| When an aircraft or a hypersonic vehicle reenters the atmosphere,the plasma sheath generated can severely attenuate the electromagnetic(EM)wave signals,causing communication interruptions in terrestrial and spacecraft telemetry,remote control,etc.Based on the principle of magnetic regulation,the ’magnetic window method’ is regarded as one of the most potential methods to mitigate the ’radio blackout’.In real application scenarios,the magnetic field used by the method must exceed 1 T.Regardless of adopting permanent magnets,electromagnets,superconducting magnets or even pulsed magnetic field,the technical specifications for reentry vehicles are extremely difficult to achieve due to the size and weight of the magnets far exceed the load capacity of a vehicle.Currently,the introduction of other physical parameters to reduce the heavy dependence of magnetic field strength(such as the E×B effect)is a development trend of magnetic field technologyFocusing on reducing the regulation field strength,this paper further proposes the notion of‘enhanced magnetic window of spatiotemporal electromagnetic field’.Specifically,by introducing the time-space varying parameters of the electric/magnetic field into the conventional ’magnetic window method’,the demand for one single variable of field strength is transformed into combination of multiple variables such as electric/magnetic field and its time-space type.This approach can improve the effect of mitigating the radio blackout while diminishing the dependence on magnetic field strength.Threefore,considering the interaction between the induced current and the magnetic field in the plasma fluid influencing on the movement of electrons,this paper applies different field configurations to regulate the induced current density J in the plasma.The ’magnetic window method’,’ExB method’ and’low frequency method’ are improved following the above principle,achieving the effect of reducing the demand for field strength.The main innovations and contributions of this paper are summarized as follows.1.The time-varying and space-varying parameters of the magnetic field are introduced in ’magnetic window method’,and a new method that is based on traveling magnetic field(TMF)of self-induced current is proposed to reduce local electron density and improve the performance of EM wave transmission.The proposed method reduces the demand for magnetic field strength by increasing the velocity of TMF.The calculation model of electron density reduction based on the TMF of the induced current is established,which improves the existing TMF-based methods.The influence of different TMF velocity and magnetic field strength on electron density distribution is analyzed according to the above model.The experimental environment for the plasma that is adapted to the TMF is designed based on the principle of low-pressure glow discharge.The effect of TMF on the electron density distribution of plasma is studied,obtaining the experimental results of different TMF velocity influencing the plasma electron density distribution.2.The time-varying parameters of electric and magnetic fields are introduced in ’Static E×B magnetic window’,and the ’time-varying E×B magnetic window’is further proposed.This method increases the coupling current in the plasma through time-varying electric field,which reduces the magnetic field strength and overcomes the Debye shielding effect.This method is applying the electric and magnetic fields with the same frequency and phase to replace the static electric/magnetic field,thereby avoiding the influence of Debye shielding effect on reducing the electron density effect.In this paper,a multi-physics numerical simulation model is established to calculate the influence of plasma with different parameters affecting the reduction of electron density in the time-varying E×B magnetic window.Simulation results show that the time-varying E×B field can reduce the local plasma density with 5 cm thickness by more than 90%,which can create a window(about 3 to 4 cm)to enable the transmission of data with the magnetic field B=0.1 T and the initial voltage U0=1000 V in the case of low collisions.Furthermore,experiments of the plasma density reduction by time-varying E×B field are performed.Experimental results suggest that the application of a 0.07 T time-varying magnetic field and a 1000 V time-varying voltage can reduce the local electron density size is as high as 6 cm,and the maximum reduction of the electron density can reach 80%.Theoretical and experimental results reveal that the timevarying E×B field can effectively reduce the electron density in low-collision circumstances.In addition,the effectiveness of time-varying E×B method reducing the magnetic field strength,as well as the feasibility of overcoming Debye shielding effect are verified.3.An annular space distributed magnetic field is introduced into ’low frequency method’,and the low frequency enhanced magnetic window with annular-space-distributed magnetic field is further proposed.This method reduces the induced circulation in the plasma by applying annular-space-distributed magnetic field to restrain free electrons,thereby alleviating the attenuation of the magnetic component of LF EM waves.In this paper,the transmission attenuation model of LF EM waves in cylindrical enveloping plasma with the effect of magnetic field is established based on shock tube,to which the corresponding experiment is conducted to verify the proposed theory.The experimental results are similar to the analytical solution,which verifies the validity of the model.The research results show that the local strong magnetic field can be replaced by an annular-space-distributed magnetic field in the low frequency method.In this method,the effect of the local strong magnetic field can be decomposed into the superposition of multiple weak magnetic fields on the induced current loop to achieve the purpose of reducing the intensity of magnetic field.In summary,the research findings of this paper provide a theoretical foundation for practical application of electric/magnetic field to mitigate radio blackout,offering technical references for the implementation of the proposed method in engineering.The thesis boosts the possibility of applying ’space-time electromagnetic field enhanced magnetic window’ in engineering,presenting a new idea and possible mean to solve the bottleneck problem of the’magnetic window method’. |
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