-based erosion diagnostic. The operating points included the nominal 300–600 V conditions on a constant 20.8 A throttle curve and conditions at other currents spanning the throttling envelope and with varying magnetic field strength, facility pressure, and discharge voltage oscillation amplitude. Surprisingly, the results show that the highest wear rates occur at the lowest voltages and currents. The wear rates were insensitive to discharge voltage ripple but increased monotonically with magnetic field strength, particularly near the inner radius of the pole cover. The inner region was also sensitive to facility pressure, showing lower rates at a higher pressure level. Separate experiments in which the energy distributions of ions generated by the hollow cathode were measured suggest that the cathode plume may be a source of energetic ions responsible for some of the erosion trends, in addition to ions originating in the thruster plume. [ABSTRACT FROM AUTHOR]

This article reports on the characterization of a laboratory model 100 A-class hollow cathode with a sintered lanthanum hexaboride ( LaB 6) emitter for high-power Hall thrusters. The cathode has been fired up to 70 A with xenon as working gas. The cathode architecture, test setup, ignition procedure, and power consumption are described first. The second part of this contribution comments on the current–voltage characteristics and the discharge modes obtained for discharge currents in the 30–70 A range and flow rates in the 15–30 SCCM range. The cathode operates in a spot mode at high discharge currents and in a plume mode with large oscillations at low currents and low gas flow rate. Spectral analysis shows that most frequencies reside in the 10–200 kHz range with flat and sharp distributions in plume and spot modes, respectively. Finally, we present electron temperatures and densities measured in the cathode plasma plume by means of incoherent Thomson scattering. The two quantities decrease along the axis. The density is large (up to ∼ 10 19 m − 3 ) and increases with both the ion current and the gas flow rate. The electron temperature increases with the current and decreases with the gas flow rate. The temperature remains relatively low (<1.5 eV) in spite of large currents and applied powers. [ABSTRACT FROM AUTHOR]

The performance of a 9-kW class magnetically shielded Hall thruster operating on xenon and krypton propellants is experimentally characterized. Thrust and efficiency measurements performed at discharge powers ranging from 4.5 to 9 kW indicate that the anode efficiency of krypton is 9%–18% lower than that of xenon. This difference is comparable to previous measurements reported for unshielded Hall thrusters, although it is found that unlike in previous studies, the efficiency ratio widens with increasing discharge voltage. Far-field probes are employed to measure the contributions to anode efficiency at conditions of 4.5 and 6 kW. These results indicate that mass utilization has the largest impact on the difference in performance between xenon and krypton. Assuming this mass utilization remains the dominant driver at higher voltages, it is proposed that the higher electron temperature of shielded thrusters along channel centerline coupled with the nonlinearity of the ionization cross section may explain why the efficiency gap widens with increasing voltage for shielded thrusters. The results are discussed in the context of optimizing magnetically shielded Hall thrusters for improved performance on krypton propellant. [ABSTRACT FROM AUTHOR]

This work presents the results of over 6500 h of wear testing completed during the maturation of the NASA 12.5 kW Hall Effect Rocket with Magnetic Shielding. Erosion of the thruster front pole covers was found to be the primary life-limiting mechanism and exhibits strong dependencies on thruster operating condition and material properties. Specifically, average pole cover erosion rates increased by 76% as discharge voltage decreased from 600 to 300 V and 42%–96% as the magnetic field increased from 0.75 to 1.25 times the nominal value. The cause of both trends is hypothesized to be ion heating from a modified two-stream instability that becomes dominant for 300 V operation and grows with magnetic field strength. Rougher pole covers were observed to have 33% lower average erosion rates than those that were polished due to local surface features that lower the effective angle of attack of eroding ions and the concomitant sputter yields. Alumina pole covers were shown to erode over 250% faster due to the higher sputter yield of alumina relative to graphite. Shifting the cathode upstream of the pole covers reduced average keeper erosion rates by 84% by reducing the view factor to high-energy beam ions. Cathode keeper erosion was also shown to exhibit azimuthal nonuniformities, which resemble the azimuthal oscillations observed in the cathode region. Taken together, these results provide in-depth insights into the life-limiting mechanisms impacting magnetically shielded Hall thrusters. [ABSTRACT FROM AUTHOR]

