The Vacuum Arc Cathode , - A Phenomenon of Many Aspects

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Abstract
Cathode spot theory provides two limiting approaches for the description of reality: The SMOOTH SURFACE MODEL and the ROUGH SURFACE MODE. For the SMOOTH SURFACE MODEL limits of the typical parameters, like current density and spot temperature can be calculated. For the SPOT AT REST we find only the (0)-MODE with typical current density values of 0(105A/cm2) on copper. For the MOVING SPOT we have beside the (0)-MODE also the (1)-MODE with typical current densities 0(109A/cm2). Which of these two modes the moving spot occupies cannot be judged at the time being since the corresponding stability considerations are to complicated. Steenbeck's principal is not applicable to the moving spot due to the non-stationarity. The theoretical limits calculated are not very narrow but measured at the experimental uncertainty the results are quite useful. Higher accuracy - as some people hope for - is not accessible yet for several reasons: First, there are serious limits in our experimental information about the accommidation coefficients of ions at a surface above boiling point, the material loss at such a surface, the temperature dependence of the material parameters and the migration of Joule energy in a high temperature cathode locally above boiling point. Second, we have no theoretical information yet about the radial and the anode side distributions of the cathodic plasma ball with its complicated kinetics. Third, the average effects from the surface roughness influence the smooth surface results and shift the data.

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