Temporal electric field and electron density behavior of a He plasma jet with pulse width closing to pulse duration

Abstract

Previously, it was observed that the plasma plume appears with three regimes (dark regime next to the nozzle, bright regime in the middle, and dim regime on the right) where the regime next to the nozzle is dark when the pulse width of the applied voltage is close to the pulse duration. In this paper, to understand such observation, the spatial and temporal resolved electron density and electric field in the three regimes are measured through the Thomson scattering and electric field-induced second harmonic method, respectively. It is found that, in the dark regime next to the nozzle, the electric field is relatively low, and it has a peak value of about 10 kV/cm, but the electron density is high, and it has a peak value of about 4.2 × 1020 m−3. So, the dark regime is like a conductive channel. On the other hand, for the bright regime, the electric field is much higher, and it has a peak value of about 17 kV/cm. However, the electron density is significantly lower than that in the dark regime, and its peak value is only about 1020 m−3. Even in the dim regime, the electric field is higher than that in the dark regime, and it has a peak value of about 13 kV/cm. Because electron temperature is related to the electric field, the results obtained in the paper indicate that the brightness of the plasma plume at different regimes is mainly decided by the electron temperature rather than the electron density. Finally, when a trace amount of O2 is mixed into working gas He, the dark regime disappears, and this is believed to be due to the fast attachment of electron to O2, which results in the decrease in the residual electron density in the regime next to the nozzle.