Energy Spectrum of Electrons in the Plasma Bullet and Its Applied Voltage Effect in Atmospheric-Pressure Argon Plasma Jets

This article presents a numerical investigation on the energy spectrum of electrons (ESE) in an atmospheric-pressure argon plasma jet, based on a typical needle-plate discharge system with a positive applied voltage and a 1-D particle-in-cell Monte Carlo collision model. The spatiotemporal evolution characteristics of the ESEs in the three characteristic areas have been revealed. The three characteristic areas refer to the plasma bullet (the bullet), the dark channel behind the bullet, and the dim area in front of the bullet. The effects of the applied voltage amplitude on the ESEs have also been shown. As the bullet propagates, the ESEs in the bullet and the dark channel present an increasingly high peak and a gradually narrowing distribution, showing a decrease in average energy $E_{\mathrm {ave}}$ of electrons, and in the dim area, the evolution of the ESE is opposite to that in the bullet. In contrast, the bullet has a substantial contribution to energetic electrons above the excitation threshold of argon, which shows that in applications of the plasma jet in biomedicine, the particles transporting into the aqueous solutions on the surface of living tissues may contain substantial non-subionization electrons. In the three characteristic areas, ESEs present an increasingly low peak and an increasingly wide distribution with the increase in the applied voltage amplitude, meaning an increasing $E_{\mathrm {ave}}$ . Compared to the other two areas, $E_{\mathrm {ave}}$ in the bullet is evidently high. In particular, a high applied voltage can induce a remarkable increase of high-energy electrons in the bullet, reaching a ratio of about 26% much higher than that in the other areas, under the applied voltage of 7 kV. This is of significance for plasma medicine.