Effective High-Frequency AC Conditioning in Vacuum

To improve the dielectric strength in a vacuum, spark conditioning through repetitive breakdowns by ac voltage application is an effective method. High-frequency ac conditioning is expected for the higher dielectric strength after conditioning, but the optimized frequency and other conditions for effective ac conditioning have not been clarified. In this article, applied ac voltage frequency and electrode material are investigated as parameters for the dielectric strength improvement by ac conditioning. High-frequency ac conditioning is conducted for the stainless steel (SUS) electrodes and compared with that for the Cu electrodes. Experimental results reveal that the high-frequency ac conditioning improves the dielectric strength by 13% at 550 Hz for the Cu electrodes, but does not almost improve for the SUS electrodes. For the electrode materials with high electrical conductivity, such as Cu, the harmful effects of unnecessary or multiple breakdowns during conventional or low-frequency ac conditioning would be relatively large. Thus, high-frequency ac conditioning can be effective for electrode materials with high electrical conductivity due to the suppression of multiple breakdowns. Second, impulse conditioning with shot-by-shot polarity reversal, i.e., no unnecessary or multiple breakdowns, is carried out in order to simulate the same condition as the optimized frequency of ac conditioning. As a result, the impulse breakdown voltage after impulse conditioning is similar to that after 550 Hz ac conditioning. This result suggests that high-frequency ac conditioning can be sufficiently effective by increasing the ac voltage frequency from 60 to 550 Hz.