Study of the Ar metastable atom population in a hollow cathode discharge by means of a hybrid model and spectrometric measurements

Abstract

The role of the metastable Ar atoms in a 1-cm-diameter cylindrical hollow cathode discharge (HCD) is studied self-consistently based on a hybrid model and experimental measurements in the pressure range of 0.3–1Torr and currents of 1–10mA. The model comprises submodels based on the principles of Monte Carlo and fluid simulations. The Monte Carlo model describes the movement of the fast electrons, fast Ar and Cu atoms, and fast Ar+ and Cu+ ions as particles, while in the fluid model, the slow electrons, Ar+, Cu+ ions, Cu, and Ar metastable atoms are treated as a continuum. The population of the two metastable states within the 3p54s configuration (P23 and P03) were combined into one collective level, for which the continuity equation was written. Typical calculation results are, among others, the two-dimensional profiles of the production and the loss rates of Ar metastable atoms, as well as the metastable atom densities and fluxes throughout the complete HCD. Moreover, the calculated radial profiles (averaged over the axial direction) of the Ar metastable atom density are compared with experimental radial density profiles recorded by laser absorption spectroscopy. The relative importance of the different processes determining the Ar metastable population is analyzed, as well as the influence of pressure and voltage on them. Experimental results evidence the presence of the metastable atom production source at the cathode surface, probably originating from fast Ar+ ions and Ar atoms impinging on it. Comparison between experimental and calculated Ar metastable atom densities shows a good agreement at low pressures, but at 1Torr the calculated values differ by a factor of 2 from the measured ones. Several possible explanations for this discrepancy are discussed.