| Physical and chemical characteristics of vacuum-insulator interface are much different from the state in vacuum and insulator. Resistibility against high voltage of a vacuum gap is reduced considerably by introduction of an insulator between electrodes. Due to rapid development of pulsed power technology, investigation of surface flashover in vacuum under nanosecond must be a significant job.Based on a Marx generator and a coaxial forming line, a set of experimental apparatus is built for study of surface flashover characteristic in vacuum under 10/30-nanosecond pulse power. Dielectrics chosen for experiment are Teflon, PMMA and Nylon, and the investigated physical factors include gas pressure, cone angle of conical frustum, diameter and length of cylindrical insulator, electrode material, electrode shape and contact style between the insulator and electrode. According to experimental data and mechanism analysis, several suggestions are recommended regarding how to design insulator used in vacuum circumstance.Experimental data are discussed through computing electric field and analyzing the course of flashover. Several conclusions are reached: (1) The important effect of CTJ (cathode triple junction) is not only the voids between insulator and electrode increase the local electric field, but also electrons emitted in the vicinity of CTJ have enough chance to come into collision with the surface of insulator and accelerate the course of electron cascade. (2) Surface flashover characteristic in vacuum are related to rising rate of voltage waveforms. When applied voltage waveforms are dc, ac and slow pulse, voltage amplitude during discharge can be ignored. Because of rapid rising rate of nanosecond pulse, flashover voltage can reach a higher value after discharge channels have been built. (3) Surface charges are necessary condition for surface flashover. At the beginning of flashover, there is no positive charge on the surface of insulator and attraction is absent for electrons to collision with the surface. Some time is needed for surface charges to be built up, and the most obstacles for flashover appear in initial stages. Flashover voltage is much influenced by the increasing rate of voltage, and the characteristic of nanosecond flashover mainly lies on the different effect caused by the quick rising speed compared with other voltage waveform.An idea is presented that the flashover voltages are independent of the gas pressure in the range of high vacuum ( 10-4 Pa -10-1 Pa )under nanosecond pulse power. Because the mean freepath of electrons is much longer than the distance between electrodes, collisions among electrons become impossible. Gas pressure at which the flashover initiates is about an order of magnitude below that required for the Paschen pd minimum (100mm · Pa ) . The key factor in the course of flashover is desorbed gases from insulator surfaces. According to Knudsen's Cosine law, the adsorbing speed of gas molecule is much fast. Energy required for desorption is not by collision between electrons and desorbed gas molecules. Stimulated by Electron, the gas molecules are changed from ground state into excited or ionization state, and the state of adsorbed gases are out of balance and then released into vacuum. Desorption begins early and outgassing plays a crucial role in the flashover development when gas pressure is in the range of high vacuum. The desorbed gases all come from insulator surface, not from background environment. Flashover is in fact a type of gaseous discharge in a high-pressure gas layer immediately above the insulator surface.Theoretic analyses are made about the mechanism of flashover. As a result of the fact that there are many differences between the experimental background of ETPR and surface flashover under nanosecond pulse, a conclusion is made that EPTR is not compatible with the phenomena of nanosecond flashover. As far as nanosecond flashover be concerned, if flashover occurs during the rising time of pulse, the whole course will never reach the balance state, but through a course of approaching anode from cathode piece by piece. The essence of flashover in vacuum is the course of electron cascade along the surface of insulator. Based on mechanism of secondary electron emission avalanche, an expression between electrical field and physical factors is proposed and the conclusion is coincident with above analysis.
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