Time evolution of plasma parameters in the jet of a low-power vacuum arc thruster

Abstract

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.