Theoreticaland Applied Research On Millimeter And Sub-millimeter Wave Transmission Devices

Millimeter and sub-millimeter wave has widespread development and application prospects in target recognition and detection, precision-guided weapons, controlled nuclear fusion, communication and so on. Millimeter and sub-millimeter wave system usually includes source, transmission devices, emission and receiving detection devices.The transmission system plays a key role in guaranteeing the system efficiency effectively. Transmission functional devices include input and output coupling devices,mode converter, passive control devices and other key components.Coaxial Bragg reflector, mode converter and taper, artificial electromagnetic material devices have been investigated in this dissertation, which will provide an effective way to solve the key technical problems for the transmission system.The main achievements and innovations are as follows:1. A coaxial Bragg reflector with sinusoidal-tapered-inner ripple is proposed and applied in the relativistic diffraction generator(RDG) successfully. The generalized scattering matrixes of the coaxial Bragg reflector are deduced by the conservation of complex power technique(CCPT). The numerical calculation and analysis program of coaxial Bragg reflector is compiled. The numerical results show that the reflector has high reflection and mode selection characteristics for 8 millimeter wave TM mode. The accordance of the simulated and the numerical calculation results verifies the correctness of the program.The three-dimensional particle-in-cell(PIC) simulations of the RDG with the reflector are given.The influences on the output power and output mode under different cathode positions, different reflector positions and different magnetic intensities are studied. The experimental results are in good agreement with the simulated results. The output power and the efficiency reached 756 MW and 20%,respectively.The experimental results show that the reflector can improve the RDG performances obviously.2. A TE0n-TE01 mode converter with dual-nonuniform period and ripple-wall in circular waveguide is proposed and studied in theory and experiment. The coupled wave equations of circular waveguide mode converter with periodic radial perturbations and periodic axial fluctuations are derived.The numerical calculation program of the mode conversion sequences using Nelder-Mead optimization algorithm is compiled. Bynumerical calculation and analysis, the optimized parameters of 0.22 THz TE03-TE01,TE01-TE11 and TE01-TM11 mode converters are obtained. The numerical results show that total length of the nonuniform TE03-TE01 converter is shortened by 44.18%, a 150%bandwidth increase of more than 95% conversion efficiency is obtained, compared to the conventional uniform perturbation structure. The experimental investigations of the nonuniform mode converter are carried out. The excellent symmetric distributions of TE02 and TE01 mode demonstrate that the converters have good performances of mode conversion.3. A quadratic sinusoidal polynomial mode taperwith a tunable radial variation rate according to different operation conditions is investigated. The numerical results show that the transmission efficiencies of 0.22 THz near cut-off and overmode TE01 mode tapers are both higher than 99.9% and the standing wave ratios(SWR) are below1.02.The numerical calculation results are in good agreement with the HFSS simulation results. The research findings show that the new-type taper for gyrotron has obvious advantages in transmission, bandwidth and compactness compared with the trigonometric function tapers and linear tapers.4. A bilayer four-split reformative complementary electric inductive-capacitive(FCELC) bandpass filter is investigated based on frequency selective surface(FSS).The experimental results show that the 3d B relative bandwidth of the broadband filter centered at 0.281 THz and the narrowband filter centered at 0.315 THz are 26.7% and7.6%, insertion losses are 2.6d B and 1.9d B, the out of band rejection are more than30 d B and 40 d B, respectively.The simulated and experimental results are in good accordance. Compared with the traditional structure, the reformative filters have obvious advantages in processing difficulties, insertion losses, bandwidths and suppressions of spurious resonances.