The author reviews surface flashover (i.e. voltage breakdown along the surface of insulators), primarily in vacuum. He discusses theories and models relating to surface flashover and pertinent experimental results. Surface flashover of insulators in vacuum generally is initiated by the emission of electrons from the cathode triple junction (the region where the electrode, insulator, and vacuum meet). These electrons usually then multiply as they traverse the insulator surface, either as a surface secondary-electron-emission avalanche or as an electron cascade in a thin surface layer, causing desorption of gas which had been adsorbed on the insulator surface. This desorbed gas is then ionized, which leads to surface flashover of the insulator. The theory and modeling of this phenomena and experimental studies of surface charging, the applied voltage waveform, prestressing, conditioning, discharge delay and speed, insulator geometry AMD material, surface treatment, surface gases, temperature, and pressure are reviewed. Some suggestions are made regarding how to choose the material geometry and processing when selecting an insulator for a particular application. >

Surface flashover of insulators

Reviews surface flashover of insulator, primarily in vacuum, although some comments are made about the effect of ambient gases on surface flashover. It presents theoretical mechanisms of surface flashover and pertinent experimental results. The holdoff voltage of insulators depends upon many insulator parameters, such as material, geometry, surface finish, and attachments to electrodes, but also on the applied voltage waveform (duration, single pulse or repetitive), the process history of the insulator operating environment, and previous applications of voltage. Several suggestions are made regarding choice of the material, geometry, and processing when selecting an insulator for a particular application. Some specific techniques for improving the holdoff voltage of insulators are recommended. >

Flashover of insulators in vacuum: review of the phenomena and techniques to improved holdoff voltage

This paper reviews the last twenty years of work concerning surface flashover of insulators in vacuum. It complements an earlier review paper [H.C. Miller, Flashover of Insulators in Vacuum, IEEE Trans. Electr .Insul., Vol. 28, pp. 512-527, 1993]. The surface flashover voltage of insulators in vacuum depends upon many parameters, such as material, geometry, surface finish, and attachments to electrodes, but also on the applied voltage waveform, duration, single pulse or repetitive, and on the process history of the insulator, operating environment, and previous applications of voltage. Suggestions are made for choosing the material, geometry, and processing when selecting an insulator for a particular application. Some specific techniques for improving the holdoff voltage of insulators are recommended.

Flashover of insulators in vacuum: the last twenty years

Radiation effects in non-metals

Desorbed gases from insulator surfaces may be much more important in flashover initiation than properties of the dielectric surface. Ambient neutral densities, immediately above the insulator surface, in the initial stages of flashover have been calculated to lie in the range from 2×1017/cm3 to 2×1021/cm3 with a mean of about 1×1019/cm3, whether flashover occurred on surfaces in air at atmospheric pressure or in vacuum as low as 10−6 Torr. The evidence presented indicates that, for dc and microsecond pulse voltages, surface flashover is a local, relatively high‐pressure phenomenon. Comparison of field strengths of surface flashover for various insulating materials perpendicular to the electrodes, operating pressures, and voltage waveforms appears to indicate that flashover values range well within an order of magnitude (∼18–80kV/cm). A filamentary nature of the flashover discharge is shown and is consistent with gas breakdown phenomena. Ionization wave front velocities are examined and these compare favorably with those obtained from gaseous breakdown studies.

Vacuum surface flashover: A high‐pressure phenomenon

Electrooptical measurements of the electric fields along insulator surfaces have been made, utilizing the Kerr and Pockels effects, to determine the mechanisms associated with fast insulator flashover in vacuum. Data are presented that show the temporal and spatial variations of the surface fields prior to and at flashover for insulator surfaces oriented at 0° and 45° with respect to the applied field. It is found that the surface field near the cathode is enhanced and the field near the anode is reduced during the excitation. The results further show a temporal reduction in the field nonuniformity as flashover is approached. The field collapse associated with flashover occurs very rapidly for 0° surfaces. The field collapse for 45° surfaces begins at the anode and propagates at 0.83 cm/ns towards the cathode. Mechanisms consistent with these experimental measurements are postulated.

Investigations of Fast Insulator Surface Flashover in Vacuum

Electro‐optical measurements of the electric field along solid insulator/vacuum interfaces have been made to determine the mechanisms associated with fast (ns) insulator flashover. Data showing the temporal and spatial performance of the insulator surface fields prior to and at flashover are presented. The results show that the intergap electric field is modified in a manner consistent with positive surface charging by field emitted electrons. The data also show that the enhanced intergap fields are reduced prior to flashover. Mechanisms consistent with this behavior are postulated.

Electro-optical surface flashover measurements

Electro-optical measurements of the electric field along insulator surfaces in vacuum during the few nanoseconds prior to insulator flashover have been made. The Pockels and the Kerr effect are used in conjunction with a polarization interferometer to measure the interfacial fields which have a risetime of a few ns. Insulator surface charging and cathode field enchancement occurs, followed by plasma formation near the cathode which propagates toward the anode at approximately one tenth the speed of light. Voltage collapse across the insulator occurs after the plasma formation has reached the anode.

https://hal.archives-ouvertes.fr/jpa-00219175/document

Electro-optical measurements of insulator surface flashover in vacuum

Electrooptical measurements oftheelectric fields along in- sulator surfaces havebeenmade, utilizing theKerr andPockels effects, todetermine themechanisms associated withfast insulator flashover invacuum.Dataarepresented that showthetemporal andspatial vari- ations ofthesurface fields prior toandatflashover forinsulator sur- faces oriented at00and450withrespect totheapplied field. Itis found thatthesurface fi'eld nearthecathode isenhanced andthefield neartheanodeisreduced during theexcitation. Theresults further showatemporal reduction inthefi'eld nonuniformity asflashover isap- proached. Thefield collapse associated withflashover occurs veryrap- idly for00surfaces. Thefi'eld collapse for450surfaces begins atthe anodeandpropagates at0.83cm/ns towards thecathode. Mechanisms consistent with these experimental measurements arepostulated.

https://ieeexplore.ieee.org/iel5/27/4317285/04317302.pdf

K. Mikkelson

Papers

Investigations of Fast Insulator Surface Flashover in Vacuum

Electro-optical surface flashover measurements

Electro-optical measurements of insulator surface flashover in vacuum

Investigations ofFastInsulator Surface