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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]