Langmuir analysis of electron beam induced plasma in environmental TEM

The characterisation of the gas state under electron impact is of high importance for the understanding of materials in-situ environmental transmission electron microscopy (ETEM) experiments. We report on the formation of a dilute plasma state of Ar and He based on the development of a Langmuir probe as a plasma diagnostic tool for the differentially pumped volume at the TEM sample position in the octagon. In order to keep stray magnetic field influences of the objective lens small, and thus the results readily interpretable within existing theory, the experiments are performed in Lorentz mode. The applicability of Langmuir type analysis for the identification of plasma parameters, such as ion density, plasma temperature, sheath extension and electron energy distribution is examined. The systematic study as a function of gas pressure and beam current reveals cold plasma characteristics with electron temperatures of around 0.4 eV and ion and electron densities that are orders of magnitude below the expected values from ionisation cross sections. The loss of charged particles appears to be due to surface collisions at the electrically conducting pole piece surfaces as well as at the conductive parts of the TEM holder.