Optical diagnostics of the characteristics of a square unipolar nanosecond pulse-driven atmospheric pressure helium plasma jet

Compared with the traditional sinusoidal voltage source, a short rising nanosecond voltage source can generate a high electron current for a short rising time. This paper investigates how the nanopulse parameters such as the voltage amplitude, pulse duration, and repetition frequency affect the radical generation and the plasma bullet propagation in an atmospheric pressure helium plasma jet. An intensified charge-coupled device was used to observe the bullet propagation in the nanosecond gate mode. The plasma bullet’s propagation speed is mainly affected by the applied voltage and externally biased electrodes rather than the pulse duration or the driving frequency. In contrast, optical emission spectroscopy diagnostics estimate that the radical density inside the atmospheric pressure plasma jet mainly increases with the repetition frequency. At the same time, the population of high-energy electrons can be controlled with the unipolar voltage amplitude. Thus, unipolar nanosecond pulses make it possible to control the emitting charges and the generated radicals independently.