# Surface Discharge Mechanism on Epoxy Resin in Electronegative Gases and Its Application
## 4 Discussion
### Surface Discharge Mechanism
However, very rarely is apparatus designed with a gas pressure exceeding 1.0 MPa [23]; therefore, it is desirable to incorporate the addition of electronegative gases into the insulation design method. An insulation design using a mixture of electronegative gases should be carried out upon analyzing the electron swarm parameters of the gases. These parameters are the electron attachment cross section and electron scattering cross section. The electron attachment cross section represents the ability of an electronegative gas to capture electrons. This ability depends on the energy of the electrons. There are certain energy ranges in which electronegative gases can effectively and easily capture electrons. In the discharge space, electrons accelerated by the applied electric field are distributed with various values of kinetic energy. The improvement in the insulation performance of the gas is attributed to the easy and effective capture of these electrons. Therefore, there is a need for a method of selecting an electronegative gas with a large electron attachment cross section spanning a wide range of electron energies. The lower the kinetic energy of the electrons, the easier the electron attachment is; thus, for insulation design, it is preferable to select an electronegative gas possessing a wide electron attachment cross section in a low electron energy range. Meanwhile, the electron scattering cross section represents the ability to reduce the kinetic energy of electrons, which are accelerated by the applied electric field. This reduction of kinetic energy effectively activates the electron attachment of electronegative gases. Therefore, electron scattering results in high-energy electrons with the sufficient ionization capacity becoming inactive, consequently resulting in effective electron attachment. Since the electron attachment process is more important than the ionization process in the prevention of a breakdown, only the addition of an electronegative gas is usually conducted in the insulation design method. However, for producing a higher insulation performance, the electron swarm parameters cannot be overlooked. Based on these two parameters (the electron attachment cross section and electron scattering cross section), the mixture gas that is more suitable for insulation design should include a gas with electron attachment abilities and the ability to suppress the electron behavior. It is preferable to include a mixture of two or more auxiliary gases with different electron attachment cross section peaks, depending on the electron energy, because one electronegative gas cannot capture all electrons in a considerably wide electron energy region. Meanwhile, the mixing of gases with an electron scattering capability that reduces the electron energy in the electron energy region corresponding to the peak of the electron attachment cross section should be recommended. Based on these electron swarm parameters, the experimental results reported that the 2.5% positive lightning impulse breakdown voltage of SF\({}_{6}\)/air and SF\({}_{6}\)/CO\({}_{2}\) in 0.05 to 0.45 MPa under a non-uniform electric field with a gap of 10 mm is higher than that of pure SF\({}_{6}\)[30]. [PERSON] et al. [31] studied the insulation properties of a mixed gas containing two or more electronegative gases based on the electron attachment cross section, depending on the electron energy. In addition to selecting the insulating gas, engineering techniques can be applied to an insulation design method, and these techniques should be integrated into high-voltage apparatus with a selection method of insulating gas that considers the electron swarm parameters. The engineering techniques are improvements in electrodes and solid dielectrics, details of which can be found in references [11; 32; 33; 34; 35; 36]. The theoretical principles governing the engineering techniques are the suppression of electron generation and relaxation of the concentrated electric field.