Investigation Of Coaxial Bragg Structures Corrugated With Sinousoidal Ripples

Bragg structure is a kind of periodic structure that exhibits so-called stopband phenomena, where the propagation of the electromagnetic wave within specific frequency range inside such a structure can be well suppressed due to the constructively Bragg scattering. In the realm of microwave electronics, a Bragg structure is usually formed by a metallic waveguide corrugated with shallowly sinusoidal or rectangular ripples on the inside surface of the conductor. As one of successful applications, a metallic Bragg structure can act as a high-reflectivity reflector in constructing the Bragg resonator for overmoded, high-power cyclotron auto-resonance maser (CARM) oscillators.In previous works, extensive theoretical and experimental studies have been devoted to the conventional cylindrical and planar Bragg structures, where only one conductor or dielectric is provided periodic perturbations. Recently, growing attention has been paid to the coaxial Bragg structures realized by coaxial metallic waveguides due to theirs attractive merits in improving the performance of high-power free-electron device. In consideration of the imperfect theoretical description of coaxial Bragg structures in available publications, comprehensive investigation is presented in this dissertation to demonstrate the peculiarities of the coaxial Bragg structure with either one or both of the conductors sinusoidally corrugated. The main contents are organize as follows:In Chapter 1, the detailed states of arts of the Bragg structure are introduced, where the problems of the recent work are revealed. The aims as well as the main contents of the present dissertation are also explained.In Chapter 2, a multiwave coupled formulation of coaxial Bragg structure corrugated with sinusoidal ripples is presented to describe all the forward and backward waves of various propagating modes within the structure, the coupled equations and the coupled coefficients are derived with the inclusion of both the band-gap overlap and the Bragg mismatch effect, and the corresponding numerical approach is also supplied. The validity of the formulation of is examined in terms of the reported experiments, and good agreement of the theoretical results with the experimental measurements is demonstrated. Comparison of the present " formulation with the previous two-modes coupled treatments reveals the band-gap overlap phenomena under overmoded operations, and confirms the significance of the multiwave coupled formulation.In Chapter 3, a coaxial Bragg reflector with a higher-order mode operation is suggested for the application in the cyclotron auto-resonance maser (CARM) oscillator, and a concept design is presented. Comparative study between the proposal and the original cylindrical Bragg reflector has been carried out in terms of two-wave coupled (single mode) treatment. It is found that, compared to the cylindrical Bragg reflector, the coaxial Bragg reflector can hold a larger eigenvalue interval with a larger dimensional size, moreover, the use of a properly corrugated inner rod provides an extra approach of affecting the reflective character. Based on this peculiarity, the results in terms of multiwave coupled treatment show that the spurious modes generation can be well suppressed by properly corrugating both the outer and inner conductors, which reveals the possibility of the high-order mode operation at a frequency of hundreds GHz in such a reflector.In Chapter 4, the band-gaps distribution of the involved modes in an overmoded coaxial Bragg structure which operates at higher-order mode at the Terahertz frequency range is generally demonstrated in various corrugation cases. It is found that, band-gap overlap always occurs in an overmoded coaxial Bragg structure, no matter if only one or both conductors are corrugated. However, it can be well separated by setting the phase difference between the outer- and inner-conductor corrugations to beπ. Furthermore, band-gaps peculiarities of the overmoded coaxial Bragg structure with Hamming-window distribution corrugations are investigated in terms of multiwave coupled treatment, where the results reveal that the residual side-lobes interaction of the involved modes can be entirely eliminated by applying Hamming-window distribution to both the outer-conductor and inner-rod corrugations.In addition, all the aforementioned numerical approaches have been programmed to several codes in FORTRAN language. Numerical examples in this dissertation are calculated employing these codes. The theoretical models as well as the numerical codes are expected to provide theory basis and engineering references for the applications of the coaxial Bragg structure.