Research On A Ka-band Overmoded High Power Microwave Cerenkov Oscillator Operated With The TM₀₁ Main Mode

Ka-band high power microwave is widely required,and the related research is significant both in theory and practice.Overmoded Cerenkov-type oscillator is featured by simple mechanism,low guiding magnetic field and high power capacity.So it is employed to generate Ka-band high power microwave.The main profile of this thesis is listed as follows:Linear theory is utilized to analyze the characteristics of the Ka-band slow wave structure(SWS).Uniform equations are deduced to calculate the dispersion relation of the Ka-band SWS,which are especially adapted to the SWS with high frequency and high overmode ratio.Based on these equations,a numeric code is developed.The dispersion relations are calculated for a typical Ka-band structure,and the impacts of the structure parameters to the dispersion relations are analyzed.The temporal growth rate,the coupling impedence,the space charge limited current and other representative parameters are discussed.Moreover,based on the analyse to the parameters influencing the temporal growth rate of the TM02 mode in the Ka-band SWSs,the possibility of utilizing the high order modes as the operating mode to improve the power capacity is investigated.Paticle in cell(PIC)simulation is used to systematically investigate the oscillator.Multiple methods are used to improve the power flux distribution and gradually changing structures are adopted to maintain the power efficiency.When the electron beam current is 3.2 kA,the diode voltage is 450 kV,and the guiding B field is 0.95 T,the power outputted in the PIC simulation is 0.52 GW,so the power efficiency is 36.1 % and the operating frequency is 32.3 GHz.The forward and backward power flux in the beam wave interaction region are 1.6 GW and 1.2 GW.The output mode component of the oscillator is evaluated using the PIC simulation data and a numeric method.The method is validated by comparing to the experiment data in a former Ka-band high power millimeter wave generating experiment.Deduced from the numeric results,the output mode component is stable after the output power becomes saturated,and the reflection characterisitcs affect the mode component significantly.Based on these findings,the structures of the oscillator investigated in this thesis are adjusted to make the power ratio of the TM01 mode in the output wave larger than 90 % in the PIC simulation,when the overmode ratio of the SWS is 4.6.The oscillator is systematically investigated on the high power millimeter experiment system.The electron transport property is tested using the witness target,and the carbon cathode with better uniformity is used in the experments.The operating frequency of the experimental oscilltor is measured to be 32.1 GHz,using a Ka-band dispersion line.This frequency is similar to the operating frequency in the PIC simulation.The output mode in the experiment is circularly symmetrical,demonstrated by an array of fluorescent lamp tubes.Comparing the radiation pattern measured in the experiment and the radiation pattern by inputting the theoretical mode component to the experimental antenna,the power ratio of the TM01 mode in the output wave is larger than 90% in the experiment,and the mode content is stable both in power ratio and phase relationship.When the electron beam current is 6.2 kA,the diode voltage is 530 kV,and the guiding B field is 0.95 T,the power outputted is 0.35 GW in the experiment,so the power efficiency is about 10.7 %.And the largest demodulation pulse width is 16.0 ns.And when the electron beam current is 9.9 kA,the diode voltage is 605 kV,and the guiding B field is 0.95 T,the largest output power got in the experiment is 0.63 GW,with the pulse width shortened to about 10.5 ns.When the voltage is lower than 600 kV,the pulse shapes keep stable in ten successive pulses with the same diode voltage and the relative output power fluctuation is less than 5 %.The experimental operating parameters influence the power efficiency in the same trend as the results got in the PIC simulation.The capability of the oscillator designed in this thesis to generate and emit GW-level Ka-band high power microwave is demonstrated by these power and pulse width measurement results.