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Vacuum breakdown voltage is greatly affected by the electrode surface conditions. To improve the hold-off voltage, a lot of surface conditioning processes were developed. Spark conditioning process is widely used commercially because of its efficiency and convenience. However the mechanism for the increase of the hold-off voltage due to spark conditioning is not completely clear. This paper describes vacuum electrical breakdown characteristics for electrodes paired with cathodes and with anodes processed by spark conditioning. Two pairs of electrodes, which were named Pair A and Pair B, were prepared. They were processed by spark conditioning procedure. The conditioning procedure was 500 times repetitive breakdowns by applying positive impulse voltage under ultra-high vacuum condition. Figure 1 shows the breakdown characteristics for two pair of oxygen free copper electrodes under 500 times spark conditioning process. As shown in Fig. 1, these gaps were almost same properties. First breakdown fields are about 10MV/m. It is found that there is a significant conditioning effect. Achieved breakdown fields by 500 times repetitive spark conditioning are about 100MV/m and 120MV/m. Using these processed electrodes, two vacuum gaps were made up by pairing of electrodes used as cathode in spark conditioning process and as anode, respectively. 50 times repetitive breakdown tests were carried out using cathode vs. cathode pair and anode vs. anode pair. Figures 2 and 3 show the breakdown characteristics for cathode vs. cathode pair and anode vs. anode pair, respectively. In case of a pair of cathode vs. cathode pair, the first breakdown field kept high breakdown field which was almost the same value for the achieved breakdown field by 500 times repetitive spark conditioning. A significant conditioning effect could not be confirmed from Fig.2. From Fig.3, the first field for anode vs. anode pair was much less than that for cathode vs. cathode pair. Moreover a significant conditioning effect is achieved. The achieved field after 50 times breakdowns is lager than that for the gap used a pair of cathodes. This is due to the difference of the surface condition between anode and cathode surfaces that were formed by spark conditioning. According to SEM images for anode and cathode after spark conditioning, the profiles had quit difference between anode and cathode surface.

Vacuum Breakdown Characteristics for Electrodes Paired with Cathodes and with Anodes Processed by Spark Conditioning