Numerical Investigation of Reactive Species Generation in He/O_2 Admixture Capacitively Coupled Plasma

摘要

We report results from a one-dimensional, numerical simulation of a capacitively coupled He/O_2 admixture plasma, operated at pressure varying from 1.5 to 7.5 Torr and over a range of molecular oxygen admixtures (He + 0.1%O_2–He + 10%O_2). This study is used to investigate the effect of the gas pressure and the addition of small fractions of molecular oxygen on some important species production which have been extensively used for various biomedical applications and several new plasma technologies. The species densities have been presented, and the dominant key production processes have been identified. At a fixed (He + 1%O_2) admixture, the electron density decreases with increasing pressure, and the densities of negative ions are relatively high at high pressures and decrease for the positive ions. However, the atomic O, the molecular metastable O_2(v), singlet-delta O_2(a~11g) and singlet-sigma O2(b~16_g~+ ) states are found to be the dominant species and their densities are weakly dependent on the gas pressure. At a fixed pressure, as the O_2 admixture increases, the electron impact with O_2 molecules becomes higher, and consequently, the electron generation decreases considerably. The formation of O, O_2(v), O_2(a~11g) and O_2(a~11g) increases until it reaches a peak at 4%O_2 and then falls significantly with increasing O_2 admixture. However, the atomic metastable O(~1D) and He∗ are less important and they decrease rapidly due to collisional quenching or Penning ionization. The ozone O_3 formation is optimal at 4%O_2. The positive and negative atomic and molecular ions are also relatively less important, and they decrease sharply with increasing O_2 admixture due to ion-ion and ion-molecule neutralization. The addition of oxygen resulted in a decrease in all plasma species due to the electronegativity property of oxygen. These results are in fairly good agreement with some experimental observations and allow for a better understanding of the main physicochemical mechanisms taking place in He/O_2 plasmas admixtures.