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Glass fibre reinforced polymers (GFRPs) have been widely used as one of the main electrical insulating structures for superconducting magnets. A new type of GFRP insulation material using cyanate ester/epoxy resin as a matrix was developed in this study, and the samples were irradiated by Co-60 for 1 MGy and 5 MGy dose. Space charge distributed within the sample were tested using the pulsed electroacoustic method, and charge concentration was found at the interfaces between glass fibre and epoxy resin. Thermally stimulated current (TSC) and dc conduction current were also tested to evaluate the irradiation effect. It was supposed that charge mobility and density were suppressed at the beginning due to the crosslinking reaction, and for a higher irradiation dose, molecular chain degradation dominated and led to more sever space charge accumulation at interfaces which enhance the internal electric field higher than the external field, and transition field for conduction current was also decreased by irradiation. Space charge dynamic at cryogenic temperature was revealed by conduction current and TSC, and space charge injection was observed for the irradiated samples at 225 K, which was more obvious for the irradiated samples.

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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 20 K 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 200 K, and reaches the minimum at 200 K. 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.

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