Electrical breakdown on surfaces of insulators was perhaps first discovered by German scientist Georg Licthenberg in about 1777, and treelike patterns evidencing such breakdown are sometimes called Lichtenberg figures.
Dielectric breakdown in high-voltage vacuum applications has been a problem for half a century, as the breakdown can limit the maximum voltage and result in catastrophic failure of expensive and complicated devices such as particle accelerators. Conventional linear particle accelerators at that time included a cylindrical insulator that separated a flat anode and a flat cathode, with the interior chamber evacuated to prevent collisions between the particles and gas molecules. The angle between the cathode and the dielectric surface exposed to the vacuum was at 90°, and the angle between the anode and the dielectric surface exposed to the vacuum was at 90°. A phenomenon called “flashover” was known to occur along the dielectric surface, which created a conductive path between the anode and the cathode and destroyed such devices.
About fifty years ago it was discovered that sloping the surface of the dielectric so that the angle between the cathode and the dielectric surface exposed to the vacuum was obtuse and the supplementary angle between the anode and the dielectric surface exposed to the vacuum was acute allowed for higher breakdown voltages. For example, in a paper from 1969 entitled “Pulsed High-Voltage Flashover of Vacuum Dielectric Interfaces,” Glock et al. attribute the discovery that cone-shaped insulator surfaces offered improvements in breakdown voltage to Smith, “Pulse Breakdown of Insulator Surfaces in a Poor Vacuum,” Proc. of the International Symposium on Insulation of High Voltages in Vacuum, (1964) and Shannon et al., “Insulation of High Voltage Across Insulators in Vacuum” Vacuum Science Tech. 2:234 (1965). Since that time, conventional high voltage particle accelerators have typically employed dielectric surfaces that are angled at 45° to the cathode and anode surfaces. Various reports have confirmed that employing a dielectric surface that is sloped so that the angle between that surface and the cathode surface across the vacuum is obtuse provides higher breakdown voltages.
While sloping the dielectric surface helped, it did not solve the problem of flashover. Over twenty years ago, in an article entitled “Review of Surface Flashover Theory,” in Proceedings of the XIVth International Symposium on Discharges and Electrical Insulation in Vacuum, Santa Fe, N. Mex. (1990) at p. 311, R. A. Anderson reviewed various studies and theories of the flashover problem. Anderson's review states the following: “Surface flashover appears to comprise at least two distinct phenomena which can be distinguished as being cathode-initiated or anode-initiated, with the former having received by far the most attention. Several models describing cathode-initiated flashover have been built on the pioneering work of Boersch and coworkers, published in 1963, and credit the breakdown mechanism to the action of an intense secondary-electron-emission avalanche on the insulator surface. Other researchers consider the electron avalanche to be only partially, if at all, responsible, and invoke various hot-carrier effects in the insulator bulk, the surface interfacial region, or in a layer of gas adsorbed on the insulator surface. Anode-initiated flashover, which contends with the cathode-initiated variety for the breakdown of insulators of conventional design, is thought to involve bulk breakdown in a way related to treeing failure.”
Regarding anode-initiated flashover, Anderson notes that flashover still occurs when the electric field is directed into the insulator surface at angles approaching normal incidence (as with conical insulators having large positive angles), where secondary-emission avalanches would be difficult to initiate. Anderson points to flashover with conical insulators having large positive angles as leading to his theory of anode-initiated flashover.
U.S. Pat. No. 5,821,705 to Caporaso et al., filed in 1996, teaches reducing dielectric flashover by providing isolated conductive layers that are alternated with insulator layers, but this approach has also not solved the problem of dielectric flashover.
About ten years ago, Chung et al. calculated, in an article entitled “Theoretical Analysis of the Enhanced Electric Field at the Triple Junction,” J. Vac. Sci. Technol. 22(3), May/June (2004), various two-dimensional triple junction geometries at which a conductor, a dielectric and a vacuum meet. Among other things, Chung et al. show that for a flat conductor geometry the electric field at such a triple point approaches infinity for an angle measured across the vacuum that is between 0° and 90°, whereas for such an angle that is greater than 90° and less than 180° no such singularity exists. One result that can be deduced from Chung et al. is that sloping the dielectric surface to create an obtuse angle at the cathode triple junction may have alleviated one type of flashover but created a singularity at the anode triple junction that could cause another mechanism of failure.
As mentioned above, a model of anode-initiated flashover was developed by R. A. Anderson in the 1970's. The mechanism proposed by Anderson is summarized in an article by Styger et al. entitled “Improved design of a high-voltage vacuum-insulator interface,” Phys. Rev. ST Accel. Beams 8, 050401 (2005), of which Anderson is a co-author, as follows: “At a sufficiently high voltage, the flashover of a 45° interface initiates at the anode junction due to emission of electrons from the insulator (citations omitted). The emission increases the electric field at the insulator surface, which in turn precipitates bulk dielectric-breakdown events at the surface. The events branch across the surface until they reach the cathode and the flashover is complete.” Styger et al. also note that the use of an anode plug to shield the anode triple junction was first proposed by D. H. McDaniel in 1975, and Styger et al. report some improvement in dielectric breakdown using such a shield.
Similarly, in an article entitled “Electron Avalanche Model of Dielectric-Vacuum Surface Breakdown,” Journal of Applied Physics, December 2007, Vol. 102 Issue 11, p113306, E. J. Lauer proposed a process by which an electron avalanche may occur, beginning near a triple point at which a dielectric meets an anode at a 45° angle. E. J. Lauer, one of the present inventors, theorized that the relatively high electric field at the anode triple junction caused field emission of electrons from the dielectric near the anode triple junction, which resulted in positive charge on the dielectric in the vicinity of the anode triple junction, which in turn caused electron avalanching to commence on the dielectric-vacuum interface. Thus, according to this theory also, providing a 45° slope to the dielectric surface to alleviate the problem of dielectric flashover that initiated at the cathode triple junction appears to have created a different type of flashover that initiates at the anode triple junction.
In short, a solution to the problem of dielectric breakdown at a vacuum interface under high voltage has eluded researchers for decades.