Effects Of Pulsed Magnetic Field On The Density Reduction Of Plasma Sheath

When the hypersonic vehicle flies at hypersonic speed in the near space,a plasma sheath will be produced.When navigation,remote control,telemetry and security control signals pass through this plasma sheath,serious attenuation will occur,that is,the so-called communication blackout problem.The attenuation amplitude of the signal mainly depends on the density of the plasma sheath.The attenuation of the signal can be reduced by reducing the density of the plasma sheath.As an effective means to reduce the density of plasma sheath,electromagnetic regulation has been studied for a long time at home and abroad.At present,it mainly focuses on orthogonal electric and magnetic field regulation,traveling wave magnetic field regulation and pulse magnetic field regulation.However,these electromagnetic control methods still exit the following problems:(1)These methods need to place the communication antenna in the center of strong magnetic field,which has the influence of mutual interference between antenna and magnetic field and stopband effect.(2)Due to the need to use electrodes to maintain a constant strong electric field,the orthogonal electric and magnetic field regulation has the problem of electrode corrosion when exposed to high-temperature gas.(3)Due to the push-away configuration of pulsed magnetic field regulation,there is still a problem of high-density region in the propagation path of electromagnetic wave.Therefore,this paper proposes a converging pulsed magnetic field method,in which the density reducing region is located at the edge of the electromagnetic coil to avoid the interference between the strong magnetic field and the antenna.In this method,the orthogonal induced electric field is generated by time-varying magnetic field,which solves the problem of electrode exposure.In addition,this configuration solves the problem that there are still high-density areas on the propagation path.Around this method,the main work of this paper is as follows:1.Aiming at the problem that there is still a high-density region in the electromagnetic wave propagation path of the push-away pulsed magnetic window,a convergent pulsed magnetic window is proposed.In order to calculate the density of static plasma under the action of convergent pulsed magnetic window,a two-dimensional axisymmetric plasma density distribution model was proposed.The influence of pulsed magnetic field on the static plasma density distribution is obtained.The simulation results show that the density reduction area of 0.5T peak convergent pulsed magnetic window in the plasma sheath can reach more than10 cm,which is beyond the thickness range corresponding to the peak density of the plasma sheath obtained in the RAM-C experiment.This is equivalent to opening a low-density channel on the electromagnetic wave propagation path.Based on this model,the influence of plasma parameters on the effect of pulsed magnetic field is studied.The results show that the greater the initial electron density,the smaller the collision frequency and electron temperature,the better the effect of pulsed magnetic field.2.In view of the effectiveness of pulsed magnetic field in reducing plasma density,static plasma density reduction experiments and electromagnetic wave attenuation improvement experiments were carried out to verify the effectiveness of the pulsed magnetic field method.Aiming at the problem that the effect of pulsed magnetic field is poor in the low-density state,the optimal value of pulsed magnetic field parameters in this state is obtained through optimization analysis.A kind of pulsed magnetic field generator with capacitor energy storage is designed,and the corresponding parameters of pulsed magnetic field are generated.By applying the pulsed magnetic field to the plasma generator,the plasma density reduction experiment was carried out in this paper.The experimental results show that the pulsed magnetic field with 0.5T peak can reduce the plasma density by 43%-76%,which verifies the effectiveness of using the pulsed magnetic field to reduce the plasma density.At the same time,the error between the calculated results of the two-dimensional axisymmetric plasma density distribution model and the experimental results is less than 10%,which verifies the accuracy of the theoretical calculation model.On this basis,the improvement experiment of L-band typical signal attenuation by pulsed magnetic field is carried out.The experimental results show that the pulse magnetic field with 0.1T peak can reduce the attenuation of typical L-band signals by 5.5d B,which verifies the effectiveness of using the pulse magnetic field to reduce the attenuation of electromagnetic waves.The density reduction experiment and the signal attenuation improvement experiment jointly verify that the convergent pulsed magnetic window opens a low-density channel on the 14 cm thick plasma,avoiding the problem that there is still a high-density region on the electromagnetic wave propagation path.3.Aiming at the problem that the plasma sheath is in a high-speed flow state in real flight and there is a large span of ambient air pressure at different flight altitudes,a threedimensional density distribution calculation model of plasma flow field electron density is proposed,and the effects of high-speed flow field and ambient air pressure on the effect of pulsed magnetic field are obtained.Using this model,the density weakening of the plasma sheath at the height of 71 km by the pulsed magnetic field is given.The simulation results show that the effect of the pulsed magnetic field will be weakened with the increase of the flow velocity,and the area with the largest density reduction will be located at 20 cm behind the coil.Based on the reduced density distribution data,by establishing the calculation model of electromagnetic wave attenuation of magnetized plasma,the improvement effect of pulsed magnetic field on electromagnetic wave attenuation is obtained.The simulation results show that the pulse magnetic field above 0.2T can attenuate the typical L-band signal less than 30 d B in the high-speed flow field,and the convergent pulse magnetic window can effectively avoid the influence of stopband effect,so it has more advantages than the static magnetic field method and the orthogonal electric / magnetic field method.On this basis,the influence of air pressure is studied.Through simulation and experiment,the corresponding effects of pulse magnetic field under different ambient air pressure are obtained.The simulation results show that the effect of pulse magnetic field decreases sharply with the increase of air pressure above 10 Pa,and the distance of density reduction above 300 Pa will decrease by less than 4cm.4.Aiming at the problem that the effect of half sine pulse magnetic field method becomes very poor at 300 Pa pressure,a method of reducing plasma density by using exponential decreasing magnetic field is proposed.In this paper,the effects of the two methods under300 Pa ambient pressure are simulated and compared by using the three-dimensional density distribution calculation model under the flow field.The simulation results show that the exponential reduced magnetic field has better density reduction effect than the semi sinusoidal pulsed magnetic field at 300 Pa.This paper gives a theoretical explanation of the advantages of the exponential decreasing magnetic field method.On this basis,aiming at the plasma sheath produced in the reentry process of typical aircraft,this paper studies and evaluates the improvement effect of half sine pulse magnetic field method and exponential reduction magnetic field method on electromagnetic wave attenuation at different heights.The simulation results show that under the peak value of 0.5T magnetic field,the pulse magnetic field method is more appropriate at the flight altitude of more than 60 km,and the exponential reduction magnetic field method is more appropriate at the flight altitude of 40-60 km.The main innovations of this paper are as follows:(1)A pulsed magnetic field method based on convergent magnetic window is proposed to reduce the plasma sheath density.(2)A twodimensional axisymmetric static plasma density calculation model is proposed.(3)A calculation model of three-dimensional plasma density distribution in flow field is proposed.(4)An exponentially decreasing magnetic field method is proposed to reduce the plasma sheath density.The research results of this paper will provide some new ideas and theoretical support for the electromagnetic regulation method of plasma sheath,and play an important role in solving the communication blackout problem as soon as possible.