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

Electron emission from ferroelectrics (FEE) is an unconventional electron emission effect. Methods of FEE excitation are quite different compared to classic electron emission from solids. Two kinds of FEE have been observed, “weak” and “strong.” “Weak” electron emission (current density 10−12–10−7 A/cm2) occurs from polar surfaces of ferroelectric materials in the ferroelectric phase only. A source of the electric field for “weak” FEE excitation is an uncompensated charge, generated by a deviation of macroscopic spontaneous polarization from its equilibrium state under a pyroelectric effect, piezoelectric effect, or polarization switching. The FEE is a tunneling emission current which screens uncompensated polarization charges. It is shown that the FEE is an effective tool for direct domain imaging and studies of electronic properties of ferroelectrics. “Strong” FEE, which is 10–12 orders of magnitude higher than “weak” FEE, achieves 100 A/cm2 and is plasma-assisted electron emission. Two modes of the sur...

Electron emission from ferroelectrics

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

The authors investigate a novel insulator concept which involves the use of alternating layers of conductors and insulators with periods less than 1 mm. These structures perform many times better (about 1.5 to 4 times higher breakdown electric field) than conventional insulators in long pulse, short pulse and alternating polarity applications. They present their ongoing studies investigating the degradation of the breakdown electric field resulting from surface roughness, the effect of gas pressure and the performance of the insulator structure under bipolar stress. Further, they present their initial modeling studies.

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Multilayer ultra-high gradient insulator technology

The health benefits and quality-of-life outcomes of a fit musculoskeletal system (musculoskeletal fitness) are reviewed in this article. The World Health Organization suggests health is a state of complete physical, mental or social well-being and not merely the absence of disease or infirmity. Physical health includes such characteristics as body size and shape, sensory acuity, susceptibility to disease and disorders, body functioning, recuperative ability and the ability to perform certain tasks.

https://powcs.med.unsw.edu.au/sites/default/files/powcs/page/Exercise%2B%2BHealth%2BOutcomes%2BHlth%2BM.pdf

Musculoskeletal fitness, health outcomes and quality of life.

The surface of a solid dielectric insulator becomes electrically charged when subjected to a high‐voltage stress in vacuum. A method for calculating the surface flashover voltage based on the assumption that the discharge occurs in a layer of desorbed gases from the insulator surface is proposed. The electric field strength required to cause surface flashover is calculated by taking into account the secondary electron emission characteristics of the dielectric material. The dependence of the surface flashover field on the insulating material is deduced. A dependence of the flashover voltage on the insulator length to a power law of 0.5 is theoretically predicted. The calculated surface flashover voltage is compared with the previously reported measurements and good agreement is obtained.

Surface flashover of solid dielectric in vacuum

The surface flashover of Teflon, plexiglass, quartz, Pyrex glass, Macor glass‐ceramic, and sapphire solid insulators has been measured in vacuum (∼10−8 Torr, ∼10−6 Pa) and in atmospheric air using dc, ac (60 Hz), and 1.2/50‐μsec lightning impulse voltages. The dependence of the flashover voltage on the following parameters is investigated: (1) spacer material, (2) diameter of the spacer, (3) spacer length, (4) number of spacers stacked in series, (5) air pressure in the range 10−6–105 Pa, (6) electrode material, (7) spark conditioning, and (8) the external resistance in series with the gap. At a fixed insulator length the flashover voltage decreases with increasing spacer diameter. The withstand voltage of spacers stacked in series increases with increasing the number of spacers. The dc flashover voltage of different insulating materials is theoretically calculated in vacuum as a function of the length of the insulator and compared with the experimentally obtained results. Good agreement is obtained.

Surface flashover of solid insulators in atmospheric air and in vacuum

The surface flashover of a Macor glass‐ceramic conical insulator in ultrahigh vacuum is investigated using direct current (dc), alternating current (ac at 60 Hz), standard lightning impulse (1.2/50 μs), and combinations of dc+1.2/50 μs and dc+ac. The dc and the 1.2/50‐μs flashover voltages are found to be at about the same level when the cone angle θ, which the surface makes with the applied field, is varied from −55° to +55°. At a fixed positive value of θ and a fixed thickness of the insulator, the lightning impulse, and the dc flashover voltages are higher than the ac. The level and the polarity of the dc prestress influence the combined dc+impulse flashover voltage in conical insulators. The ac+dc flashover depends on the dc prestress level and increases with increasing the prestress until a saturation is reached which is at a slightly higher voltage than the dc flashover. The electric field and the potential distributions at the interface of the cone insulator are calculated for different cone angles...

Surface flashover of conical insulators in vacuum

Some electrical characteristics of a triggered vacuum gap (TVG) having three different dielectric materials have been studied. Silicon carbide which possesses very high resistance (500 MΩ), steatite ceramic of medium resistance (20 MΩ), and boron nitride of very low resistance (50 Ω) have been used. The resistance of the dielectric is found to decrease with increasing trigger current and this is attributed to the deposition of metal vapor on the surface. The minimum trigger voltage and trigger current nessessary for successful operation of the TVG are measured. Deconditioning (reduction) in the minimum trigger voltage, after repeated firings, has been found and is attributed also to the deposition of metal vapor on the dielectric surface. The minimum trigger current for successful firing of the main gap decreases with increasing trigger voltage. The probability of successful firing of the TVG is found to rise rapidly with increasing trigger voltage. The delay time between the application of the trigger pulse to the breakdown of the main gap decreases with increasing trigger current and trigger voltage. A mechanism is suggested for the operation of the TVG.

Breakdown mechanisms and electrical properties of triggered vacuum gaps

The surface of a solid insulator in vacuum (and in a high gas pressure) becomes electrically charged when subjected to a high voltage stress. The surface charge density is proportional to the applied voltage. The magnitude of the surface charge density depends on the secondary electron emission characteristic, the geometrical shape, and the material of the insulator. The field enchancement, contributing to a lower withstand voltage of solid insulators in vacuum (and in high pressure gases), due to the presence of surface charge is computed along the surface of the solid insulator and at the triple electrode‐solid insulator‐vacuum junction near the anode and cathode. Different patterns of surface charge density distribution has been considered in order to evaluate their effects on the field enhancement. The polarity, the magnitude and the shape of the distribution of the surface charge density has been found to have a considerable effect on the field enhancement. The observed influence of the charge distri...

Reuben Hackam

Papers

Surface flashover of solid dielectric in vacuum

Surface flashover of solid insulators in atmospheric air and in vacuum

Surface flashover of conical insulators in vacuum

Breakdown mechanisms and electrical properties of triggered vacuum gaps

Modification of electric field at the solid insulator–vacuum interface arising from surface charges on the solid insulator