Breakdown characteristics of metal halide plasma lamps

Nowadays, metal halide plasma lamps are compact and efficient light sources. The dynamics of their breakdown stage are of actual research interest for improving reliable lamp ignition at comparatively low voltages. In order to study the influence of additives on the lamps' breakdown phase at high pressure, a simplified lamp geometry was used, containing 5 bar Xenon and some milligrams of different additives, like mercury. To avoid streamer wall interactions along the electrode axis (electrode gap ~5 mm), the model lamps exhibit a low aspect ratio. The discharges were operated by positive voltage ramps dU/dt of approximately 0.4-100 GV/s. The breakdown voltage and current were measured by fast probe techniques. Simultaneously the spatiotemporal evolution of a single streamer event was observed by a framing camera, providing sequences of four timeframes with a minimal exposure time of 10 ns. All investigated lamps were characterized by streamer propagation. At room temperature, lamps filled with 5 bar Xenon only showed one streamer, propagating directly between the two electrode tips. UV-lighting of the whole lamp body by a mercury low pressure discharge did not influence the breakdown voltage or the streamer evolution. An addition of 1 mg mercury changed this behavior. In this case the streamers branched more and the attachment on the electrodes was at different positions on and behind the electrode tips. Lighting the lamp by UV-radiation decreased the breakdown voltage very efficiently by about 4-8 kV, depending on the different lamp fillings. To enhance the mercury vapor pressure, the lamps were heated up in an oven, while the bulb temperature was measured by a thermo camera. At heating temperatures of 300 degC, mercury vaporized completely and the breakdown voltage was reduced by about 3 kV. But this effect is much less efficient than the voltage decrease of about 5-6 kV caused by UV-lighting for all bulb temperatures even for room temperature. Further experiments are planed to study the influence of other additives like NaI or iodine, which are constituents of commercial high intensity discharge lamps and can influence the ignition phase.