Zhenyu Wei's research while affiliated with The University of Tokyo and other places

In this study, we investigated the effect of various O2 concentrations, from 20% to 90%, in nitrogen–oxygen ( N2/O2 ) mixtures on the characteristics of secondary streamers. As oxygen molecules have different molecular characteristics from nitrogen molecules in terms of ionisation threshold and electron attachment property, streamer discharges generated under various nitrogen–oxygen ratios may exhibit differing characteristics such as electron density, electric field, and radical formation. We focused on the changes in these parameters in secondary streamers using simulations. Simulations were first performed under the same conditions as those in previous experiments to compare the results of the ozone production, discharge current, and discharge emission characteristics. To compare the ozone production characteristics, simulated O radicals–the precursor of ozone–were used in the simulation for simplicity. This comparison showed that, although the absolute values of each parameter were different, the simulation exhibited a similar trend in the case of the experimentally obtained oxygen concentration dependence. After the validity of the simulation was verified to some extent via a comparison with the experiment, the results obtained from the simulation were analysed in detail. The results showed that, although the electric field strength in the secondary streamer did not change much as the oxygen concentration increased, the decrease rate of the electron density was greatly accelerated by the electron attachment reaction of oxygen. As a result, many of the electrons had already dissipate during the development of the primary streamer, and few electrons remained when the secondary streamer was formed. This effect suggests that the ratio of the amount of O radicals produced in the primary streamer to that produced in the secondary streamer changes as the oxygen concentration changes.

This study aims to investigate the behaviour of positive streamer discharge in various N2/O2 mixtures with different O2 concentrations via two-dimensional simulations. The proposed model considers the effects of photoionisation caused by the photons generated by excited oxygen atoms OI, which enables the simulation of streamers in oxygen rich conditions. The simulation results indicate that the propagation velocity and diameter of the streamer increase with the oxygen concentration, while the emission intensity decreases. The result conforms to the experimental results of our previous work. Moreover, the simulation results suggest that changes in the ionisation rate caused by variations in the oxygen concentration affect the streamer propagation, while the impact of photoionisation is limited.

This study evaluates the individual effects of electron‐impact ionization, attachment, and photoionization on streamer propagation to provide a deeper understanding of the streamer propagation mechanism and useful knowledge to obtain the best set of input data for verification and validation studies. The results show that streamers with higher electron‐impact ionization rates have higher propagation velocities and lower electric fields, while those with higher attachment rates have slower propagation velocities and higher electric fields. Streamers with higher photoionization rates have lower electric fields at the head, but the velocity remains largely unaffected. The effects of different rates of ionization, attachment, and photoionization have been analyzed using an analytical solution called the diameter‐speed relation.

Limited research has been conducted on the formation mechanism of chemically active species in streamer discharges with respect to the oxygen concentration, which is critical to various applications such as ozone generation, air purification, and plasma-assisted combustion, among others. Herein, the oxygen concentration in an N2/O2 gas atmosphere is varied from 1% to 99% under atmospheric pressure and room temperature to investigate changes in the characteristics of streamer discharge propagation and generation of chemically active species. As the oxygen concentration increases from 10% to 90%, the decay rate of the discharge current, propagation velocity of the primary streamer, and ozone production efficiency increase. These phenomena are qualitatively explained by the electron attachment reaction to oxygen molecules and changes in the electron energy distribution function caused by the change in the oxygen concentration. However, the amount of discharge emission from N2(C) cannot be explained by changes in the fraction of electron energy lost in excitation of N2(C) and its quantum yield, implying that changes in the production of N2(C) in the primary and secondary streamers must be considered in a spatiotemporal manner. This study demonstrates that the ozone and N2(C) production characteristics in streamer discharges vary nonlinearly with respect to the oxygen concentration.

Photoionization is a significant source of electrons in streamer discharge. Typically, the photoionization rate is calculated using the integral model or three-exponential Helmholtz model in streamer discharge simulations. However, these models exhibit certain disadvantages, such as low calculation speed and accuracy. To overcome these drawbacks, this article proposes a novel method for calculating the photoionization rate using neural networks (NNs). Two types of datasets were used to train the NNs, namely, the random and Gaussian datasets. The results indicate that the performance of the model when trained using Gaussian datasets is better than that when random datasets are used. Furthermore, the NN model exhibits high computational accuracy under the test conditions, verifying its use as a promising technique to calculate photoionization rates. Additionally, the effects of hyperparameters and attention mechanisms are evaluated, laying the groundwork for the application of NNs in practical streamer models.