Analyzing spotless mode of current transfer to cathodes of metal-vapor arcs

The diffuse, or spotless, mode of current transfer has been observed on cathodes of vacuum arcs under conditions where the average cathode temperature was high enough, about 2000 K. It has been known for many years that none of the known mechanisms of current transfer to cathodes of vacuum arcs and, in particular, of the electron emission is capable of producing the current densities of the order of 10⁵-10⁶ Am^-2 deduced from the experiment. A fresh attempt to clarify this question is made in this work. Cathodes made of chromium are considered, on which the most of the experiments have been performed. It is shown that an account of the difference between values of thermionic and photoelectric work functions given in the reference literature allows one to significantly reduce the deviation between the theory and the experiment. Unfortunately, data on thermionic work function available in the literature refer to the cathode surface temperatures below 1400 K, which is significantly smaller than measured temperatures of the chromium cathodes of vacuum arcs operating in the spotless mode. Therefore, further experimental data are needed in order to clarify this effect.