Effect of temperature on polyimide dc flashover characteristics in different vacuum degrees

摘要

The properties of electrical insulation in vacuum and at cryogenic temperatures are important for designing the insulation of large, low-temperature superconducting devices. Particularly, gas pressure and temperature turbulence could easily lead to insulation failure. In this paper, the results of measurements that reveal the dc flashover characteristics of polyimide film from 300 to 20K at helium pressures of 10(-3)-10(3) Pa were reported. The results show that, in high vacuum, the flashover voltage is generally independent of pressure. As pressure increases, the flashover voltage first decreases and then increases gradually. Electron multiplication in helium or on the surface of PI is the main factor affecting flashover. For high to medium vacuum, the N (gas molecular density) at which electrons begin to collide with helium and reduce flashover voltage at higher temperature is less than at cryogenic temperatures. For medium to low vacuum, by comparing the flashover curve with a curve fit based on Townsend discharge theory, a result was found that when a certain specific N is exceeded, the electron impact ionization on the solid surface will no longer be the main influencing factor of flashover. This N increases as temperature decreases except 200K, and reaches the minimum at 200K. After analysis, it may be the larger mean free path of electrons increases the likelihood of electron impact ionization on the PI surface. Therefore, in order to maintain a high flashover voltage in engineering, it is necessary to maintain a higher degree of vacuum at higher temperatures; and when the pressure rises to low vacuum, the flashover is mainly determined by the gas discharge.