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An investigation has been made on the initial separation of high-current contact electrodes. Three distinct voltage-time characteristics are observed. Two of these correspond to the formation of a molten-metal bridge and the third corresponds to an abrupt transition from metallic contact to an arc. Two ideal cases are analyzed theoretically from which a model is developed. The model is then related to the three voltage-time characteristics observed.

Time-resolved optical studies were made of the cathode-fall region of a wall-bounded pulsed discharge in hydrogen. The temporal variation and spatial configuration of the electric fields within the cathode-fall region are presented as a function of the experimental parameters. The short cathode-fall region formed within 0.1 μsec and remained stable for the duration of the applied-voltage pulse. This rapid formation requires that the positive-ion space-charge cloud near the cathode face be formed locally. It is speculated that a fast initial breakdown wave leaves behind a low-density plasma, which then goes into the equilibrium configuration by an over-damped plasma oscillation mode.

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Extensive exposure to tokamak plasmas may result in significant alterations to the surface microstructure of graphite plasma-facing components. A change in microstructure from a commercial isotropic graphite to an amorphous carbon film may produce a significant change in the total sputtering yield and the level of plasma contamination. To investigate this sensitivity to surface microstructure, sputtering experiments on a variety of graphites with various surface structures were performed using the ion–surface interaction system (ISIS).1 ISIS is a computerized ion beam sputtering system equipped with twin quartz crystal microbalances capable of simultaneously monitoring both sputtering and redeposition of the beam target material. ISIS was used to obtain sputtering data on two orientations of pyrolytic graphite at seven energies between 100 eV and 10 keV. Helium bombardment perpendicular to the prism plane produced yields 2 to 7 times higher than on the basal plane. Proton bombardment perpendicular to the prism plane produced yields 45% higher than those on the basal plane. Amorphous graphite films produced from Poco AXF-5Q and Union Carbide ATJ graphites using an argon radio-frequency (rf) plasma discharge were also irradiated. Sputtering yields on the amorphous films were as much as 50% to an order of magnitude higher than those measured on commercial bulk samples. Pre and post-irradiation scanning electron microscopy of selected targets was performed to monitor surface microstructure. A structural mechanism responsible for the magnitude of physical sputtering is suggested, and an effective surface binding energy is introduced to quantify this structural dependence.