Investigation On Vacuum Breakdown Characteristic Of High Frequency Structure Of X-band High Power Microwave Generators

Over-moded O-type Cerenkov high power microwave?HPM?generation devices have shown significant advantage in high output power?long pulse and high repetitive operations.Although the mode selection problem caused by large diameter is solved to some extent by mode control,and the power capacity is improved,such devices encounter the problem of RF breakdown of metal high-frequency structures when developing to higher output power.This dissertation focuses on the key issues of high-frequency RF breakdown of overmoded O-Cerenkov devices,and carries out related theoretical,simulation and experimental research.The main contents of this dissertation are as follows:1?The heating effect of microwave pulse on the metal high frequency structure is analyzed.The heat conduction equation of microwave pulse heating is established,the heat transfer equation is solved by Green's function method,and the analytical solution of heating temperature rise is given.For the case where the analytical solution is not applicable,a numerical solution method is given.The temperature rise of the high-frequency structure of different materials of HPM source is solved by the method in this dissertation.The theoretical and Comsol simulation results are in good agreement.This part supplements the lack of research on the thermal effects of high-frequency structures in the HPM field.2?Using the single particle theory,the motion equations of electron under the circular waveguide TM₀₁ mode and the external magnetic field is solved.The trajectories of electron in different circular waveguide sections under different initial phases are calculated by numerical methods,the results show:In an overmoded O-type Cerenkov device,the initial emitted electrons have the conditions of returning to the waveguide surface.The initial phase has a greater influence on the trajectory of the electron;when the initial energy of the electron changes in a small range,it has little effect on the trajectory;the initial launch angle does not affect the trajectory too much.The above conclusion is an important theoretical support for the formation of local high pressure by electron collision desorption.The electrons accelerated by the RF field have a relatively high energy,which may cause damage to the surface structure of the metal and affect the microwave transmission.The diffusion characteristics of local gases are simulated by PEGASUS software,the results show:During the pulse time of 100 ns,the gas diffuses quite small and concentrates on the surface of the slow wave structure.This part of the content completes the single particle theory of electron motion in different types of waveguides and different mode fields.3?The dispersion relation of high frequency structure in the presence of plasma is studied.In the derivation,we assume that the high-frequency structure is totally immerged in the plasma and ignore its motion.The dispersion relation is numerically solved,and the influence of plasma frequency,plasma thickness and other parameters on the dispersion relation is analyzed.The results show that the lower limit of the plasma density affecting the dispersion is about 1×10¹⁶ m^-3,corresponding to a plasma frequency of about 900 MHz;The plasma will lift the dispersion curve,has little effect on the 0 mode frequency,and has a greater influence on the?mode frequency;The numerical results are consistent with the HFSS simulation results and the PIC simulation results.This part of the content helps to analyze and understand the influence of plasma on the operation of microwave sources.4?Detailed PIC particle simulation studies are carried out on the breakdown law of traveling wave and standing wave type??-mode?cold cavity slow wave structures?SWSs?.Explosive emission is generated in the region with the strongest field strength to generate initial electrons,and plasma is generated by gas collision ionization.The surface electric field threshold of RF breakdown is obtained by simulation.The influence of plasma on microwave transmission and the influence of parameters such as surface field and number of cycles on RF breakdown are obtained.The simulation results are in agreement with the experimental results.This part of the content is of great significance for analyzing the physical mechanism of RF breakdown.5?Experimental study on the cold cavity breakdown characteristics of?-like SWSs are carried out.Breakdown SWSs with different peak surface fields,different periods and different longitudinal modes were designed and SWSs of stainless steel,brass,TA18,TC4 and 300M steel materials were processed.The directional coupler is used to diagnose the microwaves before and after the cold structure SWSs and the effects of RF breakdown on the microwave power and pulse width are obtained;the gas composition of the system before and after breakdown is diagnosed by residual gas analyzer?RGA?;The vacuum degree of the system was finely adjusted by a gas trimmer valve to study the effect of vacuum on the breakdown law.Experimental results show:For the X-band?-like stainless steel cold cavity SWSs,when the number of periods is greater than 5and the peak surface electric field of the SWSs is greater than 1 MV/cm,the microwave pulse width will be greatly affected,radiation microwave power is also reduced,when the peak surface electric field of the SWSs continues to increase,the influence will become larger and larger;TC4 material is superior to other materials in terms of RF breakdown characteristics;When near the RF breakdown threshold?1 MV/cm?,the external magnetic field will increase the peak reflection power and narrows the microwave pulse width;The breakdown threshold of the?-like cold SWSs is higher than the traveling wave cold SWSs.The experimental results have important reference value for the design of HPM generating devices and are helpful in the analysis of RF breakdown mechanism.