This work investigates the temporal dynamics of O atoms in nonequilibrium atmospheric pressure plasma (NAPP) generated by kHz nanosecond pulsed discharge. Two‐photon laser‐induced fluorescence (TALIF) method is used to measure the time resolution of O atom density from the first discharge pulse in two gas mixtures, He + 0.4%O2 and He + 0.4%air. The discharge frequencies of 1 and 10 kHz are considered in the experiment. The results show that the O atom density does not accumulate with increasing number of pulses in both gas environments at 1 kHz. However, at 10 kHz, a cumulative effect of O atom density with the number of pulses is observed in both gas mixtures. After 10 and 300 discharge pulses in He + 0.4%O2 and He + 0.4%air, respectively, the O atom density saturates at the same moment after each discharge cycle. It was found that even after a long operating period of discharge, the decay of O atom density during each discharge cycle is not negligible. The O atom density in He + 0.4%O2 varies in the range of 2.85 × 1021 m−3–4.29 × 1021 m−3 while the O atom density in He + 0.4%air varies in the range of 2.60 × 1021 m−3–3.19 × 1021 m−3. This indicates that the choice of diagnostic time points is important for the O atom density measurements when using TALIF to diagnose kHz pulsed NAPP. In addition, 0D plasma chemical kinetics models are developed for the two gas mixtures to investigate O atoms' production and consumption processes. The causes of the cumulative effect of O atom density at 10 kHz, the saturation effect, and the formation of the periodic variation trend are also investigated. The simulation results show that the consumption rate of O atoms and the O atom density are directly correlated. As the number of pulses increases, the O atom consumption rate increases, which gradually counteracts the number of O atoms generated during the pulse discharge. This leads to delay and saturation of the cumulative effect of O atoms.

Nitric oxide (NO) is a crucial molecule for living organisms, which acts as a messenger in most mammalian organs, involves in vascular homeostasis, antimicrobial defense, immune system regulation and host defense processes, is an important element for developing and signaling the nervous system, and affects cell growth, apoptosis and senescence. Fortunately, plasma jet can generate an abundant amount of NO. In this paper, the NO generated by a helium plasma jet when it is interacting with skin is diagnosed using laser-induced fluorescence (LIF) method. The effect of the distance d between the nozzle and the skin surface, the gas parameters (gas flow rate, O2 and H2O percentage) and the applied voltage parameters (the amplitude and frequency of the voltage) on the NO density is explored. The results show that the NO density increases and then decreases as the gas flow rate increases. The gas flow rate corresponding to the maximum NO density is positively correlated with the distance d. In the case of high gas flow rate or small distance d, premixing a small amount of O2 or H2O content lead to higher NO density. Higher voltage amplitude and frequency can increase NO density, but high voltage amplitude at high flow rate and small distance d lead to a decrease in NO density. The generation pathway of NO is mainly affected by N2(A), N, O2, O, OH, H and NO2, and the consumption pathway is closely related to the particles N, O, OH, O3 and HO2. Among them, N and O atoms participate in both generation and consumption. Therefore, the synergistic effect of multi-parameters, the emergence of different effects is closely related to the synergistic effect of these two particles.