| The low temperature non-equilibrium plasmas at atmospheric pressure have shown important application values in ozone generation, material process and plasma medicine, and they can be generated by various kinds of atmospheric discharges. Atmospheric discharges are more suitable for the industrial applications without the vacuum facilities, and they have attracted more attentions in recent years.It is very difficult to diagnose the plasma parameters in atmospheric steamer discharges, mainly because the discharge evolution occurs within several hundred nanoseconds, and at the other hand numerical simulation can offer many useful information which can deepen the understanding of atmospheric streamer discharges. With respect to many other numerical simulation methods, The PIC-MCC method can provide a more accurate description for the streamers without the assumption of a given electron energy distribution function, but with a requirement of huge computation resource. Based on the simulation data from the three-dimensional PIC-MCC code, the conclusions are as follows:First we numerically study the propagation of negative streamer without considering the photo ionization under a given uniform electric field between parallel plate electrodes. The simulation show net charges with very high density can be observed both at the head and the tail of the discharge channels, and the electric field mainly generated from the net charges at the value of 20 MV/m around these regions. But within the discharge channels the field is only about 0.7 MV/m, which is much lower than the applied field of 7 MV/m. the propagation velocity of streamer is about 75 cm/μs with the radius of the discharge channel of 300 μm from the numerical data, which shows a good agreement with the given experimental observation. With the increase of the electric field strength from 6.5 MV/m to 7.5 MV/m, the maximum electron density in the head of the streamer increases from 5×1020/m3 to 11×1020/m3, at the same time, the maximum electric field strength increases from 19.8 MV/m to 26.3 MV/m, and the propagation velocity of streamer also rises from 68.9 cm/μs to 68.9 cm/μs. By reducing the electrode gap from 1.2 mm to 0.5 mm at a given applied voltage of 6.5 kV, the maximum electron density in the head of the streamer increases from 4.6×1019/m3 to 14×1020/m3, the maximum electric field strength increases from 10.9 MV/m to 43.8 MV/m, the velocity of the streamer also increases from 35 cm/μs to 166 cm/μs . Then, the results show that at a constant voltage a smaller electrode gap can provide much larger electric field and electron density, and the streamer propagates much faster.If the photo ionization is considered in the simulation code, both positive streamer and negative streamer can be observed meanwhile i.e. the propagation of double-headed streamer can be observed. In addition, the secondary electron avalanche can be clearly observed around the main discharge channel because of the non-local effect of photo ionization and background ionization, In contrast to the negative without photo ionization, the space charges show much broader distribution. The photo ionization and background ionization are comparative studied in a larger spatial scale, with respect to photo ionization. The discharges take place in a larger range due to the background ionization, but meanwhile the photo electrons are mostly generated much closes to the main discharge channel. The numerical data offered in this thesis can further deepen the insights of atmospheric streamer discharges at very small gaps. |
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