The plasma parameters temporal evolution of a 30 W-class Vacuum Arc Thruster equipped with a Ni-Cr cathode is examined in the far-field region of the plasma jet. Measurements have been performed 20 cm downstream of the arc region where the plasma is created. The thruster operated at 1 Hz with 25 μs duration current pulses. The maximum current intensity reaches 4000 A about 8 μs after the discharge ignition. The change in the electron density, electron temperature and plasma potential during the short high-current pulse has been obtained by means of time-resolved Langmuir probe measurements. A time-of-flight technique based on a planar probe has been used to determine the mean ion velocity in the plasma jet. In addition, a Faraday cup allowed the determination of the ion current density. The electron density peaks at ∼ 8×1017 m−3 at 12 μs. The electron temperature is above 25 eV at 7 μs and then it stabilizes around 5 eV. Several ion populations are identified. The lowest ion mean velocity amounts to 20 km/s. Interestingly, combining all data allows to assess the ion mean electrical charge and its temporal behavior. The mean charge reaches 12 a few μs after ignition. It decays quickly afterwards and stays unchanged at 2 beyond 10 μs. Analysis of all the results support the idea of two distinct plasma discharge regimes.

We study experimentally and theoretically the time evolution of the electron temperature Te of a photoionized barium vapor which expands freely into a vacuum. Using the gas dynamics fundamental equations and assuming the plasma expansion to be adiabatic, a model is built and analytical expressions are derived for the electron and ion temperatures, velocity, and density time evolutions. The experimental apparatus consists essentially of a vacuum chamber, a Joule effect furnace which produces the Ba vapor. A cylindrical plasma created between the two vertical plates is produced by two-step ionization of the vapor. The electron temperature is measured with a cylindrical electrostatic probe biased by a slowly variable voltage ramp. The current-voltage curve is built step by step with a boxcar averager. The results involve different parameter variations like vapor density or sampling time of the probe current for studying the time evolution of electron temperature. Finally, it was found that the adiabatic cooling model agrees well with the experimental electron temperature evolution, but was limited below 0.025 eV by the Langmuir probe accuracy.

The aim of this paper is to explore the possibility of simultaneously characterizing instabilities in vacuum arc thrusters on a broadband frequency range (kHz–GHz) in situ, meaning in a conventionally used vacuum chamber where vacuum arc thrusters are usually operated. We show in this paper that, despite the reverberant nature of the vacuum metallic chamber, useful information is extracted over a large frequency range from the measurements done with an antenna in this environment. This leads to the first characterization of the self-emission of a vacuum arc thruster in the GHz frequency range.

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