Different modes of arc attachment at HID cathodes: simulation and comparison with measurements

Based on a model for the plasma boundary layer of high intensity discharge cathodes, simulations are performed and compared with experimental results. To solve the power balance of the cathode body different methods are used, namely a 1D integral solution as well as 1D, 2D and 3D finite-element calculations. The simulations are done for cylindrical tungsten cathodes operated in different pure noble gas discharges (0.1-1.0 MPa) and with currents between 0.5 and 10 A. Under these conditions different modes of arc attachment are found, both in simulations and experiments. For the diffuse mode of arc attachment an excellent quantitative agreement between measurements and the simulations is obtained, reflecting an improved accuracy of measurements and simulation. In addition, different spot modes are found. At least one of these. modes is also observed in the experiment. Also for this spot mode the agreement between measurements and simulation for the integral quantities is good. There are still some open questions concerning the spot mode of cathodic arc attachment. Varying the geometric dimensions of the cathode, the proper simulation of the heat conduction problem of the cathode body is shown. Variations of the plasma properties, like gas type and pressure, prove the conceptional capability of the boundary layer model for the simulation of different modes of arc attachment. Evaluating the cathode fall characteristics, regions of existence for the different modes are found, which are similar to the experiments.