*2.4.4 Influence of vacuum*

As mentioned earlier, the effect of vacuum variation in non-negligible. The surface flashover voltage rarely varies in the range of vacuum going from 1.10–6 to 6.10–1 Pa. However, the decay of the surface flashover voltage is extremely important when the vacuum levels drops. The Paschen's law is controlling the surface voltage flashover above 6.10–1 Pa. It could be thought that if the system is maintained in vacuum, as it is the case for satellite environment, these effects could be neglected. However, the pressure can be modified locally above some materials due to a degassing phenomenon. Indeed, it has been shown that the degassing effect can enhance flashover process and should be taken into account in the material selection [48]. Fluorination surface treatment was tested in order to prevent hydrogen degassing from PI samples. The fluorination is realized into a reactor containing a mixed gas composed of 12.5% of fluorine and nitrogen. It was observed that after this surface treatment, the surface flashover voltage was increased by 10.5%. The C▬H and C▬O bonds are replaced by C▬F bonds at the surface which are more stable during the surface flashover and make degas process less effective. Besides, the

electronegativity of the fluorine tends to weaken the avalanche phenomenon and facilitate the electron absorption. This fluorination treatment might be considered as an effective process to reduce flashover due to degassing, but the effect over time should be considered. It might fade with time due to the surrounding aggressions in space.
