稀薄空气中圆柱腔体内系统电磁脉冲的混合模拟

系统电磁脉冲广泛存在于强电离辐射环境中,且难以有效屏蔽.为了评估稀薄空气对系统电磁脉冲的影响,本文基于粒子-流体混合模拟方法,建立了三维非稳态模型,计算并分析了稀薄空气等离子体的特性以及其与电磁场响应的相互作用.结果表明,压力越高,光电子发射面附近的次级电子数密度越高,轴向分布的梯度越大,腔体中部的电子数密度在20 Torr(1 Torr=133 Pa)下出现峰值,而电子温度随压力升高单调递减.腔体内的稀薄空气等离子体阻碍了空间电荷层的产生,电场响应峰值比真空条件下的低了一个数量级,电场脉冲宽度也显著降低.光电子运动特性决定了电流响应的峰值,压力升高,到达腔体末端的电流先增加再减小.而等离子体电流会抑制总电流的上升速率,并使电流响应出现拖尾.最后,将数值模拟结果与电子束模拟系统电磁脉冲的实验结果进行比较,验证了本文混合模拟模型的可靠性.本研究所采用的混合模拟方法相比于粒子云网格-蒙特卡罗碰撞方法,大幅减小了计算消耗.

Hybrid modelling of cavity system generated electromagnetic pulse in low pressure air

The surface of metal system exposed to ionizing radiation(X-ray andγ-ray)will emit high-energy electrons through the photoelectric effect and other processes.The transient electromagnetic field generated by the highspeed electron flow is called system generated electromagnetic pulse(SGEMP),which is difficult to shield effectively.An ongoing effort has been made to investigate the SGEMP response in vacuum by numerical simulation.However,the systems are usually operated in a gaseous environment.The objective of this paper is to investigate the effect of low-pressure air on the SGEMP.A three-dimensional hybrid simulation model is developed to calculate the characteristics of the electron beam induced air plasma and its interaction with the electromagnetic field.In the hybrid model,the high-energy photoelectrons are modelled as macroparticles,and secondary electrons are treaed as fluid for a balance between efficiency and accuracy.A cylindrical cavity with an inner diameter of 100 mm and a length of 50 mm is used.The photoelectrons are emitted from one end of the cavity and are assumed to be monoenergetic(20 keV).The photoelectron pulse follows a sine-squared distribution with a peak current density of 10 A/cm;,and its full width at half maximum is 2 ns.The results show that the number density of the secondary electrons near the photoelectron emission surface and its axial gradient increase as air pressure increases.The electron number density in the middle of the cavity shows a peak value at 20 Torr(1 Torr=133 Pa).The electron temperature decreases monotonically with the increase in pressure.The low-pressure air plasma in the cavity prevents the space charge layer from being generated.The peak value of the electric field is an order of magnitude lower than that in vacuum,and the pulse width is also significantly reduced.The emission characteristic of the photoelectrons determines the peak value of the current response.The current reaching the end of the cavity surface first increases and then decreases with pressure increasing.The plasma return current can suppress the rising rate of the total current and extend the duration of current responses.Finally,to validate the established hybrid simulation model,the calculated magnetic field is compared with that from the benchmark experiments.This paper helps to achieve a better prediction of the SGEMP response in a gaseous environment.Compared with the particle-in-cell Monte Carlo collision method,the hybrid model adopted can greatly reduce the computational cost.