*2.4.1 Effect of electron irradiation*

It was reported that after electron irradiation, the dc surface flashover of PI was increased due to the electron stored below the surface [46]. Due to the presence of electron in the bulk, the electric field at the triple junction is lowered and then the flashover initiation prevented unless the dc voltage is increased. For instance, the dc voltage flashover on raw polyimide is recorded at 19.2 and 23.9 kV after an irradiation under 20 keV. When the flashover is initiated, the secondary electrons are deviated from the surface and the propagation of the flashover is inhibited. It was noticed that electrons irradiation with an energy above 20 keV up to 30 keV seem to produce the same effect as the penetration depth increases the influence on the surface flashover initiation and propagation might have reached a limit. This tendency was also pointed out on deeply studied PI-type SKPI-MS30 provided by Changzhou

**Figure 4.**

*Surface flashover propagation and combine effect due electron beam irradiation.*

### *Polyimide for Electronic and Electrical Engineering Applications*

Sunchem High Performance Polymer company [47]. It has been observed that above a 20 keV electronic irradiation, the dc surface voltage flashover decreases slightly with the increase of the electron energy. It was also reported in this work that the previously irradiated PI presented higher flashover voltage than the initial sample. They suggest that irradiation could induce chemical-physical modifications such as degradation with the generation of low molecular compounds, gases, and crosslinking. The dielectric constant usually decreases in favor of the diminution of the electric field distortion and deep trap amount increases under low-energy irradiation. These deep traps prevent electrons from migration and favor a charge accumulation that is at the origin of the increase of the dc voltage flashover.