The dynamic properties of the Hall thruster breathing mode are investigated experimentally. Non-invasive time-resolved laser diagnostics are combined with a quasi-one-dimensional non-Maxwellian modeling technique to yield the high-speed evolution of a variety of plasma properties over the course of a breathing oscillation. The fluctuations of both ion and neutral densities are quantified in terms of amplitude and dispersion and are interpreted in the context of a simplified dispersion relation. It is found that the oscillations associated with the breathing mode are likely ion and neutral waves propagating with phase velocities commensurate with the local species drift speed. Further, the amplitudes of the waves are shown to decay with distance from the anode. This effect can be explained largely by the influence of the expansion of the background plasma and neutral gas. This monotonic decay combined with the downstream propagating nature of the waves suggests that these oscillations are influenced by conditions upstream of the acceleration and ionization regions. By comparing the presence of these waves to inferred electron temperature fluctuations, the hypothesis that the breathing mode is governed by a process in which the modulation of the neutral density near the anode sheath or the anode itself is coupled to a downstream ionization instability is qualitatively explored. The possibility of these waves relating to cyclical recombination of ions at the anode is also discussed. [ABSTRACT FROM AUTHOR]

The effect of facility background pressure on the operation of a hollow cathode in a Hall thruster-like axial magnetic field is experimentally characterized. Facility pressure was varied between 10 and 88 μTorr-Xe using a secondary flow of xenon into the test facility, and cathode operation was studied using a combination of telemetry and plasma diagnostic measurements. Increasing pressure resulted in decreased discharge voltage, cathode orifice plate temperature, and voltage and current oscillation magnitudes. The plasma diagnostics, which consisted of a radially mounted retarding potential analyzer and an ion saturation probe and emissive probe mounted to a fast reciprocating motion stage, showed that increasing pressure resulted in decreased radial high-energy ions, decreased centerline maximum plasma potential and electron temperature, and increased plasma density farther than 40 mm downstream of the cathode. The effects of background pressure on the cathode plume were largely constrained to between 50 and 100 mm downstream of the cathode, where the largest gradients of plasma potential and electron temperature occur. Experimental measurements were combined with neutral density simulations to identify that the confluence of changes in electron temperature, plasma density, and neutral density result in an increase of almost an order of magnitude in the calculated ionization rate in the same location. These results have implications for both standalone cathode tests and for improving the understanding of facility effects on cathode coupling in Hall thrusters. [ABSTRACT FROM AUTHOR]

In wall-less Hall thrusters, the ionization of the propellant and the acceleration of the ions occur outside the thruster [S. Mazouffre, S. Tsikata, and J. Vaudolon, in 50th AIAA/ASME/SAE/ASEE Joint Propulsion Conference (American Institute of Aeronautics and Astronautics, Cleveland, OH, 2014)]. This reduces interactions between the plasma and the thruster parts as compared to conventional annular and cylindrical Hall thrusters and promises a longer thruster lifetime. With a much simpler design, these non-conventional thrusters are also easier to miniaturize for operation at low power levels of a few hundred watts and lower. In this work, experiments demonstrate that a miniaturized (3 cm diameter) 200 W wall-less thruster is also able to achieve similar voltage utilization, propellant utilization, and current utilization efficiencies as conventional Hall thrusters. Yet, thruster performance of the wall-less thruster is generally lower due to a much larger plume divergence than that in conventional Hall thrusters. This plume divergence is a consequence of ion acceleration in the fringing magnetic field. Thrust and plasma measurements suggest that the thrust generated by the wall-less thruster is due to two components: ion acceleration by the JxB force in the region of the fringing magnetic field radially away from the thruster and by plasma expansion in the diverging magnetic field near the thruster axis. [ABSTRACT FROM AUTHOR]

