Improved Ignition Efficiency in Method for Laser-induced Noble Gas Plasma Light Source

High brightness and stability, a broadband spectrum, and a long lifespan characterize a laser-induced noble gas plasma light source. This study shows the effects of applying the target ignition method to a new structure for four target materials (tungsten, platinum, thoriated tungsten, and tungsten impregnated with barium oxide (BI)). Compared with previous experiments, our proposed conditions enable practical use of this high brightness light source with a one order of magnitude lower laser power. First, the proposed structure is different from a previous wire structures for the target method. Second, the ignition efficiency of each material was quantitatively evaluated according to heating time. Results showed that among the four materials, BI could be ignited with the lowest laser power (continuous wave: 35 W), and could best repeat ignition. This implies that the work function determines ignition efficiency. Finally, we clarify the ignition process. Barium atoms and ions from the target were experimentally observed under laser irradiation in the xenon gas region before ignition. Barium has a low work function as an electron emission material. Therefore, we hypothesized that the target surface emitted barium into the xenon gas region by laser heating. We expected that the barium would rapidly discharge many electrons owing to its low work function, the inverse bremsstrahlung absorption would instantaneously increase, and finally, it would ignite the xenon gas.