The excitation structure in a micro-hollow cathode discharge in the normal regime at medium argon pressure

A microplasma is generated in the micro-hole (400 mu m diameter) of a molybdenum-alumina-molybdenum sandwich (MHCD type) at medium pressure (30-200 Torr) in pure argon. Imaging and emission spectroscopy have been used to study the light emission mechanisms in the micro-discharge. We find that the emission intensities of both argon atom and argon ion lines present sharp peaks located near the cathode, and that the position of these peaks moves towards the cathode with increasing gas pressure. An ionizing-sheath model has been developed to calculate the evolution of the sheath thickness as a function of the gas pressure. This model includes the ratio between the ion flux coming from the bulk of the plasma and the ion flux created in the sheath and also accounts for the gas heating in the micro-hole. The variation of the calculated ionizing-sheath thickness as a function of pressure agrees well with the variation of the measured peak positions. The case of a sheath free of ionization has also been considered. However, the comparison between experiments and the various theories shows that a significant fraction of the ionization occurs in the sheath.