In part I of the article, we demonstrated that a variant of the dynamic mode decomposition (DMD) algorithm based on variable projection optimization, called optimized DMD (OPT-DMD), enables a robust identification of the dominant spatiotemporally coherent modes underlying the data across various test cases representing different physical parameters in an E × B simulation configuration. We emphasized that the OPT-DMD significantly improves the analysis of complex plasma processes, revealing information that cannot be derived using conventionally employed analyses such as the fast Fourier transform. As the OPT-DMD can be constrained to produce stable reduced-order models (ROMs) by construction, in this paper, we extend the application of the OPT-DMD and investigate the capabilities of the linear ROM from this algorithm toward forecasting in time of the plasma dynamics in configurations representative of the radial-azimuthal and axial-azimuthal cross-sections of a Hall thruster and over a range of simulation parameters in each test case. The predictive capacity of the OPT-DMD ROM is assessed primarily in terms of short-term dynamics forecast or, in other words, for large ratios of training-to-test data. However, the utility of the ROM for long-term dynamics forecasting is also presented for an example case in the radial-azimuthal configuration. The model's predictive performance is heterogeneous across various test cases. Nonetheless, a remarkable predictiveness is observed in the test cases that do not exhibit highly transient behaviors. Moreover, in all investigated cases, the error between the ground-truth and the reconstructed data from the OPT-DMD ROM remains bounded over time within both the training and the test window. As a result, despite its limitation in terms of generalized applicability to all plasma conditions, the OPT-DMD is proven as a reliable method to develop low computational cost and highly predictive data-driven ROMs in systems with a quasi-periodic global evolution of the plasma state. [ABSTRACT FROM AUTHOR]

A two-fluid model is used to investigate the influence of the axial component of the magnetic field on the growth rate of electrostatic resistive instabilities with cross field electron transport in a Hall thruster. The axial component of the magnetic field plays an important role in instabilities. It provides additional confinement to electrons and ions near the channel axis. Also, it helps to protect the walls from the direct impacts of particles, thereby reducing erosion and extending the operational lifetime of the system. A fourth-order dispersion equation is derived using plasma perturbed densities into Poisson's equation to observe the various effects on the growing waves in plasma. It is observed that the growth rate and the real frequency increase with axial and radial components of the magnetic field, respectively. The order of the real frequency of the wave is found to be 10 6 / s. For the fixed value of the azimuthal wavenumber ( k y = 500 / m), the amplitude of the growth rate of the instability dropped to almost 40% if the axial component of the magnetic field is considered. Similarly, the amplitude of the real frequency increases by almost 74% (at k y = 500 / m) by incorporating the contribution of the axial component of the magnetic field. In addition, it is also observed that the amplitude of the growth rate increases with low values of radial and axial components of the magnetic field, but it decreases at the higher value of the magnetic field due to the resonance of electron cyclotron frequency with plasma frequency. [ABSTRACT FROM AUTHOR]

To completely characterize the evolving state of a plasma, diagnostic tools that enable measurements of the time-resolved behavior are required. In this study, a gridded ion source with superimposed oscillations was utilized to verify the functionality of a high-speed retarding potential analyzer (HSRPA), at frequencies equivalent to the low frequency oscillations occurring in Hall effect thrusters (HETs). The verification of this device provides an effective alternative to existing diagnostics for measuring time-resolved ion energies. Retarding potential analyzers (RPAs) have established themselves as a fundamental diagnostic in the field of electric propulsion (EP), enabling the measurement of ion energy distributions within the plumes of EP thrusters. The work presented here has demonstrated the capability of a standard RPA in conjunction with high-speed circuitry and data fusion techniques to produce time-resolved ion energy distribution functions (IEDFs) at higher frequencies and beam potentials than have previously been investigated. Tested frequencies ranged between 20 and 80 kHz with 10 V peak-to peak oscillations at a mean beam potential of 570 V. In addition, measurements were conducted with several waveforms, functioning as the superimposed oscillation, including a sine wave, triangle wave, and noisy sine wave. Data from the HSRPA were successfully reconstructed into time series utilizing two data fusion techniques: the empirical transfer function method and shadow manifold interpolation. Time-resolved IEDFs were produced at all frequency set points and waveforms. This investigation has demonstrated the HSRPA effectiveness at producing time-resolved measurements under conditions similar to those occurring in HETs. [ABSTRACT FROM AUTHOR]

