Hollow Cathode Discharges for Gas Lasers

With the exception of the 632.8 nm neon line all the important visible and ultraviolet cw gas laser lines belong to the spectra of ionized atoms. To achieve proper laser oscillation the excitation of the upper levels of these lines is of primary importance. High energy electrons are needed for this process. As such electrons are in high concentration in the hollow cathode discharge, it was obvious that hollow cathode discharge should be utilized for these lasers. However, to meet laser requirements, there was a need to increase the volume of the discharge and to increase the current density as well as the pressure. Because current became very high the problem of the glow-to-arc transition in such discharge became very serious since arcing not only stops the laser oscillation but may also damage the discharge tube. The efficiency of these lasers depends on the possibility to excite the ionic spectrum. Excitation can be increased by increasing the high energy part in the electron energy distribution. This "beam" of fast electrons is produced by the cathode fall, so the possibility to increase the voltage of the discharge also became a question of interest. The present paper deals with the hollow cathode discharge applied to gas (metal vapour) lasers. First, a brief summary is given of the basic discharge phenomena important to this topic. The main constructions of different hollow cathode lasers (noble gas mixture, metal vapour, cathode sputtered) are also surveyed. Detailed measurements on the arc free discharge region and the glow-to-arc transition are presented. These measurements enabled the building of hollow cathode lasers with considerably higher current than was earlier possible. The question of increasing the voltage is also discussed and different constructions are presented