Numerical investigation of different modes of arc attachment at tungsten cathodes in dependence on the electrode geometry

Summary form only given. Diffuse and spot-like modes of arc attachment at thermionically emitting cathodes are observed in lamps and reproduced by numerical simulations. Examples of different modes on rod shaped electrodes made of pure tungsten were investigated by measurements in the Bochum model lamp being operated with argon at a pressure of p=0.26 MPa. The corresponding simulations are based on a model of the cathode boundary layer which allows the decoupling of the cathode from the bulk plasma and formulates the plasma interaction with the electrode surface as a function of the local cathode temperature and a global cathode fall. Different modes of arc attachment are simulated in dependence on the diameter, length and the edge radius of the rounded top of a cylindrical tungsten electrode for conditions which correspond to those in the model lamp. Solutions for spot modes are found in dependence on the electrode geometry which branch off from the diffuse mode as well as others which do not branch off. Additionally, the stability of the different modes is investigated. The results are compared in part with experimental findings.