Radiometric efficiency of low pressure barium discharges

Abstract

We have studied, both experimentally and numerically, the radiometric efficiency of low-pressure barium–argon positive column discharges. Using a 14 mm inner diameter alumina discharge tube to reduce chemical reactivity, we have operated discharges with Ba vapor pressures corresponding to a range of oven temperatures from 600 to 800 °C, Ar buffer gas pressures of 0.5 and 5 Torr, and currents of 100–800 mA. The spectral power distribution has been determined by examining the visible radiation emitted perpendicular to the discharge axis from a small aperture in the side of the alumina discharge tube. Absolute emitted powers were determined by comparison to a calibrated tungsten–quartz–halogen lamp. The measured efficiencies for production of visible radiation from the positive column ranged from 2.5% to 52%. These efficiencies correspond to luminous efficacies ranging from 6 to 97 lm/W. Strong emission on Ba ion lines at 455.4, 493.4, 614.2, and 649.7 nm contributed significantly to the best efficiency, 52%, and yielded a white light. A numerical model, incorporating a substantial number of excited levels and atomic processes, has been used to examine the same conditions. The model reproduces most of the major experimental trends.