时空非均匀等离子体鞘套中太赫兹波的传播特性

Propagation characteristics of terahertz waves in temporally and spatially inhomogeneous plasma sheath

高超声速飞行器再入地面的过程中,其周围等离子体的电子密度是非均匀且随时间变化的.对于不同的再入高度,飞行器周围的温度和压强也会发生改变.因此,研究电磁波在时空非均匀等离子体鞘套中的传播特性意义重大.首先建立了时变非均匀的等离子体鞘套模型,然后通过经验公式得到温度、压强与碰撞频率三者的关系.采用时域有限差分方法计算了太赫兹波段中不同电子密度弛豫时间、温度、压强时的反射系数、透射系数和吸收率.研究结果表明:在太赫兹波段中,电子密度的弛豫时间越长,温度越高,压强越大,电磁波越容易穿透等离子体;弛豫时间越短,温度越低,压强越小,等离子体对电磁波吸收率的变化越明显.这些结果为解决"黑障"问题提供了理论依据.

The plasma sheath is produced by high-temperature heating during the reentry of a hypersonic vehicle to the Earth atmosphere.Temperature around the vehicle rises rapidly because of severe friction with air.The vehicle temperature behind friction is high enough to excite various real gas effects including chemical reactions of air,which contains ablation particles of vehicle,free electrons,and ions.The plasma sheath greatly affects the transmission of electromagnetic waves and has very strong interference on the communication signals,which results in interrupt between the target and the ground station,namely,blackout.The electron density of plasma sheath surrounding the aircraft is inhomogeneous and varies with time.Temperature and pressure will also change at different altitudes.Therefore,it is meaningful to investigate the propagation characteristics of electromagnetic waves in temporally and spatially inhomogeneous plasma sheath.The temporally and spatially inhomogeneous plasma sheath model is introduced and the electron density data of the National Aeronautics and Space Administration（NASA） reentry vehicle is employed.The relationships among temperature,pressure,and collision frequency are obtained with the empirical formula of collision frequency.Then,the reflection coefficient and transmission coefficient of time-varying single layer plasma are calculated with the shift operator finite-difference time-domain（SO-FDTD） method.These results are compared to verify the correctness of the proposed method.Finally,the LTJEC-FDTD method is used to calculate the reflection coefficient,transmission coefficient and absorptivity at different relaxation time,temperature,and pressure in the terahertz（THz） band.The results show that the higher temperature and pressure will enable the electromagnetic wave to penetrate the plasma sheath at high relaxation time of electron density.If the incident wave frequency is lower than the cut-off frequency of plasma,the reflection of electromagnetic wave will be more obvious.However,when the incident wave frequency is in the THz band,the effects of temperature and pressure on the propagation of electromagnetic wave are obviously weakened.The absorption of electromagnetic wave by plasma will be more obvious when the relaxation time,temperature,and pressure decrease.If the relaxation time of electron density is shorter than or equal to the period of THz wave,more energy of electromagnetic wave will be absorbed by the plasma sheath.Contrarily,if the relaxation time of electron density is much longer than the period of THz wave,the absorption of electromagnetic energy will decrease.This study gives some insight into the temporally and spatially inhomogeneous plasma sheath,and provides a theoretical basis for solving the blackout problem.