The Insulating Properties Of The Vacuum Surface Studies

The insulation characteristic of insulator in vacuum is researched in the process of the high voltage breakdown along the insulator surface in vacuum, which is called vacuum surface flashover. The factors which influenced the process include the insulator's material, structure, the distribution of space electrical field, the way to deal with the surface, the characteristic of voltage waveform, pulse width etc. There are two kinds of theory for the vacuum surface flashover: secondary electron emission avalanche (SEEA) and electron triggered polarization relaxation (ETPR). The theory of SEEA is based on the insulator's surface emitted secondary electrons when bombarded by electron, includes the process of electron-simulated desorption (ESD), the process of desorption gas ionization and the process of the ion influencing the flashover.The empirical formula in the process of the secondary emission is analyzed; the stabilization condition of vacuum surface flashover is deduced. The physics image of the vacuum surface flashover is described with the SEEA theory. The factors which influenced the surface flashover and the way to rise the withstand voltage are also been analyzed.The observation and research are done on the long pulse accelerator (500kV, 3kA, 1μs). The way to shield the back current of foilless diode is analyzed by KARAT program. The surface flashover is raised from 300kV to 400kV when the shield is added in experiments. And the surface flashover decreased about 20% when a magnetic field of about 6000 gauss added. The surface structure of the damaged insulator by flashover is also observed.Summarized three consideration in the design of insulator.1. choose the good vacuum characteristic insulator, reduce the probable weakness;2. reduce the electrical field in the triple junction as possible3. make the electrical field distributing along the surface more uniform.A conical insulator is designed, which is operated on the maximum voltage 750kV and pulse width 200ns. The conical insulator high 15 cm,the diameter of top is 5cm and that of the bottom is 35cm. The insulator is operated for more 300 times on voltage more 600kV, and maximum voltage is 750kV. The operated is stability and no surface flashover phenomenon is happened.