Research Of Plasma Chemical Model And Analysis Of Influencing Factors Of Streamer Discharge In Air

With the development of the UHV transmission, characteristics and mechanisms oflong air gap discharge and corona discharge have drawn more and more attention.Streamer discharge which is an important stage of the long air gap discharge process isintrinsically the most difficult to investigate both experimentally and computationally,as it develops very fast, and it’s a nonlinear electrodynamic problem with multi-fieldand multi-scale. In recent years, research in streamer discharge has progressedsignificantly as computational power and sophistication of algorithms and as faster andmore sensitive detection technology has become available. e.g. the influence ofphotoionization on streamer discharge, the multiscale nature of streamers, the functionalrelations between streamers velocity and radius of the streamer channel. Althoughresearch in streamer discharge has made some progresses, our current understanding ofthe streamer discharge process remains incomplete, e.g., the space-time evolution ofparticles （the excitation and ionization of a neutral particle by an electron, therecombination of electrons and ions, and the electron thermal and non-thermalattachment and detachment processes）, the electron mean energy profile, the influenceof pressure, initial electron density and mole ratio of oxygen and nitrogen on chargedensity distribution along the axis of symmetry, streamer propagation velocity, electricfield distribution along the axis of symmetry and the mean electron energy profiles soon. A detailed research on the space-time evolution of particles is significance to revealthe microscopic physical mechanism of streamer discharge. According to the local fieldapproximation, the electric field is directly obtained from solving Poisson equation andaffected by the space charge density distribution. The electron mean energy isdetermined by the electric field at a certain gas density. The electron is the main carrierof energy-transfer in streamer discharge in streamer discharge. The energy of electricfield is transferred to molecule/atom through the collisions between electron andmolecule/atom. The rate of collision between a molecule/atom and an electron increaseswith the electron mean energy increasing. A detailed research on the influence ofpressure, initial electron density and mole ratio of oxygen and nitrogen on the meanelectron energy profiles is significance to reveal the microscopic physical mechanism ofgas discharge. In the present thesis, the mean electron energy transmission equation is firstlyintroduced into traditional hydrodynamic model of streamer discharge, and thecharacteristics of a streamer discharge in air at an atmospheric pressure are investigated.The space-time evolutions of particles are investigated by using a non-equilibriummodel based on plasma chemistry. Because of electronic parameters proposed by Steinle,Nikonov, Morrow and Kang of traditional hydrodynamic model of streamer dischargeare not applicable in non-equilibrium model based on plasma chemistry proposed in thisthesis. The transport and ionization coefficients of electron for streamer discharge arecalculated from Boltzmann equation using two terms approximation method. Toeliminate the negative electron density and to improve the grid, Convection-diffusionequation of electron is indexed. Lastly, the electron mean density, the space-timeevolutions of particles, streamer radius and influencing factors on electro mean energy,electric field and space charge density are investigated in detail. The innovative resultsobtained by this thesis are as follows:①The study found that the electron energy in the streamer head reachesmaximum value where is the main area of energy-transfer. The mean electron energy instreamer channel is constant, which is basically equivalent to the electron energy in thearea in front of the streamer.②It is found that electron ionization reaction, and ions N₂+charge transportreactions or neutralization reactions took place mainly at streamer tip. Among allreactions, R（e-N₂）which is reaction rate between electron and N₂is maximum, while theN₂+density is not. The density of O₂⁺from oxygen which is only one forth of N₂reaches the peak value.③The distribution of electron and all ions density near electrode surface is firstlyproposed in this thesis. It was found that electron and all ions density near electrodesurface plunged at0.01mm away from pin electrode and a thin low density film formed.The ions distribution at electrodes is caused by decaying between ions and electrodesurface, which offers reference to surface discharge.④The gas pressure, mole ratio of oxygen and nitrogen, initial electron density onstreamer discharge has a great effect on streamer discharge. The velocity of streamerpropagation and the mean electron energy decreases with the increasing gas pressure.With the increasing N₂ratio between N₂and O₂and the increasing initial electrondensity, the axial space charge density, electric field and electron mean energy decrease, while the streamer velocity increases with the increasing initial electron density anddecreasing N₂ratio between N₂and O₂.