Studies on arcing phenomena at high current discharges

The thesis is concerned with arcing processes and related cathode phenomena for high pressure xenon short arc lamps, TIG-welding arcs and high current pulsed light sources. Especially, the mechanisms of emission and erosion at activated cathodes are considered for different arcing modes in gaseous arcs. The microscopic arcing processes at cathodes with inhomogeneously distributed ThO2-, BaO-, La2O3-, ZrO-activators and cathodes covered by Th-layers are studied in comparison to the microscopic processes at pure metal cathodes. Discharge experiments have been carried out to determine the influence of cathode properties on the arc behaviour. The composition of the cathodes and the type of activators were changed over a wide range. Further, cathodes with thorium layers produced by means of ion beam sputter deposition in the thickness range of some hundred nm were found to show improved ignition and emission behaviour and an enhanced resistance to erosion processes. The changes of the cathodes due to arcing were studied by means of surface analysis techniques such as X-ray microanalysis, scanning electron microscopy (SEM), secondary electron spectroscopy, SIMS and cathodoluminescence (CL)-investigations characterising the cathodes before and after arcing. The distributions of emitter oxides and the chemical composition of the cathodes after different arcing times were measured. The investigations on emission and erosion phenomena have given considerable indications of the strong dependence of arcing processes on the cathode properties in micro-areas. The results of these investigations in gaseous arcs are interpreted in comparison with corresponding arcs in vacuum and the new experimental knowledge achieved in studies on microscopic arcing processes at activated cathodes is discussed and compared to the behaviour of arc spots at pure metal cathodes. (author)