Investigation On Diagnosis And Conversion Transmission Techniques Of TM_0n Mixed Modes For High-power Microwave Applications

In order to enhance the pf ² factor,scaling the high-power microwave?HPM?generators to a higher frequency range is a major focus in pushing the technology of HPM.To increase power handling capacity,an overmoded slow wave structure?SWS?may be used to reduce the power density and lengthen the output pulse width.Unfortunately,overmoded HPM generators often produce other unwanted modes,particularly TM_0n modes of higher orders.To solve this problem,in this dissertation,a novel HPM terminal system is proposed,by which high-power mixed TM_0n modes are directionally radiated by an improved radial line slot antenna in high efficiency after conversion of the mixed TM_0n modes,based on a novel analysis method of the TM_0nn mode content.Here is explored a new way of diagnosis,conversion and transmission of high-power mixed TM_0n modes.The main contents and innovative work are as follows:1.Investigation of an online device for high-power TM_0n mode content analysisA novel online diagnosis device for high-power mixed TM_0n modes is proposed,which is composed of a directional coupler of circular waveguide,a adjustable-length waveguide,a directional coupler of coaxial waveguide and a nested-horn antenna.This device can accurately determine the mode content of the mixed TM_0n modes in a steady state,laying down the foundation of the design of mTM_0n-TEM/TM₀₁ mixed mode convertor.An online device with power capacity of 1 GW,working at the centre frequency of 9.375 GHz,is designed and verified by simulation.The simulation results show that high calculation precision of the proposed"complex quantity measuring method"and"power measuring method"is verified.The estimation of the mode content deduced from the simulated data is nearly the same as the theoretical one,indicating that the data processing method proposed in this dissertation is effective.Low-power experimental investigations show that the estimation of the mode content by measurement is well consistent with the theoretical one,validating the feasibility of the mode content analysis technique in experiment.According to the experimental results up to now,the estimated measurement error of the power ratio is less than 4 percentage points,the one of the relative phase is less than 5o.High-power experimental investigations show that the power handling capacity of this device reached GW level,although the relative phase of the mixed TM_0n modes can be well determined by power measuring method?the estimated measurement error of the relative phase is less than 9o?,the estimated results of the mode content is affected by the azimuthally asymmetric mode.To minimize its impact on the mode content analysis,the measurement points with large amount of coupling should be increased,the estimated measurement error of the power ratio is less than 15 percentage points.To prevent TM_0n mode content analysis form the interference of asymmetric mode,a symmetrical coupling arm is designed,the isolation coefficiency of TE₁₁ mode is about-40 dB.2.Investigation of a high-power mTM_0n-TEM/TM₀₁ mixed mode converterIn order to converte a mixed TM_0n modes generated by an overmoded HPM device into a pure coaxial circular waveguide TEM mode or circular waveguide TM₀₁ mode,a novel mixed modes converter,with compact structure,high conversion efficiency and power handling capacity,composed of outer and inner coaxial ladder waveguide,is proposed.According to different application backgrand,a series of specially designed mTM_0n-TEM/TM₀₁ mixed mode converters working in X,Ku,Ka bands are put forward.Simulation results show that in the range of 200 MHz near each of the above centre frequencies,mode conversion efficiency reaches nearly 99%,power handling capacity meets the design demand.Tolerance features of the mode converter is analysed,conversion efficiency of which is sensitive to the relative phase fluctuation btween the main modes?when the phase of each mode fluctuates in the range from-30oto+30o,the conversion efficiency keeps above 90%?,while not sensitive to the fluctuation of the power percentage of the mixed TM_0n modes?when the power percentage of the each mode fluctuates in the range from-30%to+30%,the conversion efficiency keeps above94%?.The optimal design rule of the mode converter is presented based on the analysis of tolerance features.The simplified mTM_0n-TEM/TM₀₁ mixed mode converter operating at 12.3 GHz is designed.Simulation results shows that the total length of the optimally designed converter is almost shortened to half of the original one and the power handling capacity is improved,keeping the same bandwidth property.The mTM_0n-TEM converters applied to HPM source working at 9.735 GHz and 12.3 GHz are manufactured.Low-power experimental investigations show that in the reverse experiment,the measured patterns of the output waveguides fed by the input port of the converter are in good agreement with the simulated patterns of the output waveguides excited by the theoretical TM_0n mode mixtures.This indicates that the input port of the mode converter launches nearly the same TM_0n mode mixture as the theoretical one.The measured results of S parameters of the converter confirm that the conversion efficiency reaches 99%at the centre frequency.High-power experimental investigations show that the mode converter has high power handling capacity with GW level,the width of the tested pulse reaches 20 ns.3.Investigation of an improved high-power circularly/linearly porlarized radial line slot antenna.Based on the design approach of the original high-power circularly porlarized radial line slot antenna?RLSA?,an improved circularly/linearly porlarized one with wider bandwidth is proposed,which moves the spiral reflection cancelling slot from the lower radial waveguide to the upper one,then make room for the mTM_0n-TEM mixed mode converter.Simulation results show that the gains of both antennas reach 31.5 dBi with the same aperture diameter of 510 mm,accordinglly the aperture efficiency reaches 56%.All of the reflection coefficients are reduced to about-20 dB over the range of 200 MHz near the centre frequency of 9.42 GHz,radiation efficiency is about97%,and power handling capacity reaches GW level.Both of the improved high-power circularly and linearly porlarized radial line slot antennas are manufactured.Low-power experimental investigations show that the measured radiated far-field pattern of the circularly porlarized antenna is well agree with the simulation in the region from-30oto30oaround center axis,the sidelobe level in the other region is below-25 dB,reflection coefficient at the centre frequency is smaller than-20 dB,the measured gain reaches30.5 dBi.The bandwidth of the reflection coefficient below-20 dB doubles.As to the linearly porlarized antenna,the measured radiated far-field pattern mainly agrees with the simulation,at the center frequency the measured gain reaches 30.1 dBi,the radiation patterns indicate a reasonable cross-polarization level more than 30 dB below the co-polarization,the reflection coefficeint at this frequency point is also smaller than-20dB.High-power experimental investigations show that the power handling capacity of the improved circularly and linearly polarized RLSAs reaches GW level,the widths of tested circularly and linearly polarized pulses in 3 Hz reach 60 ns and 40 ns,respectively.4.Investigation of a high-power mTM_0n mixed mode transducing antennaTo directionally radiate a mixture of TM_0n modes with a device of short length in high efficiency,a novel high-power mTM_0n mixed mode transducing antenna composed of radial mTM_0n-TEM mixed mode converter and improved high-power RLSA is proposed.The radial mTM_0n-TEM mode conversion can be realized by utilizing radially folding middle plate to adjust the radial field intensity and change the length of the radial transmission path to match the phases between two radial TEM modes.Simulation results show that in the range of 600 MHz around the centre frequency of12.3 GHz,all of the reflection coefficients are smaller than-22 dB and mode convertion efficiency reaches nearly 99%.Compared with axial mTM_0n-TEM mixed mode converter,the total length of the radial one is reduced from 6?₀ to about 2.5?₀,moreover,this radial one keeps the same power handling capacity and mode conversion efficiency.The simulation results of the whole mTM_0n mixed mode transducing antenna show that the radiation efficiency reaches 96%and the aperture efficiency reaches 54%,keeping nearly the same as the ones of improved high-power circularly/linearly porlarized radial line slot antenna.The power handling capacity reaches GW level and the total length of the mode transducing antenna is about 3?₀.