2D numerical research on needle-to-plate discharge in atmospheric pressure helium and air mixture

Summary form only given. Previous researches have certified that diffuse plasma can be generated in some mixture gas gap between needle and plate, which may have potential application in biomedicine, film surface modification etc. In this paper, a self-consistent, multiple species, two-dimension numerical model, based on hydromechanics and electric Poisson´s equation, is established to study the properties of gas-mixture discharge in needle-to-plate structure with a 0.1mm gas gap, where the needle is connected to high voltage sources and the grounded plate electrode is covered by a 0.03mm PTFE film. Moreover, the gas-mixture is consisted of 99% helium and 1% air (79% nitrogen and 21% oxygen) while 11 kinds of particles and 27 kinds of chemical reactions are taken into account. After about one-week computation, models driven by both positive and negative 400 V pulse voltages are solved respectively, which releases that electron has similar and dispersive distribution in both positive and negative discharge when it is at the peak of discharge current pulse, differing slightly from that of streamer. Then, the spatial and temporal evolutions of neutral particles (O₃, O, N, He* etc.) are analyzed, which demonstrates that the ozone and oxygen atom have their maximum density near the cathode regardless of the polarity of voltage, and the high concentration area of nitrogen atoms and metastable helium atoms are always behind the head of ionization wave. Through the model, it can be proven precisely that the electrical field can be distorted by the electric field created by 8 kinds of charged particles, where various ions have very different spatial and temporal distribution because of the different production mechanism and ion mobility. In the process of discharge, Penning reaction and charge exchange reactions are believed to be significant in the production of some ions (e.g. N₂⁺, O₂⁺ etc.) and seeding electron.