Investigation On A Frequency-tunable Coaxial Transit-time Oscillator Across Three Bands

Coaxial transit-time oscillator has the potential of high frequency operation due to its stability,high output power,simple structure,single-mode operation and high power handling capacity.However,the research on frequency-tunable coaxial transit-time oscillator is rare.frequency-tunablity is one of the most important directions of high power microwave source.Some frequency-tunable devices have been reported,but their operation frequency bands are low and they are unable to jump across two or more bands.Therefore,a frequency-tunable coaxial transit-time oscillator across three bands is proposed and investigated which can jump across C-,X-,and Ku-band.The main content are as follows.Firstly,the coaxial transit-time oscillator is studied theoretically.High frequency characteristics of coaxial resonant cavity is analyzed in theory,the method of improving power handling capacity of device and the approximate calculation of resonant frequency are obtained.the small signal theory is derived,the electronic load conductance expression is obtained which provides a theoretical basis for mode selection of resonant cavity.Secondly,the structure design and characteristic analysis on the frequency-tunable coaxial transit-time oscillator across three bands are carried out.Two methods for the frequency-tunablity of transit-time oscillator are discussed.the bunchers of C-,X-and Ku-band is designed and their high frequency characteristics are analyzed,and the expected working mode of the bunchers is obtained.By analyzing the mode selection of bunchers,the range of working voltage for stimulating the expected longitudinal mode is obtained.Finally,he frequency-tunable coaxial transit-time oscillator across three bands are studied by using PIC simulation.Under the condition of 340 kV,9.4 kA and guiding magnetic field of 2.0 T,typical simulation results are obtained as follows: the output microwave with average power of 0.94 GW,center frequency of 6.45 GHz and beam conversion efficiency of 29.4% is obtained when working in C-band.the output microwave with average power of 0.65 GW,center frequency of 11.35 GHz and beam conversion efficiency of 20.3% is obtained when working in X-band.the output microwave with average power of 1.22 GW,center frequency of 13.23 GHz and beam conversion efficiency of 38.1% is obtained when working in Ku-band.The tuning ranges in C-,X-and Ku-bands are 6.27~6.70 GHz,11.35~11.70 GHz,12.70~13.70 GHz respectively,and the corresponding 3dB tuning bandwidths are 6.6%,3.1% and 7.6% respectively.The influence on the microwave output by the structure parameters and the electrical parameters is studied.The results accord with previous theoretical analysis and numerical results.