An experimental apparatus and diagnostics for the study of vacuum surface flashover
Young, Jacob K.
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Upper operational limits in pulsed power vacuum systems are typically determined by the maximum electrical stress the apparatus can handle. If this limit is exceeded, the first point in these systems to fail is usually the dielectric interface separating the anode and cathode. This failure is due to a conductive plasma channel developing along the surface of the insulating material, which effectively shorts both electrodes together, compromising the electrical integrity. While breakdown can initiate at the cathode, anode-initiated flashover is believed to be more dominant with 45-degree insulators, which is the geometry many modern pulsed power machines use. To investigate and characterize anode-initiated vacuum surface flashover, an experimental apparatus has been engineered to produce a repeatable flashover location. A cross-linked polystyrene (Rexolite) sample in a 45-degree wedge geometry separates a hemispherical anode from a planar cathode by 0.6 cm. The testbed housing both electrodes and the dielectric sample is located within a vacuum chamber with a nominal pressure of 10^(-6) Torr. An 8-stage Marx generator pulses the anode with up to 240 kV, resulting in flashover along the surface of the Rexolite sample. The voltage and current are measured using a calibrated capacitive voltage divider (CVD) and current viewing resistor (CVR). Additional diagnostics include time-integrated and temporally resolved imaging using an intensified charge-coupled device (ICCD) camera. Observations have shown that early light emission does originate near the anode-vacuum-insulator junction due to mechanisms that agree with the Anderson model of anode-initiated flashover. Post-test imaging also revealed dendrite pattern surface damage which appears to propagate from anode to cathode. A very similar pattern of surface scarring was observed by Anderson and is believed to be unique to anode-initiated flashover.Embargo status: Restricted until 06/2172. To request the author grant access, click on the PDF link to the left.