Kinetic study on microwave discharge plasma and its afterglow
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摘要: 研究了微秒脉冲聚焦微波束气体放电等离子体的动理学过程。数值模型基于自洽求解的微波电场亥姆霍兹方程、粒子连续性方程以及电子能量、气体分子振动能量和平动能量的平衡方程，并与等离子体动理学反应互相耦合。对比了国外报道的近期两项相关实验：次MW级X波段9.4 GHz微波氮气击穿和MW级W波段110 GHz微波大气击穿。在次MW级实验中，计算所得电子激发态N2(C3Πu)的数密度与实验所测发射光谱第二正带隙的强度一致；在MW级实验中，模拟结果重复了发射光谱测量所得振动温度和平动温度对放电气压的依赖关系。结果揭示了上述模拟和实验符合的内在物理机制。Abstract: Gas discharge plasmas generated by μs-pulse focused microwaves are investigated. The model is based on a self-consistent solution to Helmholtz equation for microwave field, particle continuity equations, and the energy balance equations, coupled with plasma kinetics. Two recent experiments are studied: a. sub-megawatt (MW) X-band 9.4 GHz microwave breakdown in nitrogen; b. MW-class W-band 110 GHz microwave breakdown in 100-10 000 Pa air. In experiment a, the tracked density of electronic states N2(C3Πu) agreed with the measured intensity from second positive system (SPS) of optical emission spectroscopy (OES). In experiment b, the simulation results reproduced the dependence of nitrogen vibrational and translational temperature on air pressure measured by OES. The underlying mechanisms for the above coincidences are unveiled.
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Key words:
- high power microwave /
- gas discharge /
- plasma /
- energy balance /
- optical emission spectroscopy
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图 1 气压200 Pa氮气放电过程中N2(C3Πu)激发态的时空分布(相对位置从0到1对应微波入射的方向)
Figure 1. Spatio-temporal distribution of N2(C3Πu) in 200 Pa nitrogen discharge(from normalized distances 0 to 1 corresponding to microwave incident direction)
图 2 不同气压下微波氮气放电余辉中等离子体电子的弛豫过程
Figure 2. Time evolution of plasma electron density in afterglow of microwave discharge in nitrogen at different pressures
图 3 微波电场在击穿阈值附近时氮气振动温度和平动温度随大气压强的变化
Figure 3. Vibrational temperature Tv and translational temperature Tg as functions of air pressure near breakdown threshold
表 1 氮气等离子体中简化的反应过程
Table 1. Reduced reaction set of nitrogen plasma used in model
No. of reactions reaction rate constant reference E1 electron elastic collisions (momentum transfer)
e+N2⇔e+N2
excitation of rotational levels
e+N2⇔e+N2(rot)
excitation\de-excitation of vibrational levels
e+N2⇔e+N2(X1Σg, v=0~8)
excitation of electronic levels
e+N2⇔e+N2(A3, B3, A′1, C3)
electron impact ionization
e+N2⇔e+e+N2+rate constant (cm3·s-1)
calculated from cross section
σ by Bolsig+[12] Q2 quenching of electronic states
N2(B3)+N2⇔N2(A3)+N23×10-11 cm3·s-1 [13] Q3 N2(A3)+N2⇔N2+N2 3×10-16 cm3·s-1 [13] Q4 N2(A′1)+N2⇔N2(B3)+N2 1.9×10-13 cm3·s-1 [13] Q5 N2(C3)+N2⇔N2(B3)+N2 10-11 cm3·s-1 [14] Q6 N2(C3)+N2⇔N2(A′1)+N2 10-11 cm3·s-1 [13] O7 optical transition*
N2(C3)⇔N2(B3)+hν1.39×107 s-1 [15] O8 N2(B3)⇔N2(A3)+hν 1.35×105 s-1 [13] P9 pooling reaction
N2(A3)+N2(A3)⇔N2(B3)+N23×10-10 cm3·s-1 [13] P10 N2(A3)+N2(A3)⇔N2(C3)+N2 1.5×10-10 cm3·s-1 [14] C11 charged particle reaction
N2(A3)+N2+⇔N3++N3×10-10 cm3·s-1 [16] C12 2N2+N2+⇔N4++N2 8.5×10-29 cm6·s-1 [17] C13 N4++N2⇔2N2+N2+ 2.1×10-16exp(Tg/121) cm3·s-1 [13] C14 N4++e⇔N2+N2(C3) 2.6×10-6(300/Te)0.53 cm3·s-1 [18] C15 N2++e⇔N+N 1.8×10-7(300/Te)0.39 cm3·s-1 [18] *Optical transitions from other electronic states with longer lifetime are neglected.
Electron temperature and gas temperature are in units of Kelvin.
For a complete air plasma chemistry, the readers may refer to our previous paper[19].
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