Research On The Diagnosis Methods Of The Output Modes Generated By Overmoded O-type Cerenkov High Power Microwave Source

With the requirement of higher power, longer pulse duration and higher pulserepetition rate, the employment of overmoded devices becomes the developing tendencyof High Power Microwave (HPM) source. Due to the restriction of the output structureand the interaction mechanism between electron beam and microwave, in practiceovermoded HPM source could not export single mode exactly. Consequently, it isnecessary to analyze the output mode content of HPM source. Considering the featuresof the complicated output mode content, high power and long pulse duration inovermoded HPM source, analysis of the output modes of O-type overmoded HPMsource is investigated in two aspects.The first is the far-field pattern method based on the far-field radiation theory andthe far-field measuring method. The radiantion field of the horn antenna is elaboratedaccording to the caliber field radiantion theory, and the patterns’ characteristics ofTMonmode is studied both in single mode and in composite modes. The study resultsshow that along with the main mode of TM01reducing the main lobe broadens and thegain declines, and that with the phase difference between adjacent modes increacingthe main lobe thins down and the gain declines. Two far-field pattern methods areinvestigated. One is based on the orthogonality of the radiantion field, and another isbased on numerical optimization. The Nonlinear Least Squares Minimization Methodis emlpoyed to determine the mode content in a horn antenna.The second is the mode coupling method which is on the bases of themode-selective coupling theroy. In order to diagnose the output modes of an X-bandlong pulse (100nanoseconds) overmoded HPM source, an online mode diagnosisdevice with high power handling capacity is investigated numerically andexperimentally. Compared with normal online measuring device this device has twocoupling arms, of which one is a mode-selective coupling arm and the other is anon-mode-selective one. By measuring the output power of the two arms, the modeconten can be obtained. The cold test resluts agree with the simulation results. Thehigh power test is performed on an X-band long pulse overmoded HPM source. Thediagnosis results show that TM01mode accounts for90%and TM02mode accounts for10%. This preparatory result means great value to diagnose the output modes of anovermoded HPM source.