Microscopic high speed investigations of vacuum arc cathode spots

The main parameters and dimensions of cathode spots have been under discussion for years. To solve these current questions, a new system was specially designed. The image converting high speed framing camera (HSFC) combines a micrometer lateral resolution with a nanosecond time resolution and a very high optical sensitivity. This camera was used to study the microscopic behavior of vacuum arc cathode spots in a pulsed high current arc discharge on copper. The direct observation of these spots with high resolution revealed that one single cathode spot, as normally observed by optical means, consists of a number of simultaneously existing microscopic subspots, each with a diameter of about 10 μm and a mean distance of 30-50 μm between them. The mean existence time of these subspots on copper was found to be about 3 μs, where the position of a subspot remains unchanged (with an upper limit of about 5 μm) during its existence time. The lower limit of the current density in the cathode spots was estimated to be on the order of 10¹⁰-10¹¹ A/m². An upper limit of the crater surface temperature was estimated by a comparison between the brightnesses of a cathode spot and of a black body radiation lamp to about 3000 K