# Surface Discharge Mechanism on Epoxy Resin in Electronegative Gases and Its Application
## 4 Discussion  
### Surface Discharge Mechanism  
This section describes the surface discharge mechanism in detail, explaining the experimental results presented in Section 3. The main outcomes were an improved surface insulation performance of insulating gases containing electronegative species and noticeable saturation of the surface insulation performance of SF\({}_{6}\) with an increasing pressure. These two results seem to be inherently related to the mechanisms of electron attachment and electron detachment, respectively, during the progression of surface discharge. The electron attachment is a mechanism by which SF\({}_{6}\) and O\({}_{2}\) trap electrons moving to the positive electrode to form negative ions between two electrodes. This reduces the number of electrons that cause collision ionization and the SEEA on the dielectric surface, thereby resulting in a higher surface insulation performance from the inclusion of an electronegative gas. However, the negative ions generated by electron attachment emit electrons from collisions among electrons, positive ions, and negative ions. As this electron detachment adds electrons between the electrodes, it may limit the rise of the surface flashover voltage. Surface discharge is developed by an initial electron, the SEEA, a surface streamer, and conductive channel formation. The formation of an initial electron depends on the polarity of the applied voltage. When the applied voltage is positive, the initial electron is generated by the electron detachment of a negative ion around the needle electrode or electron emission from the epoxy surface. Examples of electron detachment from SF\({}_{6}\)\({}^{-}\) ions can be presented as follows under an appropriate condition for each case [25]:  
Collisional detachment: \(SF_{6}^{-}+SF_{6}\to SF_{6}+SF_{6}+e\),  
Photodetachment: \(SF_{6}^{-}+hv\to SF_{6}+e\),  
Thermally-induced detachment: \(SF_{6}^{-}+heat\to SF_{6}+e\). When the applied voltage is negative, an initial electron is generated by field emission from the cathode at the triple junction, where the electrode, solid dielectric, and insulating gas make contact. This electron collides with the epoxy surface. Then, the generated initial electron contributes to the emission of secondary electrons from the surface. The secondary electrons again lead to an SEEA at the epoxy surface and collision ionization in the gas. During the process of collision ionization, photons emitted from the excited molecules are irradiated to both the epoxy surface and other molecules. This irradiation results in photoemission and photoionization and can supply additional electrons in the discharge space between the electrodes. The electrons generated by field emission, the SEEA, ionization collision, photoemission, and photoionization can attach themselves to an electronegative gas. During the development of surface discharge, negative ions can form from the electron attachment process. However, these ions can also disappear due to the electron detachment mechanism during the same surface discharge development. As the surface discharge develops with an increase in the applied voltage, the SEEA and photoemission can become more prominent. Owing to the active SEEA and photoemission processes, a surface streamer develops along the epoxy surface. The head of the streamer advances based on what the electron source is (photoionization/photoemission), and the surface streamer intersects between the electrodes. When a conductive channel is formed between the electrodes due to the surface streamer and the presence of many electrons, surface flashover finally occurs. The most evident difference between an electronegative gas and a non-electronegative gas is the occurrence of electron attachment or detachment during the process of surface discharge development. The authors believe that these processes are the key determining factors that explain the experimental results.