| Summary: | 碩士 === 國立澎湖科技大學 === 電資研究所 === 99 === The gyrotron traveling-wave tube (gyro-TWT) amplifier is a coherent electromagnetic radiation source with the capabilities of high power, high frequency, and broad bandwidth. It is a promising source for applications in communication, military radar , plasma heating and satellite. The operating frequency of a gyro-TWT is near the grazing intersection of the beam-wave resonance line and the dispersion curve of a waveguide mode. The bandwidth of a gyro-TWT can be increased by tapering both the waveguide cross section and the magnetic field to connect the disparate bandwidths of each section with different cutoff frequencies. A two-stage gyro-TWT with tapered waveguides was reported to have considerable increased bandwidth, but its output power was limited due to the stability problem. Accordingly, the wide-band gyro-TWT in this paper employs two-stage tapered coaxial waveguides with distributed wall losses as the interaction structure to enhance the stability. This study firstly investigates the start-oscillation conditions and the suppression of likely oscillating modes in the wide-band coaxial gyro-TWT. Then the bandwidth of the wide-band coaxial gyro-TWT will be optimized under stable operation conditions. The wide-band gyro-TWT with distributed losses is predicted to generate 111 kW in the Ka band with efficiency 53 %, a saturated gain of 62.5 dB, and a bandwidth of 29 GHz~43 Ghz (%) for a 70 kV, 3 A electron beam with an =0.95 and an axial velocity spread of 0 %, but is predicted to generate 100 kW in the Ka band with efficiency 45 %, a saturated gain of 39.86 dB, and a bandwidth of 36.5 GHz~4.5 Ghz (%) for a 70 kV, and an axial velocity spread of 3 %.
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