One of the limiting factors to developing plasma thrusters on alternative propellants is the cost associated with changing the diagnostic tools, which are often propellant-dependent. For laser induced fluorescence (LIF), which is typically used for ion velocity distribution measurements to determine ion trajectories and potential profiles, either new lasers need to be bought, which are tuned to the wavelength of the new element's excitation level, or a costly tunable laser is required. A method to use existing LIF setups designed for xenon on any propellant has been demonstrated on a Hall thruster operating on krypton. In the demonstration test, a small amount of xenon (0.01%–4%) was mixed with the main krypton propellant for use as a diagnostic tracer, and xenon ion velocities were measured while also monitoring changes in the mean discharge current and oscillations. High signal-to-noise ratios in LIF data acquired along the channel centerline were obtained with tracer gas fractions ≤1% that negligibly affected the thruster operation. These results and comparison of the emission spectra of xenon and other common propellants suggest that the tracer LIF method should be broadly applicable to LIF measurements in Hall thrusters operating on alternative propellants. [ABSTRACT FROM AUTHOR]

The discharge current and potential difference between cathode and ground of a Hall thruster were predicted sequentially by Recurrent Neural Network (RNN) in order to optimize operating conditions. The prediction accuracy and calculation cost for three RNN models, the standard Echo State Network (simpleESN), cycle-groupedESN, and Long Short-Term Memory (LSTM) were compared. The ESN model structures were chosen using Bayesian optimization. We calculated the normalized root mean square error (NRMSE) of the model output against the experimental results of a 200W class Hall thruster developed at Kyushu University. The NRMSE of the simpleESN model output against discharge current was 0.0407, about 1/10 that of the LSTM. The NRMSE of the simpleESN model against the potential difference between cathode and ground was 0.0981, about 1/7 that of the LSTM. Moreover, cycle-groupedESN reduced the calculation time for the optimization process to 24 seconds, compared to 303 seconds with simpleESN, though the NRMSE against discharge current of the cycle-grouped ESN was 0.1179. These results show that the simpleESN and cycle-groupedESN models are superior to the LSTM in both prediction accuracy and calculation costs for prediction of discharge current and voltage between ground and cathode in Hall thrusters in the considered settings. [ABSTRACT FROM AUTHOR]

The Japan Aerospace Exploration Agency has developed a power processing unit for a 6 kW Hall thruster, a candidate for propulsion systems for geostationary satellites. Hall thrusters have a discharge instability of tens of kHz, which may damage thrusters, power processing units, and satellite systems. This paper proposes our system for detecting discharge current amplitudes that exceed the specified limit of the thruster. The discharge current is converted into a voltage by the current sensor and is output to a digital controller through an AD converter. The system's innovation includes a frequency prediction model and a damping correction algorithm, which allow the system to accurately detect the peak amplitude of the discharge current without testing to measure the frequency characters of the Hall thruster. The design of the amplitude detection system considers the frequencies of the ionization oscillation and the switching frequency of the power processing unit. The frequency prediction model estimates the frequency of discharge current oscillations from simple parameters: the output voltage of the discharge power supply and the thruster dimensions. The amplitude of the discharge current is damped slightly by the frequency characteristics of the system, but the algorithm corrects this. The results of the frequency prediction model agreed well with the actual measurements. The predicted value was 13.2 kHz, while the actual measurement was 8–15 kHz. The integration test showed that the amplitude detection system identified 36 peaks exceeding the specified limit of 80% peak-to-peak during a 100 ms segment, although the actual number was 34. This difference is acceptable. The proposed amplitude detection system can accurately count the peaks in the discharge current that exceed specifications. • An amplitude detection system detects discharge current oscillations that exceed the specifications for protecting satellites. • The algorithm and frequency response of the system are designed considering the frequency of ionization oscillations. • The frequency is estimated by a simple model with only the output voltage of the discharge power supply and thruster dimensions. • The system detected the amplitude of the discharge current oscillations with an error of 5.8%. [ABSTRACT FROM AUTHOR]

