Transient non-classical transport in the hollow cathode plume II: evaluation of models for the anomalous collision frequency

The ability of fluid-based closure models to describe the non-classical electron collision frequency in the plume of a hollow cathode is experimentally investigated. Six models-all predicated on the assumption that the non-classical collision frequency can be attributed to ion acoustic turbulence (IAT)-are considered. Experimental measurements of the time-resolved plasma properties in the cathode plume (Georgin M P, Jorns B A and Gallimore A D 2020 PlasmaSources Sci. Technol, 29 105010) are used to evaluate each closure model and compare it to experimental measurements of the effective electron collision frequency. Though more than one of the considered closures can predict the time-average behavior of the plasma in the cathode plume, it is found that only one model accurately predicts the measurements in both space and time for the cathode and operating conditions that were studied. This new highest fidelity model is derived using a single-equation approach based on modeling the average frequency of the IAT as it evolves in space and time. The implications of the success of this model are discussed in the context of the understanding of the dynamics of the IAT in the cathode plume as well as on-going fluid-based modeling efforts related to cathode plumes.