Effect of Cathode Length on Electrical Characteristics of a Microhollow Cathode Discharge in Helium

Abstract

The electrical characteristics of a microhollow cathode discharge (MHCD) have been measured over a wide range of helium gas pressures from 10 to 900 Torr, by using cathodes of 0.5 mm diameter and 0.5–3.0 mm length. A relatively high voltage of the Paschen minimum at high pressure for the shortest cathode and a conventional hollow cathode mode for longer ones were observed. These are explained by a change of the plasma loss owing to the difference in cathode length. The radial extent of the negative glow inside the cathode hole increased with discharge current, and eventually spread along the outer surface. The threshold current for the extension of the plasma outside the cathode hole increased in proportion to the cathode length and to the square of the gas pressure. Thus, the longer cathode provides a favorable condition for the generation of reactive MHCD plasma at high-pressure and high-current operation. On the basis of these results and spectroscopic observation, the sustaining mechanism of an MHCD is briefly discussed.