A combined method for measuring the thrust vector and roll torque of low-power Hall-effect thrusters is proposed, using a conventional torsional thrust pendulum, with a measurement accuracy of a few tens of micro-Newton meters. The 300–500 W class Hall-effect thruster is mounted horizontally on the pendulum, whose motion is determined only by the radial thrust vector and rolling torque of the thrusters. The contribution of the two components of a Hall-effect thruster is analyzed through the method of changing the position of accelerator and external hollow cathode, with the accelerator contributing a significant portion of the radial thrust vector and increasing linearly with thruster anode power. And the external hollow cathode contributing at a magnitude of one hundred micro-Newtons, in the direction opposite to the cathode. The combining method provides a way to measure the deviation angle of the stationary EP thrusters, an analysis of the contribution of the accelerator and the external hollow cathode, and a method for decreasing the deviation angle of a Hall-effect thruster by counteracting the effects of these two components. • Proposed a new direct measurement method of thrust vector and rolling torque for Hall effect thrusters with conventional torsional pendulum. • The first attempt to measure and analyze the thrust vector contribution of the accelerator and the external hollow cathode. • Firstly proposed a method for decreasing the deviation angle of a Hall-effect thruster by counteracting the effects of the accelerator and the cathode. [ABSTRACT FROM AUTHOR]

Using both 0D and 1D fluid models, we revisit the formation of the breathing mode in Hall thrusters and show that it is an ionization instability associated with nonlinearity in the electron power absorption. As the plasma density increases, the axial electric field profile changes and the magnitude of the electric field is enhanced in the ionization zone. This causes a nonlinear increase in the power absorbed by electrons and an increase in the electron temperature and ionization rate factor that is able to partially compensate for the decreasing neutral density to keep the ionization rate high. This sets up a positive feedback mechanism where the electric field continues to be enhanced as the plasma density increases and, consequently, the neutral density needs to decrease even further before plasma growth can be halted. At this point, the neutral density is so low that the plasma can no longer be "sustained," and time is needed for neutrals to refill the thruster channel before "reignition" can occur and the process repeated. By treating the breathing mode as an AC excitation, a carefully designed external circuit can be used to counteract the change in the axial electric field by appropriately varying the anode voltage to stabilize the discharge. [ABSTRACT FROM AUTHOR]

Air-breathing electric propulsion is a promising concept that may allow sustained access to very low earth orbit by guaranteeing continuous drag compensation through in situ harvesting of propellant. For earth observation, such low orbits benefit image resolution and reduce signal latency and transmission power requirements, easing link budgets. Moreover, not having to carry typical electrostatic thruster propellants such as xenon and tanks positively impact overall mission costs. However, the low molecular mass and high first ionization energies of principal atmospheric constituents hinder efficient thruster operation and result in strongly degraded performance. In this study, we examine the performance of an extended channel Hall thruster design for air-breathing operations. An extended channel compensates for the reduced residence times of lighter neutrals. The longer channel, coupled with an extended radial magnetic field region, allows for an increased length of the ionization zone, enhancing molecular ionization probability. Here, we present direct thrust measurements of this thruster operating with a pure nitrogen flow. Analysis of the results show performance in the 500–800 W anode power range, with thrust, specific impulse, and total thrust efficiency ranging from 17 to 22 mN, 1000 to 1100 s, and 14% to 18%, respectively, with a constant 2 mg/s pure molecular nitrogen propellant flow. Performance tends to degrade with increased voltage, suggesting increased contributions of electron-molecule kinetics to performance losses, contrary to what is typically seen in the Hall thrusters operating with propellants such as xenon. [ABSTRACT FROM AUTHOR]

