| Surface flashover across the insulators in vacuum is the major limitation of the performance of the high power system, such as high power microwave tube, pulse power switch, high-energy linear accelerator, etc. Flashover studies are of considerable importance in many applications.Alumina ceramics are used widely in vacuum insulation system. The flashover and charging performance have been systematically investigated on Alumina ceramics in vacuum under pulse voltage (0.7/4^8). Different ceramics samples were prepared under different sintering temperature, different additives, and with different surface condition. The influence of the manufacturing technique and the surface treatment of materials on the flashover performance have been studied in detail. The mechanism of flashover of insulators in vacuum has also been developed.The main contributions to the study on the flashover of insulators in vacuum are as follows:1. A set of test system for the studies on flashover of insulators in vacuum has been developed, which consists of vacuum system, electrode system, E-dot pulse voltage measurement system, surface charge measurement system, current monitor system and luminescence signal measurement system. This is the only test system for investigating the flashover in vacuum in domestic. This set system can monitor and record the waveforms of pulse voltage, current and luminescence signals in real time, and can also measure the surface charge distribution on insulators in vacuum.2. The pre-flashover phenomena of Alumina insulators in vacuum have been investigated. According to the experimental results, a reversed pre-flashover current was observed repetitively, which often occurs at the tail of pulse voltage close to zero (sometimes after the applied pulse voltage). The experimental results show that the surface charging is related with the pre-flashover events, the pre-flashover events can bring the change of surface charge distribution. These may be attributed to the micro-discharge caused by the traps in insulator. The charge carriers can be captured by traps, a space electric field will be set up by the trap centers, and the combined electric field may exceed the breakdown electric field of local area, then the micro-discharge will be initiated.3. The influence of microstructure on the flashover performance of Alumina ceramics has been studied systematically through investigating the flashover performance of different Alumina ceramic samples. Experimental results reveal that the flashover performance is affected by the grain sizes and its dispersiveness. The smaller grain size is, the higher flashover performance; the less dispersiveness of grain sizes is, the higher flashover performance. It has also been found that the pre-flashover events and the charging characteristic are related to the flashover performance, the samples with seldom pre-flashover events and lower charging ability show a better flashoverperformance. The flashover performance of Alumina ceramics can be evaluated based on the above results, and it is helpful for choosing a proper material used in vacuum insulation system.4. The influence of surface treatment (surface polish) on the flashover performance has been investigated with different kinds of Alumina ceramics. It was found that the surface roughness could be used as a reference value for evaluating the influence of surface polish to a certain degree. The experimental results show that the samples with a rougher surface (polished by SiC) have a worse flashover performance. This is important to conduct the surface treatment of material for a better flashover performance.5. A new model for the growth stage of surface flashover has been developed according to the experimental results, which is based on the solid band theory. It is suggested that the electron multiplication could be attributed to two processes: one is the secondary electron emission avalanche caused by collisional ionization, the other is the micro-discharge caused by the trap centers of insulator. Th...
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