Numerical Simulations Of Coaxial Bragg Structures With Tapered Ripples

High-power radiation sources in millimeter and sub-millimeter waves have a great prospect of applications to radar, plasma heating, high-gradient linear colliders, and communications, and therefore, attract more and more attention in many countries. Microwave devices play an important role in high-power microwave technology. And as one of microwave devices, Bragg reflector is focused by researchers in the world. In the past years, the Bragg resonators have been widely employed in many microwave and optical devices. Especially, it is the most suitable cavity for over-mode, high-power cyclotron auto-resonance maser (CARM) and free-electron laser (FEL) in millimeter and sub-millimeter wave ranges.Recently, growing attention has been paid to the coaxial Bragg reflectors due to the merits for potential application in high-power cyclotron auto-resonance maser. Results show that coaxial Bragg reflector with both the outer wall and inner rod corrugated with sinusoidal ripples has some peculiarities: the operating mode can get strong reflectivity and the spurious modes are well suppressed, the structure has large dimensional size and great power-capacity, excellent mode selectivity for higher-order mode operation at frequency of hundreds of GHz.Studies about ordinary waveguide with tapered border show, compared with the uniformity border, that the taper can improve the performance of selecting mode and selecting frequency. Therefore, adding appropriate gradient to the coaxial Bragg structure may further improve its performance of selecting frequency. This dissertation is devoted to a novel coaxial Bragg reflector with tapered ripples, where both the outer wall and inner rod are corrugated with weakly sinusoidal ripple. Comprehensive study is made, including theoretical analyses and CST software numerical simulations. Several valuable academic achievements are obtained.Firstly, the background and developments of high-power microwave are briefly reviewed, and the significance, content and innovation points are explained in Chapter 1.Then, in Chapter 2, two methods, the coupling theory and exciting theory, are introduced. And the general coupled equations and all explicit formulas of the coupling coefficients are introduced, which are based on the coupled theory and orthogonally of the eigen-modes. The CST software is be introduced simply.In Chapter 3, the validity of the model we made using CST software has been verified. Then comparative Study of coaxial bragg structure with three different ripples are carried out, the results we obtain can offer the engineering references for the selecting ripple.In Chapter 4, Characteristics of frequency response in a coaxial Bragg structure with different parameters are presented, and the concept of coaxial Bragg structure with tapered ripples is introduced, then the frequency response infections of tapered are simulated by CST software.In Chapter 5, to optimize the characteristics and to suppress the residual side-lobes of the frequency response, of the slot depth which is based on a common technique in filter theory is introduced for the coaxial Bragg structure with tapered ripples. Results show that the pass-band is effectively smoothened by the windowing distribution.In Chapter 6, the effect of tapered ripples on the coupled modes and the higher-order modes is studied.Finally, conclusions are summarized in Chapter 7.The main innovation points in the dissertation are as follows:1. Comparative study of coaxial Bragg structure with three different ripple shapes is performed by making use of the CST software. One can select the ripples and size of coaxial Bragg structure according to the required reflectivity and output power.2. The conception, a coaxial Bragg structure with tapered ripples, is introduced. The simulation model is constructed, and the influence of positive taper and negative taper in different gradient angles on a single lower-order with lower frequency is simulated. The residual side-lobes of a coaxial Bragg structure with tapered ripples can be suppressed by adding window functions. The results show that the bandwidth of the coaxial Bragg structure with positive taper can be narrowed as the tapering angle increases, whereas the bandwidth of the coaxial Bragg structure with negative taper can be expanded as the tapering angle increases; The residual side-lobes of the frequency response can be effectively suppressed by employing the windowing-function technique.3. The influence of positive taper and negative taper in different gradient angles on the coupled higher-order with higher frequency has been investigated. Results show that the influence of different gradient angles on the operating mode is inconspicuous, no matter if the taper is positive or negative; but the influence on the competing modes is notable.