电磁波在大面积等离子体片中传播特性的分析

Analysis of propagation properties of electromagnetic waves through large planar plasma sheets

脉冲磁约束线形空心阴极放电形成的大面积等离子体片可应用于等离子体天线、隐身及模拟超音速飞行器表面的等离子体鞘套.本文首次利用实测等离子体片电子密度时空分布和横向场传播矩阵法,研究了电磁波在等离子体片中反射率、透射率、吸收率随频率及脉冲放电时间的变化特征.结果表明:极化方向平行磁场的电磁波,在小于截止频率的低频带内具有较高的反射率和吸收率,增大电流,反射率增加,吸收率下降,在大于截止频率的高频带内反射率和吸收率较低,增大电流,透射率下降,吸收率升高;极化方向垂直磁场的电磁波在高混杂谐振频率附近存在吸收率明显增强的吸收带,谐振吸收峰值与放电电流无关;脉冲放电期间,电磁波的反射率、透射率与吸收率由不稳定过渡到稳定的时间约为100μs,过渡时间随着放电电流的增加而增大,极化方向垂直磁场、小于截止频率的电磁波在稳定放电阶段谐振吸收较强.本文的研究成果对利用等离子体片实现对电磁波的稳定高反射作用具有重要意义.

Large planar plasma sheets, generated by a linear hollow cathode in pulse discharge mode under magnetic con-finement, can be used in the field of plasma antenna, plasma stealth, and simulation of a plasma layer surrounding vehicles traveling at hypersonic velocities within the Earth’s atmosphere. Firstly, to investigate the propagation prop-erties of electromagnetic waves at different frequencies and polarization, the transverse field transfer matrix method is introduced. Secondly, the measured electron density temporal and spatial distribution and the transverse field transfer matrix method are utilized to calculate the reflection, transmission and absorption of electromagnetic waves by large planar plasma sheets with different currents. Finally, 1 GHz （less than the critical cut-off frequency） electromagnetic waves and 4 GHz （greater than the critical frequency） electromagnetic waves are chosen to investigate the evolution of propagation properties during the pulsed discharge period. Results show that both the reflection and absorption of the electromagnetic waves are greater for their polarization direction parallel to that of magnetic field, and their frequencies lower than the critical cut-off frequency, and as the discharge currents rise, the reflection increases while the absorption decreases. However both the reflection and absorption of the electromagnetic waves with their polarization direction perpendicular to the magnetic field direction and their frequency greater than the critical cut-off frequency become less, and as the discharge currents rise, both the reflection and absorption will increase. For the electromagnetic waves with their polarization direction perpendicular to the magnetic field direction, there is an upper hybrid resonance absorption band near the upper hybrid resonance frequencies, in which the absorption is significant but the absorption peak value is not affected by the discharge current. The propagation characteristics of the electromagnetic waves with polariza-tion direction perpendicular to the magnetic field direction are the same as that of the electromagnetic waves with the polarization direction parallel to the magnetic field direction, except the upper hybrid resonance absorption. During the pulse discharge period, the propagation characteristic of the electromagnetic waves experiences an unstable phase before reaching steady states. The transition time is about 100 μs and increases as the discharge current rises. The upper hybrid resonance absorption is significant during the phase of steady state for waves with frequency lower than the critical cut-off frequency and polarization direction parallel to the magnetic field direction. For the applications of a large planar plasma sheet to reflect electromagnetic waves effectively and steadily, the pulse discharge period should be larger than 100 μs, and its discharge current should be large enough to make the critical cut-off frequency greater than the frequency of incident wave, and its polarization direction should be parallel to the magnetic field direction.