Study On The Low-frequency EM Wave Transmission Characteristics Under Plasma Sheath And Blackout Mitigation By E×B

When high speed vehicles fly in the rarefied atmosphere at high altitude with high speed, plasma will generate around the vehicle and attenuate the electromagnetic wave signals seriously such as navigation, TT&C and safety telecommand, even interrupt the communication. In this paper, the propagation characteristics of low frequency EM wave in the plasma sheath are studied. The mechanisms of EM wave attenuation through plasma sheath emitted from the aircraft in the near field and EM wave attenuation through plasma sheath emitted from the earth in the far field are emphatically analyzed. The theoretical calculation model is put forward, and the attenuation characteristics of electromagnetic wave in different plasma are calculated and verified by experiments. In the case of large EM wave attenuation when the ratio of electron density and collision frequency is high, the mechanism of the electron density manipulation method by E×B fields is studied. The theoretical simulation model is established, and the effects of the electron density manipulation at different conditions are studied by numerical simulation. The research results of this paper will provide a theoretical basis to communicate through plasma sheath, and provide a theoretical method and application evaluation for reducing electron density with E×B fields and improving the communication quality. The main research contents are as follows:1. The mechanism of low frequency EM wave interaction with plasma sheath in the near field is studied. Two models to calculate the EM wave attenuation through plasma in the near-field region of loop antenna are proposed: one model based on the EMI shielding theory to calculate the EM wave attenuation through uniform plasma and the other based on the plane wave spectrum expansion method to calculate the EM wave attenuation through non-uniform plasma layers. Attenuations at different plasma densities, collision frequencies, EM wave frequencies and distributions of plasma densities are calculated with the above two models, and the relationship between EM wave attenuation and plasma parameters in the near field is present.2. Low frequency attenuation experiments in the near field region are conducted under glow discharge excited by low frequency power supply and the experiment results verify the effectiveness of low frequency EM wave propagation theory in the near field region. In order to strengthen the reliability of the experiment results, interference factors of Langmuir probe in the low pressure glow discharge are analyzed, and the corresponding improvement methods are put forward. And then the plasma density is diagnosed by both microwave method and the Langmuir probe, experiment results show that the estimation error between the above two methods is lowered than 30%.3. In the case of low frequency EM wave received on board, because the plasma sheath’s size is much smaller than the wave length of low frequency electromagnetic wave, the plasma sheath is simplified as a sphere shell with uniform parameters. Equivalent circuit models to calculate the EM wave field components attenuation covered by plasma shell are proposed. With the equivalent circuit models, the electric and magnetic fields attenuation are calculated. Experiments are carried out with plasma-like medium and the experiment results verify the theoretical expectations that the magnetic field component attenuation is smaller than electric field component attenuation.4. To lower the EM wave attenuation at high ratio of electron density and collision frequency, the mechanism of reducing the electron density in plasma sheath by E×B fields is studied, and the electromagnetic fluid model is established. Based on this model, the electron manipulation effects under different electrode structures and different conditions(electron density, pressure, voltage, magnetic field intensity, velocity, etc.) are simulated with the finite volume method. Simulation results show that the electron manipulation effect by U-shaped electrode is better than that by the plate-shaped electrode. Under the condition of low electron density and low pressure, the electron density can be decreased to 20% with the length of electron manipulation area up to 2cm, but increasing the voltage or the magnetic field intensity will not improve the electron manipulation effect significantly. When the pressure increases, the electron manipulation effect will decrease sharply.5. The low frequency propagation characteristics and the electron manipulation effects by E×B fields during the reentry of a typical aircraft are evaluated. The results show that by combining the low frequency and E×B fields, EM wave at 3 MHz may attenuate less than 30 d B during the entire reentry phase between 20~100 km.In summary, the research results of this paper provide a theoretical research method for the application of low frequency EM wave and E×B fields. It will be meaningful to explore the methods to mitigate the blackout.