Investigation On Ka-band Super Overmoded Coaxial Cerenkov High Power Microwave Ocsillator

Ka-band high power microwave（HPM）is widely researched due to its high Pf ²factor and location in the“atmosphere window”of microwave transmission.However,the development of Ka-band HPM devices is restricted by the power capacity limit.Super overmoded Cerenkov devices have the superiority of achieving high power capacity and mode controlling synchronously.Therefore,in this paper,we adopts super overmoded Cerenkov devices to carry out the reseaches on the Ka-band HPM generator.The research contents include the following aspects:Numerical investigations on the power capacity of coaxial slow wave structure（SWS）are implemented.Based on Flquet harmonic expansion,a MATLAB code is composed and used to study the power capacity characteristics of the sinusoidal outer ripple coaxial SWS.The following conclusions are obtained:the power capacity of the TM₀₁ mode is significantly higher than that of the quasi-TEM mode,when the distance between the inner and outer conductors is less than a critical value.The comparison of the power capacities of the TM₀₁ mode in coaxial and hollow SWS shows that when the distance between the inner and outer conductors of coaxial SWSs is small enough or the outer radius is large enough,the power capacity of the TM₀₁ mode in coaxial SWSs is higher than that of hollow SWSs with the same outer radius.A high power capacity parameter region is determined.In this region,the power capacity of TM₀₁ mode in coaxial SWS is higher than both of that of quasi-TEM mode in coaxial SWS and that of TM₀₁ mode in hollow SWS.Two methods of mode decomposition in coaxial waveguide are proposed based on the data extracted from the particle-in-cell（PIC）simulation:space beat wave method and mode orthogonal method.The space beat wave method is applyed in the simuation that there are only two mode in the ouput whose wavelengths are close.By mesuering the peak value and valley value,the power of each mode can be calculated.This method is simple and intuitionistic,but the adapted situations are limited and the phase difference can not be obtained.The mode orthogonal method is based on the field distribution data in the output waveguide,using the orthogonality of the eigenmodes in the regular waveguide to quantify the mode composition.On the foundation of deriving the normalized field distribution in the coaxial waveguide,the method of solving the power amplitude is obtained.The method is verified by the coaxial waveguide model in HFSS and the device model in PIC simulation.By comparing with the results of the model composition obtained by the space beat method,the validity of the two methods is proved.The Ka-band coaxial super overmoded Cerenkov oscillator is designed,optimized and studied by 2-D PIC simulation.A Ka-band coaxial overmoded Cerenkov oscillator with a front reflector and two-stage SWS is designed.The typical PIC simulation results are:the output power is 601 MW with the efficiency of 39.5%;the microwave frequency is 32.26 GHz,and maximum surface field strength is 1.38 MV/cm,much lower than those of the same band Cerenkov devices.Through the comparison of the electron conductivity,transmission characteristics,external Q values and field distributions,phase space distributions,power flow distributions,and electron beam energy distribution in the SWS of the two operating modes,the mechanism of the device is known as the dual-mode operation mechanism,that is,the synchronous interaction between the 7π/8 mode of quasi-TEM mode and the non-synchronous interaction of the 3π/8 mode of TM₀₁ mode.The sensitivity of the device is also studied,when the device is stable,and the efficiency is more than 30%,the diode voltage range is 360 kV to 480 kV,the guiding magnetic field range is 0.7 T to 3.2 T;the electron beam thickness range is 0.7 mm to 1.5 mm,the electron beam radius range is 26.4 mm to 27.4 mm,and the electron beam collector range is 135 mm to 155 mm.Considering the limited conductivity characteristics of the conductor material,the output power of the device is 403 MW,down by about 33%,under semi-stainless steel conditions.Aiming at the dual-mode output problem of the device,two kinds of schemes,such as corragated waveguide mode purification and resonant cavity mode purification,are obtained.The purification effect can reach more than 99.3%,the tolerance of the diode voltage is（407±30）kV,the tolerance of the guiding magnetic field is greater than±0.5 T.3-D PIC simulation is used to validate the results of 2-D PIC simulation,and the asymmetric mode competition is studied and suppressed.In the 3-D PIC simulation,serious asymmetric mode competition will occur in the original device;therefore the device has to be improved.After improving,the working characteristics of the device in the 2-D and 3-D PIC simulation cases are basically the same.Analyzing the problem of asymmetric mode competition in the device,we pointe out that in the super overmoded coaxial SWS,serious band overlapping is the fundamental reason that the asymmetric mode competition is difficult to be suppressed.Even if there is only a slight asymmetric mode in the startup of oscillation,when there is reflection downstream,the impact on the working state is fatal.In order to solve this problem,the corresponding measures are put forward:adjusting the position of the downstream cone waveguide so that TM₁₁mode achieves destructive interference,and the special support posts are designed so that TE₁₁ mode is transmitted without reflection.Finally the influence of the asymmetric mode reflection is eliminated and the device keeps in a stable state.The high power experimental study of the designed Ka-band super overmoded coaxial Cerenkov HPM oscillator is carried out.After designing the engineering structure of the device,and setting up the experiment and measurement system,the high power experiment is carried out.In the preliminary experiment,the electron beam quality of five cathode materials are compared,and the dielectric copper cathode and the graphite wedge-shaped cathode are selected for experimental study.When the dielectric copper cathode is used,the microwave frequency is 32.62 GHz,and the output microwave mode is mainly symmetric mode with a diode voltage of about 416kV,current of about 5.10 kA and a guiding magnetic field of 0.94 T.EH₁₁ mode competition is very slight.When the graphite wedge-shaped cathode is used,the microwave frequency is 32.72 GHz measured by a 35 GHz bandwidth oscilloscope with a diode voltage of about 401 kV,a current of about 4.13 kA,and a guiding magnetic field of 0.95 T.A competition frequency of 32.83 GHz can be found,which is inferred corresponding to EH₁₁ mode.In order to solve the problem of asymmetric mode competition,the position of the cone waveguide and the support posts are improved.After the improvement,under the condition of diode voltage about 390 kV,current about 4.42 kA and guiding magnetic field 1.06 T,the microwave output is 291 MW,the microwave pulse width is about 25 ns,the waveform is square wave,and the working state is stable.The far field radiation pattern is basically consistent with the simulation results.The radiation output microwave mode is basically symmetric mode,and there is very little TE₁₁ mode,which does not have a significant effect on the operation state of the device.In the range of 365 kV to 420 kV,the output power is greater than 200 MW and the efficiency is more than 10%.In the range of 0.8 T to 1.55 T,the output power is greater than 250 MW and the efficiency becomes more than 15%.