Characterization of hollow cathode plasma turbulence using coherent Thomson scattering

Abstract

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. Lastly, 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 which 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.

Nonlinear interactions of ion acoustic waves explored using fast imaging decompositions

“…IAW may also be useful tools to probe sheath criteria in multiple ion plasmas [11][12][13] . Technological plasmas may also trigger IAW, that, in return, affect their operation, as reported for Hollow Cathodes 14,15 , Hall thrusters 16 and diverging magnetic nozzle thrusters 17 .…”

Effects of multi-dimensionality and energy exchange on electrostatic current-driven plasma instabilities and turbulence

“…2011; Hall et al. 2019), and recent laser Thomson scattering measurements show evidence of bi-directional plasma waves at oblique orientations to the local magnetic field (and uni-directional along the parallel direction) in the plume of a hollow cathode (Tsikata, Hara & Mazouffre 2021). Despite the increasing experimental evidence of multi-dimensional electrostatic plasma waves amplified due to kinetic instabilities, there have not been many detailed numerical studies simulating and analysing the long-term behaviour of such waves to date.…”

Full fluid moment modeling of rotating spokes in Penning-type configuration

“…Spokes are usually observed as a low-frequency propagating oscillation, compared to the fast electron motion such as the E × B and diamagnetic drifts. Such self-organized plasma structures traveling at speeds on the order of the ion acoustic speed have been observed in a variety of cross-field discharge systems, including Penning discharges [7], Hall-effect thrusters (HET) [1,5,8,9], magnetron discharges [4,10], hollow cathodes [11,12], among others [3,13].…”