Electron properties of an emissive cathode: analysis with incoherent thomson scattering, fluid simulations and Langmuir probe measurements

Abstract

Emissive cathodes serve a key function as the electron source for Hall plasma thrusters. Development of reliable numerical models for their operation and information on plasma behavior requires the use of sensitive and accurate diagnostics. This paper discusses results from a combined experimental and numerical study of the plasma regimes and properties of a 5 A LaB 6 pellet emitter cathode. Electron properties across a range of currents (2-16 A) are determined for the first time using incoherent Thomson scattering. A 2D axisymmetric fluid code is adapted to the study of the electron properties of this cathode and the nature of regime transitions. Similar trends in the numerical and experimental results are found, such as transitions to plume-like behavior with increasing current; quantitative agreement would require the use of a combined thermal-plasma model for the cathode under study. Comparisons of results from non-invasive Thomson scattering to Langmuir probe measurements reveal limitations of the latter diagnostic for plasma characterization.

Discharge and plasma plume characterization of a 100 A-class LaB6 hollow cathode

“…Experimental data plotted in Fig. 11 indeed shows an increase of n e with I d , in agreement with other studies [36]. Interestingly, two regions can be distinguished in the graph according to the slope of the n e against I d curve, the frontier being around I d ≈ 35 A.…”

“…The electron density decreases along the cathode plume centerline in all cases as the cathode plasma expands into vacuum behind the orifice. Similar results have been obtaied in the jet of a 5 A LaB 6 hollow cathode by means of ITS and Langmuir probe [20,36]. The density decay is steeper at large discharge current: the decrease in n e reaches almost one order of magnitude at 70 A, whereas n e drops by a factor of 5 at 30 A.…”

“…Figure 10 shows the electron temperature decreases along the cathode axis whatever the operating conditions, similarly to what was found experimentally and with computer simulations for a 5 A cathode [36] and for a 25 A cathode [29]. The electron cooling downstream the cathode orifice for all currents originates in energy transfer through collisions and plasma flow expansion [37].…”

“…In that case individual electron fluctuations can be observed, allowing electron density and temperature to be determined. Details about the compact high-sensitivity ITS bench used in this study can be found in reference [34][35][36]. Only a short description is given here.…”

“…9 gives values of electron temperature and density of, respectively, 1.1 eV and 7.6 × 10 17 m −3 . A detailed systematic uncertainty analysis of the ITS spectra has been conducted in a previous work [36]. It was explained that the two main sources of uncertainties originate from the calibration through the uncertainty on the Raman scattering cross-section and from the Gaussian fitting procedure.…”