Abstract
Atmospheric Pressure Plasma Jets (APPJs) have been paid much attention and developed rapidly in recent years due to no necessity for expensive vacuum systems. As can be seen, APPJs produce non-thermal equilibrium plasmas as RF powers are applied. Extremely high electron temperature (around 1 eV) and active plasma particle densities (e.g., e- /above1017 m-3, O, O2* and O3) are generated, but the gas temperature is still maintained at room temperature. Therefore, they are widely used in biomedical applications, sterilization, heat-sensitive surface treatment, and depositing film on soft electric boards. Besides, among all type of APPJs, the linear slot structure APPJ is much suitable for industry applications which allows for continuous in-line processes and is easy to scale-up. Nevertheless, a question, i.e., very rare experimental equipments could measure the plasma characteristics such as plasma temperature and plasma particle densities in the bulk plasma, arise. That’s why many physical/chemical mechanisms in APPJs can’t be explained clearly. Hence, in this thesis, we not only investigate the α mode discharge characteristics of a linear slot structure He/O2 APPJ by using a two-dimensional fluid model numerical analysis (Numerical simulation software, ESI CFD-ACE+ package, is employed.), but also indicate that the importance of Penning ionization and the O3 formation mechanism by calculating the gas phase reaction rates. Simulation results point out that the penning ionization, i.e., He*+O2□He+O2++e-, He*+O□He+O++e-, is a main source of producing oxygen positive ions. The main reason is that He metastables (19.82) have sufficient energy to ionize oxygen species comparing with other metastables. Then, the main O3 formation mechanism is also clarified by gas phase reaction rates in our simulation. We find that O3 formation mechanism is different in the discharge region (DR) and Effluent Region (ER), respectively. In DR, O- and O2* are the most important species to ozone formation (O-+O2*□O3+e-). However, in ER, the O is the key particle for ozone generation due to the decrease of electrons, i.e., He+O2+O□He+O3 is the dominant gas phase reaction for O3 formation in the effluent. Further, we analyze the distribution of the average electron temperature and plasma particle density. Simulation results are presented by changing plasma parameters, such as different applied powers, flow rates and fraction of oxygen. According to the simulation results, as the applied power increases, the electron density increases and the plasma sheath thickness shrinks due to higher electric field in the discharge gap. At the same time, the most dominant species are O(3P), O2*(1△g),O3 in the ER and O(3P) (around 2×1020 m-3), O2*(1△g) (around 6×1019 m-3), O*(1D) in the DR besides He and O2. The atomic oxygen (O) density is even up to 2×1020 m-3. However, to increase the flow rate, the average electron density slightly decreases (5-10%) in the discharge gap due to the insufficient ionization energy. But, the mass flux of atomic oxygen increases with the flow rate. This simulation result is a great benefit to surface treatments. Moreover, by changing simulation parameter, different fraction of O2, we observe the He/O2 discharge characteristics are determined by the ionization reactions and higher attachment rates of electrons with O2 and O. The electron density decreases to form negative ions (O-, O2-) due to attachment reaction. And, the competing reactions, the Penning and electron impact ionizations, produce a lot of electrons and positive ions (O2+, O+). Besides, because the Penning ionization, He* number density decreases as the fraction of oxygen increases. Meanwhile, the positive oxygen ions increase markedly. One may notice that, an interesting phenomenon, the negative ion-dominated property of He/O2 discharge is more obvious as the fraction of oxygen increases which has shown in our simulations. In summation, the simulation results not only provide an index to improve the He/O2 APPJ operation for better control, but also clarify some physical/chemical mechanisms, e.g., penning ionization and O3 formation mechanism, which few studies have been reported. We think the investigation of the thesis is a significant contribution to He/O2 APPJ.