An Investigation of the Control of Electron Energy in the Atmospheric-Pressure Helium Plasma Jet

Abstract

In this paper, an investigation of the control of electron energy in the atmospheric-pressure helium plasma jet produced by a needle–plane discharge system has been performed by means of the numerical simulation based on a 2-D self-consistent fluid model. Due to the consideration of practical application of the cold atmospheric-pressure plasma jet (CAPPJ) in the fields of biomedicine, in this paper, the mean electron energy (MEE) nearby the cathode plane is accounted for. The parameter dependences of the MEE have been systematically calculated. The considered parameters include the voltage amplitude and rising time $t_{r}$ of the applied voltage pulse as well as needle electrode radius. The voltage amplitude is changed in the range from 3 to 19 kV, and $t_{r}$ is from 1 to 200 ns. The spatial electric field has also been analyzed in detail because of its dominant role in controlling electron energy. In addition, the effect of electron energy on the production of the reactive species OH, O3, and H2O2 has also been investigated when considering the addition of humid air to the working gas helium. This paper gives the following significant results. For a given $t_{r}$ , the MEE increases almost linearly with increasing voltage amplitude. The effect of voltage amplitude on the MEE becomes much more evident for smaller $t_{r}$ and is obviously weak for the $t_{r}$ with a few hundreds of nanoseconds. Fixing the voltage amplitude, the MEE decreases with the increase in $t_{r}$ . The dependence of the MEE on $t_{r}$ is evident for a high voltage amplitude and the reverse is true for the lower voltage amplitudes. As a whole, the higher voltage amplitude and smaller rising time $t_{r}$ or their synergism should be considered for obtaining high electron energy. Especially, an MEE of about 10 eV can be obtained, which covers the energy thresholds inducing all the processes of dissociative electron attachment to water molecule and thus is of essential importance for the production of the reactive species such as OH, H2O2, and O3 in the interactions of plasmas with the aqueous solution embedded in biofilms or tissues. The needle electrodes with different radii result in a minor variation of the MEE under the fixed voltage amplitude and $t_{r}$ . Therefore, changing the radius of the needle electrode is not an effective solution to the control of electron energy in the CAPPJ. Furthermore, the parameter dependence of the spatial electric field is similar to that of the MEE, which reveals the dominant role played by the spatial electric field in controlling electron energy in CAPPJs. Considering the addition of humid air to the working gas helium, with the increase in electron energy, the averaged densities of OH and O3 increase, but that of H2O2 decreases.