Effects of oxygen concentration on the electron energy distribution functions in atmospheric pressure helium/oxygen and argon/oxygen needle-electrode plasmas

In this work, a numerical investigation on the oxygen concentration effects of the electron energy distribution function (EEDF) in the atmospheric pressure helium/oxygen and argon/oxygen plasmas has been performed based on a typical needle-electrode discharge system and a Particle-in-cell Monte-Carlo collision model. Specially, the corresponding mechanisms have been analyzed in detail with the use of the energy conservation of electrons in the development of discharge. The present work gives the significant results below. In helium/oxygen mixture, the oxygen concentration dependence of EEDF presents the different behaviors in the three phases of the discharge. Increasing oxygen concentration, in the first phase, the peak of EEDF decreases and its distribution becomes wide, and then, EEDF presents a transition to the third phase, where the peak of EEDF increases, but its distribution gets narrow. The mechanisms governing these dependences can be attributed to the different effects of oxygen concentration on the electron energy obtained from the electric field, the energy loss of electrons due to the inelastic collisions, and the contribution of the electron density to reducing the average energy of electrons, at the different phases of the discharge. In argon/oxygen mixture, for different time ts in the development of discharge, the oxygen concentration dependences of EEDF are similar, i.e. the rising peak and the distribution narrowing down with the increase in oxygen concentration, but the dependence is weak. In comparison with helium/oxygen mixture, this weak dependence is mainly due to the especial distributions of the excitation and ionization thresholds of O₂, O, and Ar, namely, the ionization thresholds of O₂ and O are located between the excitation and ionization thresholds of argon, which leads to the unobvious increase of the inelastic collision frequencies of electrons with O₂ and O, when increasing oxygen concentration, and thus leads to weak effect on the EEDF in argon/oxygen mixture. (paper)