Modelling and experiment on microdischarges in metal vapour of Cadmium

Microdisharges in Cadmium metal vapour occur in a testing equipment for the safety assessment of electric devices for explosion protection, where they represent an ignition source in a hydrogen/air mixture ($\varphi=21 \% \mathrm{H}_{2}$). In this work, electric discharges in a metal vapour of Cadmium released from the cathode during contact opening are studied by means of a unified one-dimensional model [1]. The model employs the drift-diffusion approximation of the electron kinetics, the general species equations for atoms and ions of Cadmium, as well as the heat transfer in the plasma and the electrodes. The electron current density due to a field emission from the cathode is computed applying the transfer matrix method [2]. Results are obtained for electrode gap lengths of ($60-160) \mu \mathrm{m}$ and compared with the voltage measured in related experiments [3]. For a constant electric current of 60 mA, the discharge voltage amounts to about (12-30) V depending on the gap length. The model provides the spatial structure of the discharge: the lengths of the space-charge sheath adjacent to the electrodes, species densities, different temperatures of electrons and heavy particles indicating a deviation from local thermodynamic equilibrium.