Comparison Study of Spatiotemporally Resolved Emissions of Nanosecond Pulsed Microplasma Jets

Abstract

A single needle-electrode helium microplasma jet powered by 5- or 164-ns 8-kV pulses at 500 Hz is investigated to elucidate the impact of nanosecond pulse rising rate and pulse duration on emission behaviors of plasmas. For the 5-ns pulsed plasma, the energy per pulse was measured to be 83 $\mu \text{J}$ , and it is 1.3 times of the 164-ns pulsed plasma. Time-integrated, spatially resolved emissions revealed that the shorter pulsed plasma generated higher production of excited $N_{2}^{+}$ (by a factor of 1.3) but less excited $N_{2}$ , He, O, and OH productions. In addition, all of these species maintained their maximal emissions near the needle nozzle, implying the importance of direct electron impact on generation of these excited species. Temporally resolved emissions showed that comparable or more excited species were produced by the 5-ns pulsed plasma for the first 100 ns, and the additional productions of excited species by the longer pulsed plasma occurred during the failing phase of the 164-ns voltage pulse. These findings suggested that rising and falling rates (≥ 1011 V/s) of a voltage pulse may not only play important roles in energy deposition during the plasma initiation but also influence the plasma chemistry that is associated with the production of excited species. The voltage pulse duration became important for the streamer development and charge transfer, which critically contributed to the total production of the reactive plasma species.