Low-frequency ionization oscillations involving plasma and neutral density (breathing modes) are the most violent perturbations in Hall thrusters for electric propulsion. Because of its simplicity, the zero-dimensional (0D) predator–prey model of two nonlinearly coupled ordinary differential equations for plasma and neutral density has often been used for the characterization of such oscillations and scaling estimates. We investigate the properties of its continuum analog, the one-dimensional (1D) system of two nonlinearly coupled equations in partial derivatives (PDEs) for plasma and neutral density. This is a more general model, of which the standard 0D predator–prey model is a special limit case. We show that the 1D model is stable and does not show any oscillations for the boundary conditions relevant to Hall thrusters and the uniform ion velocity. We then propose a reduced 1D model based on two coupled PDEs for plasma and neutral densities that is unstable and exhibit oscillations if the ion velocity profile with the near-the-anode back-flow (toward the anode) region is used. Comparisons of the reduced model with the predictions of the full model that takes into account the self-consistent plasma response show that the main properties of the breathing mode are well captured. In particular, it is shown that the frequency of the breathing mode oscillations is weakly dependent on the final ion velocity but shows a strong correlation with the width of the ion back-flow region. [ABSTRACT FROM AUTHOR]

Analytical and numerical investigations have been carried out that show the lower hybrid drift instability is active in a region of low-β plasma outside the discharge channel of a magnetically shielded Hall thruster. The region is located downstream of the inner front pole magnet and is contained by the thruster ion beam and hollow cathode plume. It is found that large enough E × B and diamagnetic drifts in the out-of-plane (azimuthal) direction are produced in this region to destabilize lower hybrid waves, even in the presence of electron collisions. The results have been obtained by solving an electrostatic dispersion relation in which the density inhomogeneity is accounted for in the velocity distribution function of the strongly magnetized electrons. Ions in the azimuthal direction have been treated as stationary and unmagnetized. The plasma properties in the dispersion have been provided independently by large-scale 2D multi-fluid/particle-in-cell simulations. The growth rates are found to be even greater than those associated with the modified two-stream instability, which was also predicted recently to be active in this region. 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 perpendicular to the applied magnetic field, thereby altering the velocity field and enhancing the energy of those ions that strike nearby thruster surfaces. [ABSTRACT FROM AUTHOR]

Three-dimensional simulations of partially magnetized plasma are real challenges that actually limit the understanding of the discharge operations such as the role of kinetic instabilities using explicit Particle-In-Cell (PIC) schemes. The transition to high performance computing cannot overcome all the limits inherent to very high plasma densities and thin mesh sizes employed to avoid numerical heating. We have applied a recent method proposed in the literature [L. F. Ricketson and A. J. Cerfon, Plasma Phys. Controlled Fusion 59, 024002 (2017)] to model low temperature plasmas. This new approach, namely, the sparse grid combination technique, offers a gain in computational time by solving the problem on a reduced number of grid cells, hence allowing also the reduction of the total number of macroparticles in the system. We have modeled the example of the two-dimensional electron drift instability, which was extensively studied in the literature to explain the anomalous electron transport in a Hall thruster. Comparisons between standard and sparse grid PIC methods show an encouraging gain in the computational time with an acceptable level of error. This method offers a unique opportunity for future three-dimensional simulations of instabilities in partially magnetized low temperature plasmas. [ABSTRACT FROM AUTHOR]

During development testing of a 12.5 kW magnetically shielded Hall thruster, direct evidence of counterstreaming ions eroding the pole covers was found. One stream of ions appears to originate from the discharge channel while the other stream appears to originate from the centrally mounted cathode. Velocity distribution measurement indicates that each stream impacts the poles of the thruster at high oblique angles of incidence. While the average energy of each stream was in the tens of eV, the energy distributions contained high-energy tails that can be a major contributor to erosion. Starting with the physical picture of high oblique angle bombardment, predictions of change in erosion behavior over time are in good agreement with pole cover erosion measurements taken during wear testing. The new evidence points to a need to study these ions that are traditionally considered "low-energy" ions and the role they play in the erosion of the poles of magnetically shielded Hall thrusters. [ABSTRACT FROM AUTHOR]

