Investigation On The X-band Cerenkov Type High-power Microwave Oscillator Without Guiding Magnetic Field

Driven by the application requirements,the compactness and miniaturization of High-Power Microwave?HPM?generators are drawing more and more attentions.Cerenkov type HPM oscillators without guiding magnetic field have the advantages of compact and lightweight,while keeping a relatively high efficiency at the same time.However,at present,few researches for some higher bands such as the X-band of this type devices were made,and the experimental investigations have been rarely carried out.Accordingly,this thesis proposed theoretical,simulational,and experimental studies on an X-band Cerenkov type HPM oscillator without guiding magnetic field.The main contents of this thesis are listed as follows.Firstly,theoretical and simulational studies are carried out on the device.The dispersion relation of the slow-wave-structure?SWS?is investigated,and the dispersion lines of the infinite uniform SWS are drew to get the approximate working frequency and modes of the device.By the standing-wave field simulation and the particle-in-cell?PIC?simulation,the field distribution and the working modes of the actual nonuniform SWS in the device are analyzed.It is shown that the working mode is the approximate7?/8 mode of TM₀₁ mode,and the frequency is about 9.1GHz,around which a high order hybrid?/2 mode of TM₀₂ mode exists.The reflection of the microwave by the tapered collector is the main factor inducing the high order hybrid mode.So the structure parameters of the collector influence the output microwave power strongly in the PIC results,and this influence is strongly correlated with the impacts of the parameters on the extraction effficency of the microwave.Based on these investigations,the PIC optimized results show that the device achieves an efficiency of about 40%in the simulation.When the diode voltage and the current are 620kV and 9.0kA,respectively,the output microwave power is about 2.2GW,frequency is 9.1GHz.At last,the beam-wave interaction is analyzed with the relevant PIC results.Through the modulation of the radial beam-wave interaction,bunched electrons get nearer to the SWS,which benefits the energy conversion.Then,it is demonstrated that the phase of the radial electric field should be modulated at right value to make the electrons convert their radial dynamic energy back into the microwave.Only by this,the power conversion efficiency of the device can reach a relatively high value.Secondly,this device is investigated through preliminary experimental study.The output structure is designed,which includes the surpporters,and the attena.In the preliminary experimental studies,it is found that the confining structures of the cathode are easily broken down,and the confining effect on the electrons is also unsatisfactory.The relatively low transmittance of the anode grid and the undesireable emitting state of the cathode also limit the output power of the device.In this experiment,the output power and the frequency of the microwave are 110MW and 9.1GHz,respectively,when the diode voltage is 620kV,and the current is 14kA.Thirdly,the diode structure is optimized,and the emitting states of different cathodes are investigated.The related problems of the diode structure are analyzed to achieve an optimized cathode structure which adopts outer and inner quarter-circular shielding electrodes to restrain the edge-enhanced emission and confine the electrons.The maximum surface electric field of these shielding electrodes is decreased from450kV/cm to 350kV/cm,and the confining effect on the electrons is obviously improved.Made of the tungsten wire which has a diameter of 0.05mm,the improved anode grid can achieve a space transmittance of more than 90%,and a good resistance to beam bombardment,which shows that it can sustain more than 150 shots when a electron beam has a current density of several kA/cm² and an energy of 500keV.Results of the fast framing optical photograph experiments show that the quarter-circular shielding electrode can improve the uniformity of the electron emission obviously,and the carbon fiber cathode is validated to be an appropriate choice,which shows better emission characteristics of the uniformity,the start-up speed,the emitting ability,and the stability than the graphite cathode and the polymer velvet cathode.Finally,the improved device is studied through the experiments.The witness target is used to get the distribution of the electrons,and the Faraday cup is used to test the high transmittance of the anode grid.By using the optimized diode structure,the carbon fiber cathode,the improved anode grid,the HPM experiments show that the improved device achieves an output microwave power of 310MW with a frequency of 9.05GHz,when the diode voltage and current are 430kV and 8.6kA,respectively.The diode votage pulse width is 50ns,and the microwave pulse width is 20ns.The power conversion efficiency is about 8.3%.The difference between the results of the simulation and experiments may be caused by the nonuniformity of the electron emittance and some other parameters.