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This work provides measurements of the ion-induced electron emission yield caused by keV-range energy indium ions (3–8 keV). In our experiment, the ion sources, fueled with indium, are based on the working principle of field-emission electric propulsion (FEEP) technologies. The measured yields are given for different material properties (plain structure vs foam) and geometries (flat surface vs conical). Study outcomes show that electrons induced by keV-range indium ions can lead to overestimating the measured ion current by 30%–200% depending on the collector materials and geometry. We spotlighted that molybdenum, tungsten, and stainless steel 316L have relatively low yields compared to aluminum 2017A. Moreover, it is possible to lower down a specific material yield by two-thirds when using a foam structure. Finally, it is shown that off-axis ion influence on a collector increases the resulting electron emission yield. In the case of an indium-FEEP thruster, ion-induced electron emissions are a major perturbation when measuring ion current. Mainly, due to the energy range of ions studied, yields retrieved are orders of magnitudes larger than what is usually measured when studying the plume of an electrostatic electric propulsion system. It is, therefore, strongly recommended to use a so-called Faraday cup to study the ion beam from an indium-FEEP thruster as the probe's close architecture allows us to greatly mitigate these perturbations. [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]

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]

An alternative Hall thruster architecture that shifts the ionization and acceleration regions outside the plasma chamber is demonstrated. This unconventional design is here termed a "wall-less Hall thruster," as the bulk of the magnetized discharge is no longer limited by solid boundaries. A 200W prototype with permanent magnets has been developed and characterized. Experimental results concerning the thruster operation, discharge oscillations, electric field distribution, and ionization zone characteristics are presented and discussed. Our first experiments show that the crossfield discharge can be moved outside the cavity without drastically disturbing the ion production and acceleration mechanisms. This design offers the benefit of reduced plasma-wall interaction and lower wall losses, while also greatly facilitating diagnostic access to the entire discharge ionization and acceleration regions. [ABSTRACT FROM AUTHOR]

The characterization of an electric propulsion device cathode is performed in the so-called diode configuration with an external anode. The anode acts as a physical boundary for the cathode plasma discharge; therefore, it influences cathode operation and performances. In this study, four different anodes—namely, a disk, a plate, a long cylinder, and a short cylinder—have been used with a flat disk LaB6 emitter 5 A-class cathode to examine the anode geometry impact on cathode discharge properties. Current–voltage curves, discharge oscillations, electron parameters, and ion velocities have been measured for currents in the 2 A to 12 A range and xenon mass flow rates varied from 0.4 mg/s to 1 mg/s with a fixed cathode-to-anode distance. The set of results clearly supports the fact that the anode geometry strongly influences the cathode characteristics both at the macroscopic and the microscopic scale. [ABSTRACT FROM AUTHOR]

We compare time-averaged and time-varying measured and computed ion velocity distribution functions in a Hall effect thruster for typical operating conditions. The ion properties are measured by means of laser induced fluorescence spectroscopy. Simulations of the plasma properties are performed with a two-dimensional hybrid model. In the electron fluid description of the hybrid model, the anomalous transport responsible for the electron diffusion across the magnetic field barrier is deduced from the experimental profile of the time-averaged electric field. The use of a steady state anomalous mobility profile allows the hybrid model to capture some properties like the time-averaged ion mean velocity. Yet, the model fails at reproducing the time evolution of the ion velocity. This fact reveals a complex underlying physics that necessitates to account for the electron dynamics over a short time-scale. This study also shows the necessity for electron temperature measurements. Moreover, the strength of the self-magnetic field due to the rotating Hall current is found negligible. [ABSTRACT FROM AUTHOR]

A semiempirical time-dependent thermal model of Hall effect thrusters was developed to compute evolution of the thruster channel wall temperature during a transient regime. The model accounts for heat conduction through dielectric walls and radiative transfer as the channel is considered as a thermal enclosure. The graybody view factors are experimentally determined during the cooling period that follows a power shutdown. The model permits the assessment of the amount of power delivered to walls by fitting calculated temperature profiles to experimental profiles obtained by means of calibrated infrared imaging. Plasma-surface interactions are examined in this contribution for three types of thrusters, namely, SPT100-ML, PPS®1350-G, and the 5-kW-class PPSX000-ML. The electrical input power range that is probed stretches out from 500 W to 5 kW. The energy flux passed to the discharge chamber dielectric walls varies between 0.1 and 2 W/cm2. The overall amount of power delivered to the external and internal walls is found to vary linearly with the applied power. It is in favor of an energy loss mechanism driven by ion bombardment. A departure from the linear trend is observed at high discharge voltage. Production of multiply charged ion species could be an explanation. The impact of thruster design and wall material on power losses inside the thruster channel is also investigated from both a technological and a physics standpoint. [ABSTRACT FROM AUTHOR]

Electron temperature and plasma density, as well as the electron energy distribution function (EEDF), have been obtained inside and outside the dielectric channel of a 200 W permanent magnet Hall thruster. Measurements were carried out by means of a cylindrical Langmuir probe mounted onto a compact fast moving translation stage. The 3D particle-in cell numerical simulations complement experiments. The model accounts for the crossed electric and magnetic field configuration in a weakly collisional regime where only electrons are magnetized. Since only the electron dynamics is of interest in this study, an artificial mass of ions corresponding to mi = 30 000me was used to ensure ions could be assumed at rest. The simulation domain is located at the thruster exit plane and does not include the cathode. The measured EEDF evidences a high-energy electron population that is superimposed onto the low energy bulk population outside the channel. Inside the channel, the EEDF is close to Maxwellian. Both the experimental and numerical EEDF depart from an equilibrium distribution at the channel exit plane, a region of high magnetic field. We therefore conclude that the fast electron group found in the experiment corresponds to the electrons emitted by the external cathode that reach the thruster discharge without experiencing collision events. [ABSTRACT FROM AUTHOR]

The dynamic behavior of the Hall effect thruster plasma plume is investigated after ultrafast current ignition driven by a metal-oxide-semiconductor field-effect transistor MOSFET power switch. Time-resolved optical measurements performed with a gated intensified charged coupled device (CCD) camera allow to reconstruct the plume temporal features. Images of the ion beam reveal oscillations in plasma light intensity linked to 'breathing' instabilities. The observed periodic variations of the beam divergence originate from the displacement of the ionization layer within the thruster magnetic barrier. Index Terms--Breathing modes, Hall thruster, optical emission, plasma instabilities.

The amount of ions and atomic radicals contained in an expanding thermal hydrogen plasma, which is produced by a wall-stabilized dc arc, depends on the source nozzle geometry and material. The nature of the radiation emanating from the plasma jet is also influenced by the nozzle properties. In the light of experimental data obtained by means of laser-aided diagnostics, these striking facts can be explained by the wall-association processes of hydrogen atoms to form vibrationally excited hydrogen molecules. Index Terms--Expansion, hydrogen, plasma, recombination, wall.