Temporally resolved light emission and optical emission spectroscopy of surface flashover in vacuum

Early light emission provides information about the dominant mechanisms culminating in vacuum surface flashover (anode-initiated vs. cathode-initiated) for particular geometries. From experimental evidence gathered elsewhere, for the case of an insulator oriented at 45° with respect to the anode, anode-initiated flashover is believed to dominate since the field at the anode triple point is roughly three times that of the cathode. Similar to previous work performed on cathode-initiated flashover, light emission from the voltage rise through the impedance collapse is collected into two optical fibers focused on light emanating from the insulator in regions near the anode and cathode. The optical fibers are either connected to PMTs for spectrally integrated localized light intensity information or to a spectrograph used in conjunction with an ICCD camera. Challenges associated with localizing the flashover for optical diagnostics and incorporating the optical diagnostics into the high-field environment are discussed. Initial results for cross-linked polystyrene (Rexolite 1422) support the premise that flashover is initiated from the anode for these geometries, as early light from the anode leads cathode light up to photocathode saturation. Early spectroscopy results show promise for future characterization of the spatio-temporal development of emission from desorbed gas species across the insulator surface and identification of bulk insulator involvement if it occurs.

I. Introduction

Surface flashover is of primary concern in high voltage vacuum systems because it often limits the maximum energy deposition into a load. Cathode-initiated surface flashover, the dominant mechanism for most symmetrical anode-cathode-insulator geometries, is relatively well understood. However, an examination of anode-initiated flashover, which is suspected to predominate for geometries where the insulator is oriented at 45° with respect to the anode (positive 45° geometry) due to the enhanced field at the anode triple junction, is less mature. It is noted that this constitutes a very typical geometry for large-scale pulsed power devices, and thus there is interest in gathering experimental evidence for a more complete theoretical basis for anode-initiated flashover. Spectroscopic techniques have previously been used to interrogate cathode-initiated flashovers across planar and cylindrical geometries (0° geometries), leading to the identification of desorbed gasses and cathodoluminescence [1] and quantification of plasma temperatures [2].