Study of early light emission from anode-initiated surface flashover in vacuum

Date

2022-08

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Abstract

The physical phenomena leading to high voltage surface flashover are investigated. Surface flashover is a possible failure mechanism in high voltage vacuum systems which involves electrical breakdown across the surface of an insulator. Positive angled insulator geometries, i.e. insulators which are narrower near the anode, are known to withstand higher voltages because the field at the cathode triple junction (CTJ) is suppressed; such geometries are common in large pulsed power machines due to their superior performance. However, the field at the anode triple junction (ATJ) is significantly enhanced, so when flashover does occur in these geometries it is believed to be anode-initiated. A deeper physical understanding of the mechanisms of anode-initiated flashover is necessary to continue to improve the performance of these systems.

An experimental setup for the study of vacuum surface flashover is introduced which is capable of greater than 200 kV pulses with rise times on the order of a few tens of nanoseconds. The positive 45° flashover geometry is designed to provide similar field conditions to the insulator stacks of large scale pulsed power machines while improving diagnostic access. Specifically, an experimental apparatus for collecting the flashover self-luminosity is introduced. It enables spatiotemporally resolved photodetection from regions adjacent to the ATJ and CTJ. The apparatus further permits collecting temporally resolved optical emission spectra in the spectral range from 200-800 nm, yielding additional insight into the flashover dynamics. Light intensity waveforms reveal that the geometry was successful in promoting anode-initiated flashover, since early anode light precedes early cathode light by a few to ten nanoseconds during the voltage rise. Then, a spectral progression of the flashover plasma is presented, spanning from the earliest detectable light to tens of nanoseconds after the impedance collapse. The earliest light is a broadband background, which is joined by hydrogen and carbon lines prior to the impedance collapse. Metal lines are also detected near the anode prior to the impedance collapse, which appears to be unique to anode-initiated flashover.


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Keywords

Spectroscopy, Surface Flashover, Anode-Initiation

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