Effects of Magnetic Field on Surface Flashover of Polyimide Film for Superconducting Magnet Insulation

Polyimide (PI) film is widely used as insulation material in superconducting magnet systems. While operating, the PI film is subjected to the interaction of electric and magnetic fields. The existence of magnetic field will affect charge behaviors leading to the changes in flashover voltage. In this paper, a steady magnetic field up to 1.2 T was applied to the PI films. The flashover voltages were investigated under DC condition and AC condition. Different angles between magnetic and electric fields were set to explore the influence of magnetic field direction on the flashover voltage of PI film. Experimental results show that the $E\times B$ drifting into the surface will block the secondary electron emission, resulting in an increase in the flashover voltage. When the $E\times B$ drifts out of the surface the result is just the opposite. The promoting effect of magnetic field on the changes of flashover voltage is weakened under the AC condition, while the suppressive effect is strengthened. The failure mechanism of PI film under strong magnetic field will provide a theoretical basis for improving the insulation reliability of superconducting magnets.