*2.3.1 Effect of proton and electron irradiation*

Usually the photoemission induced a positive charging at the surface of dielectric that can influence the emission as photoelectron can be attracted back to the surface. That is why during measurements, it is important to choose new area each time by shifting the sample and using short-time exposure to UV pulse. It was shown on Kapton® 100H sample [39] that the proton ageing (in the range 1**–**15 years equivalent exposure on GEO with 50 keV proton in which penetration depth is estimated by Casino software to be of 600 nm and various flux) makes the PEY to raise. On the contrary, the same ageing with electrons (of 500 keV that can cross the 25 μm sample thickness) seems to make the PEY decay slightly. In both cases, the energy loss by the particles along the way is transmitted to the material, however in the case of proton, this energy is concentrated into the surface shallow layer that might induce high ionization and act in favor of photoemission. In the case of highenergy electron, the energy is deposited in the entire bulk and the surface ionization that might occur is not predominant. It is also important to remember that air exposure can help the sample recovery in a few hours [40]; it is recommended to perform all these measurements while the sample is maintained under vacuum.

### *2.3.2 Effect atomic oxygen and UV exposure*

The UV ageing in the range 25–1000 ESH seems to make the PEY increase but with a saturation above 500 ESH. The same saturation was noticed on secondary electron emission experiments [39]. A study on Kapton®HN, Kapton®E, and Upilex®S showed increases in solar absorptance and the *α/ε* ratio under VUV radiation exposure, whereas emittance changes were not significant [41]. As more photons can be absorbed by damaged PI, the PEY is expected to increase. This effect of absorbance is probably coupled with the chemical degradation and the production of free radical during the UV irradiation [42]. Actually free radicals provide activated electrons that can enhance PEY. At last the AO effect seems to be limited even with the equivalent exposure of 1 year on LEO. In fact, AO increases the surface roughness of the PI film and the photoelectron that are emitted are probably recapture before they can really escape and be detected that is why the PEY tends to decrease with long AO beam exposure.

### **2.4 Surface flashover**

Surface flashover has been identified at triple junction locations on spacecraft [43]. A triple junction is characterized by a metal electrode, a dielectric, and the surrounding environment (air or vacuum). The main theory is based on the secondary electron emission avalanche (SEEA) [44]. The electrons are emitted from the cathode due to field emission. The electrons hit the dielectric surface and
