A Ka-band High Power Coaxial Transit-time Oscillator

Ka band microwave technology has great application prospects in the fields of satellite communication,radar,plasma diagnosis,controlled thermonuclear fusion,medical health and other fields.At the same time,researches on Ka band high power microwave devices are in accordance with the trend of higher power and higher frequency development of high power microwave devices.Therefore,the research on Ka band high power microwave devices has important significance.There are several problems in the traditional Ka band high power microwave sources.First of all,the channel of the electron beam is narrower,and the electron beam needs to be confined in a smaller range,so the magnetic field required by the device is generally high.On the one hand,the size of the single-mode device is small,so the power capacity is relatively small.On the other hand,pattern competition often exists in the over-moded device.However,the coaxial transit-time oscillator?TTO?can effectively avoid these problems above.Firstly,the induced current on the inner conductor of the coaxial structure weakens the space charge effect and allows the device to work under lower guiding magnetic field.Then,by using the coaxial structure,the power capacity of the device can be improved by increasing the radial dimension of the inner and outer conductors.Besides,coaxial transit time oscillator has the advantage of single and stable working mode,high power and high efficiency.Based on the analysis above,a Ka band coaxial TTO is studied in this paper to realize a high power microwave source with low guiding magnetic field,high power,high efficiency and single working mode.The main contents of this paper are as follows.Theoretical research on Ka band coaxial TTO has been carried out.To begin with,the space limit current of coaxial structure and hollow waveguide is analyzed.Analysis shows that the coaxial structure has the potential of lower guiding magnetic field.Then,the working modes in the buncher are analyzed.Study shows that the buncher with four gaps has a potential of more efficient.Furtheron,the coupling coefficient between the buncher and extractor is introduced.It is found that the efficiency of the device increases with the coupling coefficient.Particle simulation research on Ka band coaxial TTO has been carried out.The model of Ka band TTO with low guiding magnetic field,high power capacity and high efficiency is set up,and the over-moded ratio of this device is about 8.9.With a diode voltage of 447 kV and a current of 7.4 kA,output microwaves with a power of 1.27 GW and a frequency of 26.2 GHz are realized.The efficiency is 38.5%,the guiding magnetic field is 0.6 T,and the impedance is about 60?.Furthermore,the method to reduce the electric field intensity in the device is studied.The maximum field intensity on the surface of the device is reduced to 1.4 MV/cm by adding the radial dimension of the device and the chamfering process.And then,a compact Ka band coaxial TTO packaged with permanent magnet is studied.To reduce the demand for the guiding magnetic field,a new focused cathode is introduced at first.With a electric power of3.28 GW,the output power of the microwaves is 0.8 GW when the guiding magnetic field is 0.35 T,and the output power is 1.1 GW when the guiding magnetic field exceeds 0.48 T.A permanent magnetic structure with a mass of 66.3 kg is designed to provide guiding magnetic field for the device.In simulation,output microwaves with a power of 1 GW and a frequency of 26.2 GHz is obtained,the efficiency is higher than30%.The engineering design of the Ka band coaxial TTO has been carried out.The magnetic coil,support rod and mode converter for such a Ka band coaxial TTO is designed.Simulation has been done on the device chosen for experiment.With a voltage of 450 kV and a current of 9.1kA,output microwaves with a power of 1.57 GW can be gotten.Moreover,the ohmic loss of the device under stainless steel is studied.Particle-in-cell simulation shows that the ohm loss accounts for about 36.3%of the output power in such a Ka band TTO.Experimental research on Ka band coaxial TTO has been carried out.In the preliminary experimental research,output microwaves with a frequency of 26.7 GHz were obtained.The near field fluorescent screen shows that the output microwaves contain TE₁₁ and TE_29,1 modes.For this purpose,novel supporting posts with no reflection on the TE₁₁ mode is designed.Then,experimental research is carried out with the improved support posts and diode,which shows that the asymmetric modes are effectively suppressed.With a diode voltage of 450 kV and a beam current of 9.1 kA,output microwaves with a power of 650 MW and a frequency of 26.5 GHz are obtained.The power efficiency is 16%.The development of the TTO with frequency tunability has been carried out.In order to study the frequency tunability of TTO,researches have been done on the L band TTO at first.Three kinds of TTO are studied:coaxial structure with gaps on the inner conductor,coaxial structure with a smooth inner conductor,and the hollow structure.Then,comparasion has been done on these three structures.The power efficiency of the coaxial one with gaps on the inner conductor has a maxiamal efficiency of 41%,the frequency range is from 1.53 to 1.93 GHz.The coaxial structure with a smooth conductor has a maxiamal efficiency of 41%.The frequency modulation range is from1.38 to 1.52 GHz.The power efficiency of the hollow device is lower and the maximum is 25%.Furtherly,the mechanical modulation to realize frequency modulation is studies.The operation is simple and easy to be realized.Last but not least,the frequency tunability of the Ka band TTO is studied.By changing the radial size of the buncher,output microwaves with power higher than 200 MW can be get from 23.83 to 26.2 GHz with a diode voltage of 450kV and a current of 7.4kA.The mechanical modulation studied above is real-time and continuous.