Diagnosis Of Dc Grid Discharge Plasma And Its Effect On Microwave Transmission Characteristics

Plasma is a multi-particle system containing a large number of unbound charged particles with unique properties that distinguish it from solids,liquids and gases.The charged particles in the plasma are in an unbound state and are easily affected by the electromagnetic field,so they have various effects on electromagnetic waves,such as absorption,cut-off,reflection,resonance and collision dissipation.These characteristics make plasma,as a special electromagnetic transmission medium,attract more and more scientific workers’attention.The transmission of electromagnetic waves in plasma has a wide range of application scenarios,such as the propagation of radio waves in the ionosphere of the earth,the transmission of radio waves in interstellar space,the communication between space and the earth,the communication"black barrier"of hypersonic vehicles,plasma stealth,etc.The wide range of applications has promoted the research and development of the interaction between electromagnetic waves and plasma.It is of great scientific significance and application value to carry out experimental research on electromagnetic waves and plasma interaction.Based on the above analysis,in order to experimentally study the effect of plasma on the transmission characteristics of electromagnetic waves,a new type of large-area DC grid electrode discharge plasma generator was designed and built in this thesis.The device utilizes a grid electrode structure to generate plasma,and the charged particles in the electrode gap enter the space behind the anode to form a plasma layer under the action of an electric field.The plasma parameters were diagnosed by probe,microwave and spectroscopic methods,and the experimental study of microwave transmission characteristics in plasma was carried out.The influence of plasma on electromagnetic wave transmission was analyzed by numerical simulation.The main research contents of this paper are as follows:A DC grid electrode discharge device was built based on the DC discharge theory,and its discharge characteristics were experimentally studied.By measuring the volt-ampere characteristic curve of the DC grid discharge plasma,combined with the modified Paschen curve,the device parameters were optimized,and the plasma in a stable discharge state was obtained.In the case of determining the optimal discharge conditions of the plasma,the parameter characteristics of the space plasma behind the grid electrode are further diagnosed.Firstly,the probe measured the electron density in the electrode gap.Combined with the microwave phase method,the average electron density in the space behind the grid electrode was measured,and its magnitude was 10¹⁵ m^-3-10¹⁷ m^-3.In order to obtain the plasma electron density distribution characteristics of the space behind the grid electrode more accurately,a DC grid discharge plasma scaling device is designed,which can be more convenient to use spectral technology for diagnosis.the effective thickness of the plasma in the space behind the mesh.By analyzing the diagnostic data,the effects of discharge voltage,discharge gas pressure,electrode gap and source voltage on the spatial distribution of plasma electron density are further discussed.The microwave transmission characteristics of the obtained DC grid electrode discharge plasma were studied by using the spatial reflection method.The reflection characteristics of microwaves in the plasma layer under different conditions such as grid anode opening shape,grid transmittance and electrode material were measured experimentally,and numerical simulation was carried out by Finite Difference Time Domain（FDTD）method.,it was found that when the secondary excitation coefficient of the cathode material is small,the shape of the opening of the grid anode is a regular hexagon,and the electrode gap is small,the plasma layer formed in the space behind the grid anode is thicker,which has better microwave resistance.This study provides a platform and experimental data support for the study of the interaction between electromagnetic waves and plasma.