Recent experiments on vacuum breakdown of oxygen-free copper electrodes

The effects of the gas content, in situ sputter cleaning, annealing and a mirror finish of OFC (oxygen-free copper) electrodes on the electrical breakdown in a vacuum were investigated. The grade of the OFC meets the ASTM-F-68 class 1 standard. ECB (electrochemical buffing) or diamond turning was employed for obtaining a mirror finish. It was found that every one of parameters investigated, namely, low gas content, in situ sputter cleaning and annealing, increases the breakdown fields after conditioning due to repetitive breakdowns. In addition, a higher annealing temperature produced higher breakdown fields. The mirror finish reduced the required number of breakdowns (conditioning procedure by repetitive breakdowns) to improve the insulating capability of the vacuum gap. A breakdown field of ~250 MV/m was obtained by a combination of annealing at 700°C and a mirror finish by diamond turning. It has been pointed out that the combination of these treatments is necessary for obtaining higher breakdown fields. The effect of annealing and diamond turning is discussed on the basis of the recrystallization of copper and residual stresses on the surface