Solidification of hat craters on the cathode of vacuum arcs

A dynamic model of the cool-down phase of an inactive cathode crater is used for the determination of the solidification time tau c . Results are presented for Cu and W. Heat conduction, phase change, the motion of the melt and surface cooling due to evaporation and radiation are taken into consideration. Because of the large values of tau c (large in comparison with the crater formation time tau f ) the afterglow of solidifying craters will influence optical measurements of the spot area and thus falsify estimations of the current density in the spot. Solidification time tau c and with it the mass flow from the crater depend on the cathode bulk temperature T 0 . This effect could be one reason for the observed increase of crater size and erosion rate with T 0 .