Electron optical observations of cathode protrusions formed during pre-breakdown conditioning

The surface structure of a hair-pin tungsten cathode has been studied during and after conditioning with a large plane anode. Microscopic protrusions whose shape is related to the anode material have been observed to grow on the cathode during conditioning. Metals of low melting point and mechanical strength such as Al, Cu and Ni lead to smooth hemispherical protrusions, whilst tungsten results in sharp conical whiskers. To observe the electrodes in situ, the experiments were conducted in the modified specimen chamber of an electron microscope; the ambient pressure was about 10 ?torr.