A Comparative Investigation on the Behaviors of Electrons in the Plasma Bullets in Atmospheric-Pressure Argon and Helium Plasma Jets

This article presents a comparative investigation on behaviors of electrons in the Ar and He plasma jets by means of the numerical simulation based on a typical needle-plate discharge configuration with positive applied pulsed voltage and a 1-D particle-in-cell Monte-Carlo collision (PIC-MCC) method. Emphasis is placed on the electrons in the bullet in the jets. In Ar, the bullet velocity is low and the electron density in the bullet is high, compared to He. In He, the energy spectrum of electrons (ESEs) in the bullet is of a low peak and a wide distribution, and the corresponding average electron energy is evidently high, contrary to Ar. The reason for this evident difference is mainly due to their different excitation energies and ionization energies as well as their different cross sections. In Ar and He, two electron densities N _e and Next in the bullet have been calculated. The former is the electron density in the bullet and the latter refers to a density of electrons in the bullet with energy above the lowest excitation threshold inducing the dissociative electron attachment (DEA) to water molecules. Consequently, not only N _e but also N _ext in Ar is higher than in He, meaning more generation of reactive species in aqueous solution via DEA mechanism. The ESEs in the bullet in Ar are more sensitive to the applied voltage amplitude than in He, thus in Ar, the increase in the applied voltage amplitude induces more evident increase in the average electron energy in the bullet than in He.