Spectroscopic and comparative study of He-Cd plasma created in perforated, slitted and helical hollow cathodes for trichromic laser applications

Abstract

Presents a comparative study of the He-Cd discharges created in five hollow cathodes with different geometries. The cell comprises an outer cylindrical anode and an inner cathode. All the cathodes used are cylindrical (4 mm inner OE, 6 mm outer OE); the first is perforated, the second slitted and the last three are helical with 2, 3 and 4 mm pitches, respectively. The five plasmas are studied by spectroscopy. The electron energy distribution function is deduced from the measurements. It appears that the discharge created in the helical cathode with pitch>or=3 mm has many advantages. Indeed, this discharge is very stable; the cadmium vapour diffuses easily because of the great transparency (>50%) of the cathode. The discharge is not a real hollow cathode type because there is a glow outside the cathode. However, the negative glows overlap inside the cathode and the plasma intensity is strong. The Cd+ laser lines are the most intense in such a structure. The particular geometry of the helix gives rise to a mixing of the active species of the plasma which increases their efficiency. There are fewer high energy electrons in this helical structure relative to the perforated or slitted ones, which have a weak transparency, leading to the hollow cathode effect.