| Summary: | 碩士 === 國立澎湖科技大學 === 電資研究所 === 101 === High power terahertz (THz) wave can be applied to weather radar, remote detection of explosives, metal and non-metallic weapons, space communications, plasma diagnostics, DNP technique, material processing, electron spin , and resonance spectrum. However, high power and coherent THz sources are difficult to obtain. The gyrotron backward-wave oscillator, studied in this thesis, is capable of generating high-power radiation in the THz band, and its oscillation frequency can be continuously tuned by changing the voltage or magnetic field. A THz gyrotron backward-wave oscillator must operate at a high-order waveguide mode to enlarge the cross-section dimension of the waveguide for high power operation. In order to avoid the mode competition problems resulting from the high-order mode operation, this thesis adopts the coaxial waveguide as the interaction structure and selects a smaller radius ratio to reduce the number of competing modes. In addition, the outer radius of the coaxial waveguide is tapered to shorten the effective interaction length and increase the start-oscillation currents of the competing modes. The simulation results show that tapering the outer radius can not only effectively suppress the competing modes, but also change the operating magnetic field range of the operating mode to avoid the mode competition. Moreover, tapering the outer radius can also enhance the efficiency and increase the tuning bandwidth of the operating mode. Finally, the coaxial-waveguide gyrotron backward-wave oscillator, operating at a voltage of 30 kV and a current of 5 A, can generate an output power of 17 kW (efficiency 11.5%), a stable 3dB tuning bandwidth of 3.2 GHz (305.5 GHz ~ 308.7 GHz, 1.1 % ) .
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