Study On The Cherenkov Radiation Sources With Metal Photonic Band Gap Cells

High power millimeter wave and sub-millimeter wave have extensive applications in military and civil fields such as nuclear fusion heating, radar, communication, biomedical and material sciences. Cherenkov devices are always the effective means to produce high power electromagnetic wave. However, as the operation frequency increases to millimeter-wave and sub-millimeter range, the slow wave system (SWS) has small dimension so that power capacity will decreas correspondly. It is also very difficult to pass high current electron beam through such small structure. In relativistic Cherenkov device, the overmoded structure has been successfully employed to increase current capacity and radiation power but mode competition is difficult to be suppressed.To remove the mode competition of Cherenkov device in millimeter and sub-millimeter range, a metal photonic band gap (PBG) structure is employed to construct the novel SWS in this dissertation. Attributing to the mode selectivity of the PBG structure, the device can operate on single mode. The major achievements are follows:1. The global band gaps of TM polarization wave for general two-dimensional (2D) PBG structures formed by square or triangular arrays of metal posts are studied using finite-difference-time-domain (FDTD) code. The calculated results show that many band gaps of the TM polarization wave appear in triangular and square lattices and there is only one band gap in both lattices when the values of the ratio of the post radius (r) to the post spacing (a) in a certain range.2. The properties of a PBG cavity are investigated. The results show when the reasonable'r'and'a'are chosen, the PBG cavity only supports TM01-like mode and several other nonsymmetrical modes such as TM11-like and TM21-like modes. The higher-order TM0n-like modes are disappeared in the cavity. The Q value of TM01-like mode is determined by the inner posts of the cavity. Then, it is effective to decrease the Q value of high-order passing band mode by reducing the number of outer layer of lattice.3. A Ka-band SWS consisting of PBG cavity is proposed. The dispersion characteristic and the distribution of electric field of TM01-like mode in the PBG SWS are studied. To obtain the dispersion curve from the analytical theory, the opening boundary of the PBG structure is replaced by a closed boundary model with an appropriate radius equal to the average radius of the defect; and a dispersion equation is derived. According to the numerical calculations, the dispersion curve of TM01-like mode from the dispersion equation is in good agreement with the results of the simulation and experiment. Due to hexagonal boundary in defect, the Ez field of TM01-like mode has poor azimuthal symmetry and furthermore results in the poor azimuthal asymmetry of the coupling impedance. The results of 3-dimensional particle-in-cell (3D PIC) simulation show that the mode competition is depressed greatly in the Cherenkov source with PBG SWS. Single operation mode and radiation frequency are observed in PIC simulation.4. To improve the azimuthal symmetry of electric filed and coupling impedance of TM01-like mode, a SWS with modified PBG cavity is proposed. The inner post in modified PBG cavity is arranged in a circle. The interaction between electron beam and electromagnetic wave in the modified PBG SWS is analyzed by 3D PIC simulation. The results show that the azimuthal symmetry of the electric filed of TM01-like mode improves greatly. This improvement increases the efficiency of the device and restrains the asymmetry modes.5. Cold test experiments of the PBG and modified PBG SWSs are accomplished. The dispersive characteristics of the both SWSs obtained from cold test experiment are in good agreement with the numerical results, which demonstrates the reliability of the dispersion curve. In order to preserve the impedance matching between pulse power source and SWS, an electron gun with two-cathode is proposed to produce two-electron-beam. Only inner beam is used to drive the SWS and the outer beam is obsorbed by graphite plate. The characteristic of the gun is performed by 3D PIC simulation. On the other hand, the parameters of output system of high frequency structure are obtained by HFSS code. Based on these researches, the whole system for hot test is fabricated.