Research On The Regulation Of Plasma Sheath Based On A Strong Pulsed Magnetic Field

For spacecraft passing through the atmosphere,such as the reentry space vehicle,the near-space hypersonic spacecraft,and the Mars probe,due to an intensive friction with the air there is a layer of dense plasma named by plasma sheath surrounding them.The sheath can attenuate the electromagnetic wave for communication,and it may be totally interrupted when the sheath is severe,casuing the so-called communication blackout.It is of great significane to alleviate or remove the blackout in order to precisely control and track the spacecraft for a safe mission.So far,using a manetic field to actively regulate the plasma sheath is an innovative and promising method to mitigate the blackout.However,a large-scale magnetic field with a Tesla magnitude is needed to make the method take effect,which implies that the system will be heavy and the effective load of the spacecraft will be low when the strong magnetic field is achieved by the static or steady magnetic technology.Thus,in order to reduce the energy requirement and weight of the system,the thesis proposes a novel method to alleviate the blackout by regulating the plasma sheath with a pulsed magnetic field.The dissertation will carry on experiments of regulating the plasma sheath by a strong pulsed magnetic field for revealing the mechanism of the interaction between the plasma sheath and the pulsed magnetic field,finding the matching relation between the magnetic field and the plasma sheath,and then exploring the feasibility of the method to alleviate the spacecraft communication blackout.Based on reviewing the physics and characteristic parameters of the plasma sheath,the key parameters for simulating its electromagnetic performance is shown by analyzing the scaling laws for ground simulation of the plasma sheath.A cascade arc plasma source with three channels is used to generate a plasma flow of high density,strong electron-neutral collision,and large radius to simulate the plasma sheath.Based on the principle of the magnetohydrodynamics,the requirement for the pulsed magnetic field to regulate the plasma sheath is raised,which is that its intensity shoule be larger than 2 T,its top width should surpass 10 ms,and it must be perpendicular to the plasma jet.Afterwards,a pulsed magnet is designed to generate the magnetic field,and multi-physics coupling analysis is done to assure its reliability.Test result shows that the designed magnet can fulfil the requirement.A pulsed current supply with a flat top is developed to drive the magnet for generating the magnetic field.A new shceme based on a modified SFPFN（Sequentially Fired Pulse Forming Network）is proposed to improve the energy efficiencies of the capacitor bank and PFN（Pulse Forming Network）and reducing the stress on the switching elements of the SFPFN with a heavily inductive load.Key technologies of the new power supply are studied,including the technology of reliable operation of the thyristor under intensive pulsed current,the technology to make the crowbar diode work reliably under the mode of repetitive turn-on and turn-off,and the strategy of preventing the power supply from overcurrent of high dynamic range.The critical technologies are verified by the experiments,and the modified SFPFN can achieve a pusled current with peak 30 k A and top width 12 ms on the pulsed magnet load.A plasma diagnostic platform is constructed for studing the mechanism behind the regulation of the plasma sheath by the pulsed magnetic field.The initial unregulated plasma density is obtained using the HβStark broading method,and the temporal evolution of the plasma density distribution under the magnetic field action is characterized by the evolution of the plasma optical intensity collected by a high-speed camera.The relationship between the plasma density and optical intensity is built according to the regression analysis.Then,the evolution process of the plasma sheath regulated by the pulsed magnetic field and the dynamic mechanism of the process is studied,and the effect of the pulsed magnetic field is analyzed in terms of reducing the local plasma density.Under the magnetic field action,the plasma is constrained upstream of the interaction region,and the local plasma density downstream declines considerably.When the initial plasma density is in the order of 1×10¹⁹ m^-3,the density can decrease by more than 50%during the interal of 1 ms～13 ms after the magnetic field is applied,which verifies the validity of mitigating the spacecraft communication blackout using a pulsed magnetic field.The drift wave and transport of the plasma sheath regulated by the pulsed magnetic field is studied based on the inter-frame difference method and two-dimensional Fourier analysis,and then the influence of the magnetic field and plasma parameters on the regulation effect is investigated.It is found that with the magnetic field increasing,the confinement of the plasma becomes stronger,the local plasma density reduces more,and the reduction region is enlarged.However,when the magnetic field intensity increases to some value,its action tends to saturate.If the initial plasma density is 1.8×10¹⁹ m^-3,the saturation magnetic field is about 1.5T.The velocity of the drift wave and plasma transport is in inverse proportion to the magnetic field intensity.The effect of the pulsed magnetic field can be strengthened by increasing the plasma density,decreasing the collision frequency,and inhibiting the drift wave and transport,which can be realized by raising the magnetic field and reducing the field ripple.The research of the thesis laies theoretical and technological foundations for spacecraft blackout mitigation by regulating the plasma sheath with a pulsed magnetic field.