High time resolution diagnosis of electron density in helium plasma jets with impurity gas

Atmospheric pressure helium plasma jets are widely used in biomedical applications. Researchers normally introduce small amounts of nitrogen and oxygen (0.2-1.0%) into helium to enhance the electron density and electron energy, thus increasing the concentration of active species in plasma. To further explore why the combination of impurity gases N-2/O-2 leads to an increase in the electron density from the discharge mechanism, we used a microwave Rayleigh scattering method with excellent time-varying characteristics to monitor the temporal electron density changes when different concentrations of N-2/O-2 were mixed. The research revealed that even trace amounts of N-2/O-2 (0.2%) can increase the peak electron density, with this effect being more pronounced when N-2 is added, increasing from 3.3 x 10(19) to 4.6 x 10(19) m(-3) in pure helium. As the concentration increases, the introduction of O-2 leads to a rapid decrease in the electron density. When 1.0% oxygen is mixed, the electron density decreases from 3.3 x 10(19) to 2.4 x 10(19) m(-3). However, the situation is different when N-2 is added, at 0.5% proportion of nitrogen, the electron density increases to its maximum at 6.5 x 10(19) m(-3). These effects are due to the electronegativity of the oxygen-containing particles or the Penning ionization related to excited nitrogen species.