The objective of this paper is to determine the transient
characteristics and sites of the pre-breakdown (pre-BD) in the spark
conditioning under power frequency voltage (PFV) in vacuum. A
multi-angle optical path system was built to record the arc of pre-BD at
two viewing angles by one camera, realizing the determination of the
pre-BD sites in spark conditioning. The recording rate of high-speed
camera was 580,000 frames/s. The experimental results indicate that, in
the spark conditioning under PFV, the pre-BDs continuously occurred at a
random moment. Within several half-waves, the number of the pre-BDs
could reach dozens of times, which was the key for the treatment of the
electrode surface in conditioning. Under the application of PFV, the
single pre-BD occurred at a random moment, in which would last for
several microseconds and the peak value of the current would reach
several amperes. Moreover, the pre-BD sites randomly distributed in the
surface of the electrode and almost covered the whole surface of the
electrode. The number of the pre-BDs in edge region of the electrode
tent to be more. The pre-BDs took place independently in both time and
space.
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