The gas desorbed from the dielectric surface has a great influence on the characteristics of microwave breakdown on the vacuum side of the dielectric window. In this paper, the dielectric surface breakdown is describe...The gas desorbed from the dielectric surface has a great influence on the characteristics of microwave breakdown on the vacuum side of the dielectric window. In this paper, the dielectric surface breakdown is described by using the electromagnetic particle-in-cell-Monte Carlo collision(PIC-MCC) model. The process of desorption of gas and its influence on the breakdown characteristics are studied. The simulation results show that, due to the accumulation of desorbed gas, the pressure near the dielectric surface increases in time, and the breakdown mechanism transitions from secondary electron multipactor to collision ionization. More and more electrons generated by collision ionization drift to the dielectric surface, so that the amplitude of self-organized normal electric field increases in time and sometimes points to the dielectric surface. Nevertheless, the number of secondary electrons emitted in each microwave cycle is approximately equal to the number of primary electrons. In the early and middle stages of breakdown, the attenuation of the microwave electric field near the dielectric surface is very small. However, the collision ionization causes a sharp increase in the number density of electrons,and the microwave electric field decays rapidly in the later stage of breakdown. Compared with the electromagnetic PIC-MCC simulation results, the mean energy and number of electrons obtained by the electrostatic PIC-MCC model are overestimated in the later stage of breakdown because it does not take into account the attenuation of microwave electric field. The pressure of the desorbed gas predicted by the electromagnetic PIC-MCC model is close to the measured value,when the number of gas atoms desorbed by an incident electron is taken as 0.4.展开更多
The structure and propagation of the plasma in air breakdown driven by high-power microwave have attracted great interest.This paper focuses on the microwave amplitude and frequency dependence of plasma formation at a...The structure and propagation of the plasma in air breakdown driven by high-power microwave have attracted great interest.This paper focuses on the microwave amplitude and frequency dependence of plasma formation at atmospheric pressure using one two-dimensional model,which is based on Maxwell’s equations coupled with plasma fluid equations.In this model,we adopt the effective electron diffusion coefficient,which can describe well the change from free diffusion in a plasma front to ambipolar diffusion in the bulk plasma.The filamentary plasma arrays observed in experiments are well reproduced in the simulations.The density and propagation speed of the plasma from the simulations are also close to the corresponding experimental data.The size of plasma filament parallel to the electric field decreases with increasing frequency,and it increases with the electric field amplitude.The distance between adjacent plasma filaments is close to one-quarter wavelength under different frequencies and amplitudes.The plasma propagation speed shows little change with the frequency,and it increases with the amplitude.The variations of plasma structure and propagation with the amplitude and frequency are due to the change in the distribution of the electric field.展开更多
The energy transmission of the long microwave pulse for the frequency of 2.45 GHz and 5.8 GHz is studied by using the electron fluid model, where the rate coefficients are deduced from the Boltzmann equation solver na...The energy transmission of the long microwave pulse for the frequency of 2.45 GHz and 5.8 GHz is studied by using the electron fluid model, where the rate coefficients are deduced from the Boltzmann equation solver named BOLSIG＋. The breakdown thresholds for different air pressures and incident pulse parameters are predicted, which show good agreement with the experimental data. Below the breakdown threshold, the transmitted pulse energy is proportional to the square of the incident electric field amplitude. When the incident electric field amplitude higher than the breakdown threshold increases,the transmitted pulse energy decreases monotonously at a high air pressure, while at a low air pressure it first decreases and then increases. We also compare the pulse energy transmission for the frequency of 2.45 GHz with the case of 5.8 GHz.展开更多
基金supported by the National Key Laboratory Foundation 2021-JCJQ-LB-006,China(No.6142411132116)the Natural Science Basic Research Program of Shaanxi Province,China(Nos.2023-JC-YB-512 and 2023-JC-YB-042)+1 种基金the Fundamental Research Funds for the Central Universities,China(No.ZYTS23075)the China Postdoctoral Science Foundation(No.2019M653545)。
文摘The gas desorbed from the dielectric surface has a great influence on the characteristics of microwave breakdown on the vacuum side of the dielectric window. In this paper, the dielectric surface breakdown is described by using the electromagnetic particle-in-cell-Monte Carlo collision(PIC-MCC) model. The process of desorption of gas and its influence on the breakdown characteristics are studied. The simulation results show that, due to the accumulation of desorbed gas, the pressure near the dielectric surface increases in time, and the breakdown mechanism transitions from secondary electron multipactor to collision ionization. More and more electrons generated by collision ionization drift to the dielectric surface, so that the amplitude of self-organized normal electric field increases in time and sometimes points to the dielectric surface. Nevertheless, the number of secondary electrons emitted in each microwave cycle is approximately equal to the number of primary electrons. In the early and middle stages of breakdown, the attenuation of the microwave electric field near the dielectric surface is very small. However, the collision ionization causes a sharp increase in the number density of electrons,and the microwave electric field decays rapidly in the later stage of breakdown. Compared with the electromagnetic PIC-MCC simulation results, the mean energy and number of electrons obtained by the electrostatic PIC-MCC model are overestimated in the later stage of breakdown because it does not take into account the attenuation of microwave electric field. The pressure of the desorbed gas predicted by the electromagnetic PIC-MCC model is close to the measured value,when the number of gas atoms desorbed by an incident electron is taken as 0.4.
基金supported by China National Natural Science Foundation of Shaanxi Province(No.2020JQ-643)China Postdoctoral Science Foundation funded project(No.2019M653545)the Fundamental Research Funds for the Central Universities,China(No.JB210510)。
文摘The structure and propagation of the plasma in air breakdown driven by high-power microwave have attracted great interest.This paper focuses on the microwave amplitude and frequency dependence of plasma formation at atmospheric pressure using one two-dimensional model,which is based on Maxwell’s equations coupled with plasma fluid equations.In this model,we adopt the effective electron diffusion coefficient,which can describe well the change from free diffusion in a plasma front to ambipolar diffusion in the bulk plasma.The filamentary plasma arrays observed in experiments are well reproduced in the simulations.The density and propagation speed of the plasma from the simulations are also close to the corresponding experimental data.The size of plasma filament parallel to the electric field decreases with increasing frequency,and it increases with the electric field amplitude.The distance between adjacent plasma filaments is close to one-quarter wavelength under different frequencies and amplitudes.The plasma propagation speed shows little change with the frequency,and it increases with the amplitude.The variations of plasma structure and propagation with the amplitude and frequency are due to the change in the distribution of the electric field.
基金Project supported by the National Natural Science Foundation of China(Grant No.61501358)the Fundamental Research Funds for the Central Universities,China
文摘The energy transmission of the long microwave pulse for the frequency of 2.45 GHz and 5.8 GHz is studied by using the electron fluid model, where the rate coefficients are deduced from the Boltzmann equation solver named BOLSIG＋. The breakdown thresholds for different air pressures and incident pulse parameters are predicted, which show good agreement with the experimental data. Below the breakdown threshold, the transmitted pulse energy is proportional to the square of the incident electric field amplitude. When the incident electric field amplitude higher than the breakdown threshold increases,the transmitted pulse energy decreases monotonously at a high air pressure, while at a low air pressure it first decreases and then increases. We also compare the pulse energy transmission for the frequency of 2.45 GHz with the case of 5.8 GHz.