• On-orbit non-intrusive perturbation measurement of Hall effect thrusters. • 10 kHz to 100 kHz analysis better than full frequency domain. • Perturbation signal amplified by scatter numbers in the far-zero region. • The physical core represented by the scatter numbers is the dwell time. • Dwell time is mainly related to breathing oscillations. The failure of Hall effect thruster (HET) is usually the result of the accumulation of several tiny abnormal disturbances during operation. Timely and accurate early warning of perturbations is an important guarantee for the normal operation of thrusters. In this study, when the perturbation rate of HET signal (anode voltage or anode flow rate) is −4% to 4%, the time scale signal of breathing oscillation (BO) in the frequency range of 10 kHz to 100 kHz is statistically analyzed. Experiments show that when the signal is perturbed, the balance between ionization and acceleration of plasma in the discharge channel is destroyed, resulting in the dwell time of the wave peaks and valleys of BO in the far-zero region with the perturbation law, which visually shows that the number of scattered points at the corresponding current value changes regularly. According to the above statistical characteristics, this paper proposes a new condition monitoring index, namely the number of scattered points in the wave peaks side of the far-zero point area, and designs a perturbation indicator for the operating condition of the HET. Compared with direct monitoring of discharge current in engineering, this indicator can amplify the influence of the perturbation signal on BO, and has better identification and accuracy. Meanwhile, the indicator does not use invasive measurement and complex equipment, and is promising to be applied to the monitoring and diagnosis of the perturbation operation state of the on-orbit thruster. [ABSTRACT FROM AUTHOR]

A modified Rayleigh-Taylor (RT) instability equation is derived using a three-fluid Hall thruster plasma model by including multi-ionized ions to study the growth rate of the RT instability. For a simplified plasma density and electron drift velocity axial profile, the growth rate for an unstable wave and the condition leading to this instability are discussed. A possible analytical solution for the modified Rayleigh-Taylor equation is discussed along with the condition that leads to it. For double-ionized ions, the behavior of growth rate and perturbed potential for different densities of double-ionized ions are studied. [ABSTRACT FROM AUTHOR]

Presence of dust in cross-field plasma systems such as Hall thrusters or in plasma processing units changes the whole physical phenomenon. Owing to the density gradient, pressure gradient and spatial variation of the magnetic field, the plasma in these devices is often found to be unstable and hence, instability evolves in these systems. Considering dust contamination and ionization in the plasma, we make theoretical study on the growth rate of the instability under the effect of dust density, dust mass and dust charge in addition to the effect of ion temperature, ion temperature gradient, scale length of density inhomogeneity and profile of the ion velocity. For their effective use, the said plasma systems need to be instability free, and the model developed in the present article will help experimentalists to optimize the plasma parameters, external magnetic field and biasing voltage. [ABSTRACT FROM AUTHOR]

Hall thrusters, the most common type of electric propulsion system, typically use xenon as the propellant, given its inertness, its ability to be stored at a high density under pressure, and good thrust to power ratio coupled with a high specific impulse compared to chemical propulsion. However, the number of satellites utilizing electric propulsion units and particularly Hall thrusters is dramatically increasing, resulting in a strain on the availability of xenon propellant in the context of a volatile noble gas market. This phenomenon is seen with the dawn of large satellite constellations and the accelerated launch rate of satellite units, the majority of which now use a Hall thruster as their primary propulsion system. Alternatives to xenon are available in the form of other noble gases, molecular propellants and condensable elements. Such propellants offer certain advantages in terms of specific mission scenarios, or for certain propulsion system sizes. This paper represents a review of alternatives to the conventional xenon propellant for Hall thrusters, providing a comparative study of the most feasible alternatives. Various considerations of using alternative propellants are outlined, and a comprehensive database of experimentally measured Hall thruster performance is compiled to pair the measured performance using various propellants to the results of a theoretical propellant performance estimation. • Condensable propellants may replace xenon in Hall thrusters. • Power requirements for condensable propellants estimated. • Iodine shown to offer similar performance to xenon at all power levels. • Molecular propellants: air, carbon dioxide and water performance in Hall thrusters. • Extensive Hall thruster performance database. [ABSTRACT FROM AUTHOR]