Experimental Study On The Role Of Seed Electrons In Atmospheric Pressure Non-equilibrium Plasma Jets

According to the streamer theory or ionization wave theory, seed electrons are necessary for the propagation of atmospheric-pressure non-equilibrium plasma jets (APNP-Js). However, due to the limitation of methods and techniques, the research of seed electrons is an extremely tough issue in the field of gas discharge or plasma science, which largely constrains the development of streamer theory or ionization wave theory. In this work, to investigate the mechanism of seed electrons production and its effects on the propagation of APNP-Js, a self-designed charge measurement system based on drift and diffusion principle and a diagnostic system with μm spatial resolution and ns time resolution are built.Because the streamer theory or ionization wave theory is based on seed electron hypothesis, it is urgent to carrier out experimental studies on the mechanism (photo-ionization and background ionization) of seed electrons production.(1) Photo-ionization. First, to confirm the existence of photo-ionization in APNP-Js for the first time, a fA level photo-current measurement device is carefully designed and the interference of discharge charges is totally excluded. Results show that photo-ionization indeed exists in APNP-Js. Second, to detect the photon with high energy emitted from photo-ionization process, the noise ratio is strongly improved by combination of high-speed photography and spectroscopy diagnostics. Two extreme ultraviolet light bands (94.1-95.3nm and101-101.7nm) are observed, which are close to the theoretical prediction. Third, a two dimensional streamer model is used to calculate the density of seed electrons generated by photo-ionization. The results show that the seed electron density in front of the stream head is close to108cm-3. Finally, it interesting to find that photo-ionization could largely in favor of the generation of APNP-Js.(2) Background ionization. The absolute density of seed electrons produced by background ionization is measured by the self-designed charge measurement system. The density of background seed electrons is linearly dependent on the pulse repetition frequency. When the pulse frequency is fixed at8kHz, the background charge density is equal to or greater than3.4×109cm-3. On the other hand, when the pulse frequency is reduced to0.2kHz or even lower, the background charge density in APNP-Js is approaching to the seed electrons density generated by photo-ionization. It’s reasonable to predict that a mode transition in the propagation of APNP-Js will happen.The propagation of APNP-Js is of high repeatability, which is one of the key features distinguished from the propagating streamer in open space. However, the physical mechanism of this phenomenon is not well understood. Therefore, in this work, the role of residual charges in the repeatability of the dynamics of APNP-Js is investigated.(1) It is the first time to observe that the dynamics of APNP-Js is stochastic, rather than repeatable as people known, when the pulse frequency is reduced to0.2kHz or lower. This result is in good agreement with the prediction of background seed electrons. Thus, it is the first time to propose that the repeatability of dynamics of APNP-Js is closely related to the density of background seed electrons. Because the background seed electrons is greatly dependent on the working gas species and discharge pulse numbers, in the next, the relationship between the background seed electrons and the propagation of APNP-Js is investigated by varying the gas species or discharge pulse numbers.(2) It is found that the repeatability of APNP-Js propagation is dependent on the nitrogen additive gas but independent on the oxygen additive gas.(3) When the discharge pulse numbers increase from1to over100, the density of background seed electron increases, leading to the propagating mode transition of APNP-Js from stochastic mode to repeatable mode. Detail analysis show that, when the density of background seed electrons is109cm-3or higher, the propagation of APNP-Js is repeatable; in the contrast, the propagation of APNP-Js is not repeatable.Based on the interaction of seed electrons and the propagation of APNP-Js, two counter-propagating APNP-Js in a tube is specially designed and investigated. By controlling the fluid state of gas flow or perimeters of pulse voltage, it is the first time to observe "snake-like" propagation of APNP-Js and the ignition of a third APNP-J by residual charges without external applied voltage.(1) When the gas flow is turbulent, APNP-Js propagate snake-like in a curve, rather than "bullet-like propagating forward" generally. Detail analysis shows that, as the gas flow switches to the turbulent mode, the density of background seed electrons becomes non-uniform, resulting in the snake-like propagation of APNP-Js.(2) To affect the level of seed electron density, the symmetry of external applied pulse voltage is adjusted. When two voltage pulses are unsymmetrical, a third APNP-J presents between the two APNP-Js, instead of a dark zone. High-speed photographs depict that the third APNP-J is ignited at300ns after the pulse voltage falling to zero, which indicates that the ignition of the third APNP-J is no directly related to the external applied voltage. Further analysis shows that the high electric field caused by the high background charges density should be responsible for the ignition of the third APNP-J.In conclusion, the mechanism of seed electrons production, the key role of seed electrons in the repeatability of the propagation of APNP-Js, and the two new phenomena are investigated. This work not only stands back experimentally for gas discharge theory, plasma simulation and diagnostics, but also provides new ideas for the transportation of reactive species in APNP-Js.