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We report measurement of electron-emission yield (EEY) under the impact of electrons on materials of Hall-effect-thruster (HET) interest: BN, BN-SiO 2 , and Al 2 O 3 . The effects of the material aging (under electron irradiation) on the yield of BN and Al 2 O 3 are investigated. The EEY of BN grows with electron exposure, whereas that of Al 2 O 3 reduces. A simple analysis of our experimental results indicates that these variations are most likely because of surface and near surface composition changes caused by the electron beam. The representativeness of EEY measurements on ceramics that have not suffered from the specific environment of a HET (ion and electron bombardment) is discussed.

Experiments are performed on polycrystalline MgO in order to investigate the internal effects of positive charging on the total electron emission yield (TEEY) under low incident electron fluence (a few to hundreds of fC mm−2) at 200 eV electron irradiation energy. It is experimentally shown that the Coulomb-attractive recombination of electrons undergoing emission with holes generated by the previously emitted electrons significantly affects the TEEY. Furthermore, quantitative analyses of experimental data are done to deduce the electron–hole recombination cross-section which is found to be of a few 10−11 cm−2.

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A review of the contamination physics and of the most widespread engineering approaches to contamination assessment was carried out. The two main approaches are the physical and the empirical one. The main questions still open to validate the physical approach to outgassing and deposit physics were then studied. Among others, special attention was paid to the important point of a realistic separation of chemical species, probably a prerequisite for a physical modeling. Several original results were obtained. Some lead to a quite clear conclusion, like the preeminence of the limitation by desorption over the limitation by diffusion for outgassing. This observed trend needs yet to be validated on other materials. Other major results are progress on the validation of the physical approach and on the ambitious species separation program. Nomenclature D = Fick's law diffusion coefficient, cm 2 :s 1 E A = activation energy, J:K:mol 1 f = contaminant flux density, g:cm 2 :s 1 k = outgassing kinetic constant, s 1 L = effective diffusion length of the sample, m M = contaminant molar mass, g=mol m, m evap = deposited, evaporated or outgassed surfacic mass, g:cm 2 m 0 = contaminant monolayer mass, g:cm 2 n vol = contaminant volume density, g:cm 3 n surf = contaminant surface density, g:cm 2 P S = saturation vapor pressure, mbar R = gas constant, 8:314 J:K:mol 1 T = temperature, K t = time, s W = relative contaminant mass, % = outgassing or deposit evaporation characteristic time, s

A technique for the determination of the total electron emission yield (TEEY) of insulators or insulated conductors, due to low energy electron irradiation is described. It is based on the measurement of the surface potential variation induced by a pulsed electron beam with the help of a high-sensitivity Kelvin probe. The results obtained on a floating copper sample are in agreement with those obtained by the well-known electron-collector method, on a grounded copper sample. We illustrate the use of the technique on insulators with measurements on Kapton-HN. The measured TEEY of Kapton-HN is compared with those given in the literature with a relatively good consistency.

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