Investigation On Ku-band Magnetically Insulated Transmission Line Oscillator

The magnetically insulated transmission line oscillator?MILO?is a kind of cross-field device,which can generate gigawatt-class high power microwave?HPM?.The MILO is compact and lightweight because it operates successfully without external applied magnetic field.Due to the potential use of the Ku-band HPM in communication,radar,remote sensing and other applications,the investigation of the Ku-band HPM generater technology has been one of the hottest topics in the HPM field.As a result,a Ku-band MILO is proposed and designed in this dissertation on the basis of investigating the conventional MILOs in low frequency bands.The main content and results of the dissertation are described as follows.Firstly,the theoretical analysis and physical design of the Ku-band MILO are conducted.The dispersion curve of the slow-wave-structure?SWS?is calculated by a high frequency electromagnetic field simulation program HFSS.The resonant frequency of the TM₀₁ mode close to the electron beam line is 12.45 GHz and the frequency of the?-mode(TM₀₁ mode)is about 12.7 GHz,which certify that the device can operate at the near?-mode of the TM₀₁ mode.The resonant characteristics of the overall structure of the Ku-band MILO are studied.The main resonant frequency is 12.4 GHz,which is identified as the?-mode of the TM₀₁ mode by analyzing the corresponding electric field distribution.And the other resonant modes are suppressed well.The operation characteristics of MILO such as operation frequency,efficiency,impedance,B-H and Hull cut-off condition,the drift velocity of electron beam and the movement of individual electron in the plate electrode model are theoretical analyzed.According to the theoretical analysis,the physical design of the Ku-band MILO is conducted and the design results are presented.Secondly,the structure design of Ku-band MILO is introduced.The design of cathode structure is optimized.When the launch point of the cathode is 1.5 cm in front of the first choke vane and the right end point of the cathode is 2 cm in front of the first extracting vane,the device can achieve higher power output and faster microwave start-up.The investigation of Ku-band MILO with different number of SWS cavities is conducted and the results show that when the MILO is with 8 SWS cavities,the device can acquire high power and high efficiency microwave output with fast microwave start-up and without modes competition.In order to avoid the electrodes erosion in the load region,a limited load is used,in which the load cathode length and the electrode spacing can be designed large sufficiently to reduce the energy deposition density.The triple-cavities extracting structure is optimized with higher extracting efficiency in comparison of single-cavity,double-cavities and fourth-cavities extracting structure.The coaxial output structure is designed.When the radius of the inner conductor is increased to 39 mm,the TM₀₁ mode is cut off in the coaxial output waveguide and the device achieves efficient pure TEM mode output.Thirdly,particle-in-cell?PIC?code is used to investigate the mechanism of the proposed Ku-band MILO.The basic model of Ku-band MILO is constructed and the related physical images in operation process such as the output microwave power,operation frequency,the electric field distribution and the electron distribution are presented.The typical simulation results show that under the input voltage of 476 kV and current of 42 kA,an HPM pulse with power of 1.7 GW at frequency of 12.3 GHz is generated,yielding a conversion efficiency of about 8.5%.The maximum surface electric field of the SWS is about 1.5 MV/cm,and the operation mode is near?-mode of the TM₀₁ mode.Fourthly,the experiments are carried out on a low impedance accelerator of our laboratory.Under the diode voltage of 505 kV and the current of 47.5 kA,the radiated microwave with frequency of 12.35 GHz,power of 1.05 GW,pulse duration of 33 ns and conversion efficiency of 4%is generated.The far-field radiation pattern is measured and the good agreement between the experimental far-field radiation pattern and the theoretical far-field radiation pattern calculated by a 3D electromagnetic simulation program verifys that the operation mode is TM₀₁ mode.The asymmetric modes generated in the initial experiments are identified as HEM₁₁ mode and suppressed effectively by improving the concentricity of the device and the electron emission uniformity of the cathode.Finally,an improved Ku-band MILO with fast start-up and high power capacity is proposed and investigated theoretically and experimentally.Through the improvements,the maximal axial electric field of the SWS is decreased from 1.2 MV/cm to 1 MV/cm,and the current density of the cathode is reduced by 13%.Moreover,the microwave start-up and saturation times are decreased 2.5 ns and 5.5 ns,respectively.In the experiment,under the diode voltage of 515 kV and current of 48 kA,the improved Ku-band MILO device produces microwave pulse with a duration of 32 ns,at a central frequency of 12.36 GHz,and with a power as high as 1.4 GW,corresponding to power conversion efficiency of about 5.6%.The experimental results show that the output microwave power and efficiency is efficient enhanced by the structures improvements.