Heat transfer studies of strontium recombination lasers

Abstract

Thermal loading models have been developed in order to predict the optimum input powers of free-convection cooled strontium recombination lasers (SRLs) and forced-convection cooled SRLs. Gas temperature models have also been developed for circular bore and rectangular bore SRLs, to complement the thermal loading models, so that the performances of circular and rectangular bore SRLs can be compared. It has been shown, in theory, that rectangular bore tubes offer operation at lower gas temperatures than do circular bore tubes of the same cross-sectional area, with the same input power per unit length. Laser performance is, therefore, expected to be improved through the use of rectangular bores in the construction of SRLs. The results of the thermal loading models are validated by experiments. The need to oven process alumina tubes in forming gas, prior to their use in the SRL, is demonstrated by an increase in output power from tubes processed using this technique. Enhanced radiative heat extraction, accompanied by an increase in average laser output power, is obtained from a free-convection cooled SRL by increasing the emissivity of the outer surface of the ceramic discharge tube. Three rectangular bore water-cooled SRLs, optimising at different input powers, have been constructed, operated and compared in performance to a free-convection cooled SRL, constructed using the same ceramic discharge tube. An average output power of 2.3 W has been obtained from a 40 cm long rectangular bore beryllia discharge tube through the use of active water cooling. This represents an improvement of over 200% when compared to the performance of a free-convection cooled SRL utilizing the same ceramic discharge tube.