Hollow cathode plasma discharges are a fundamental part of a large variety of applications in industry, academia, and space. From surface processing and coatings deposition to plasma–surface interaction research and electric propulsion, advances in hollow cathode modeling and performance are critically important to the progress and evolution of these and other areas of technology. This paper describes perspectives on the progress that has been made in recent years in the capabilities and modeling of hollow cathodes used in plasma discharges. While many of the developments have been driven by the demanding requirements of electric propulsion applications, the information provided applies to all thermionic hollow cathodes and their applications. In the paper, we describe the status of 2D global simulations of hollow cathode plasmas, hollow cathode plume instabilities, and the development of higher current cathodes and low-current heaterless cathode technologies. Advances in our understanding and technology in these areas and some of the challenges that still need to be addressed and solved are discussed.
We present results from 2D axisymmetric fluid simulations of the partially-ionized Xe gas in a 25-A cathode discharge, for a range of flow rates (5-20 sccm) in which transition between the so-called spot and plume modes has been observed in laboratory experiments. The simulations for the first time capture very well the characteristic rise of the peak-to-peak amplitude in the keeper voltage oscillations with decreasing flow rate-the so-called 'plume margin'-thereby allowing for a closer interrogation of the processes that drive them. The oscillations are found to be due to plasma dynamics in the near-plume region that increase in amplitude as the flow rate is reduced. The cathode interior remains largely quiescent and changes in the emitter peak temperature do not exceed 2% across the entire range of flow rates examined. Along the cathode centerline the amplitude of the electron density oscillations peak at a location where the number density of the Xe atoms has diminished to its minimum value and the degree of ionization is close to one. The computed spectra reveal that most of the oscillation power resides in the frequency range of 100-300 kHz. These frequencies are much greater than the local ionization collision frequency (n eA I < 10 kHz), and are associated with wavelengths of several centimeters. Such wavelengths are many orders of magnitude greater than λ D . The waves are largely longitudinal with wave vector in the axial direction, and phase velocity ω/k>10 km s −1 , which is more than 6×greater than the ion acoustic speed. Though the computed and measured plume margins are in good agreement the measured spectra show that most of the power resides between ∼60 and 90 kHz, lower than the computed range (100-300 kHz), and exhibits a sharper distribution. It is argued this is due to the presence of a rotational mode in the experiment that is coupled with the longitudinal mode, yielding a complex 3D plasma motion that cannot be captured by the simulations due to the assumption of azimuthal axisymmetry, but that it is indeed the longitudinal plasma motion that drives the transition from spot to plume modes. An anomalous resistivity has been invoked in the simulations that is based on the formulations of Sagdeev and Galeev for the saturation of IAT which is long known to exist in these discharges. Though the simulations do not account for possible anomalous heating of the ions, the idealized model for the electrons is found to be sufficient in capturing the abovementioned dynamics, yielding a multiplication coefficient α of the anomalous collision frequency ν α =αω pe (T e /T i )(u e /u Te ) that is within a factor of two of the estimated theoretical value.
High-current hollow cathodes are widely used in electric propulsion as well as for laboratory plasma generation applications. The plasma region just outside of high-current cathodes is characterized by the presence of many fluctuations, which can develop coherent plasma structures affecting the operation and life of the cathode. The properties of plasma oscillations have been investigated in the exterior region of a high-current hollow cathode operating at 25–150 A of discharge current with an applied axial magnetic field. Electrostatic and magnetic probes were used to measure the electromagnetic fluctuations, and correlation analysis between each of the probes signals provides the spatiotemporal characterization of the generated waves. The results of this investigation reveal the presence of a coherent magnetohydrodynamic (MHD) azimuthal mode with a fundamental frequency of 58 kHz. This mode has features of a helical kink instability with azimuthal wave number m=1 and axial wavenumber kz=50m−1 that is readily observed in high-speed Fast-Cam images. The occurrence of this mode, the frequency, and the wavenumber at onset are found to be predicted well by the ideal MHD theory, considering the boundary condition of a non-line-tying cylindrical anode. Investigation of the MHD mode properties at different discharge currents and mass flow rate shows additional features that could qualitatively agree with resistive MHD modes. In particular, when the plasma has a finite conductivity associated with anomalous resistivity in the cathode plume, a gradual emergence of modes at relatively low discharge currents are observed.
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