The laboratory characterization of the interaction between a hollow cathode plasma source and an ambient plasma of ionospheric type is reported. The investigation was carried out by means of Langmuir probes for various positive polarization levels of the source with respect to the ambient plasma in absence of magnetic field. Measurements put into evidence the formation of a double layer that dominates the current collection process on the hollow cathode source. The electron energy distribution function, derived along the axis of the current path, indicates the presence of an anomalous collisional mechanism due to the bumphyphen;inhyphen;tail instability in the region between the double layer and the source. The experimental data were also compared with the results of a onehyphen;dimensional theoretical model of the interaction; taking into account the approximation of the model, such a comparison indicates that both the position and the voltage drop associated to the double layer match rather well the theoretical predictions.
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