Conference Paper

Dynamics of Axial Mode Competition in the Gyrotron Backward-Wave Oscillator

Dept. of Phys., Nat. Tsing Hua Univ., Hsinchu
DOI: 10.1109/PLASMA.2005.359068 Conference: Plasma Science, 2005. ICOPS '05. IEEE Conference Record - Abstracts. IEEE International Conference on
Source: IEEE Xplore

ABSTRACT Summary form only given. The dynamics of axial mode competition in the gyrotron backward-wave oscillator (gyro-BWO) has been investigated in theory and experiment. Because of the difference in field-shaping processes, the axial modes of the gyro-BWO each exhibit a distinctive asymmetry in field distribution along the axis. Multi-mode particle simulations reveal a consistent pattern of axial mode competition in which a fast-growing and well-established mode is subsequently suppressed by a slowly-growing mode with a favorable field profile. This is verified in a Ka-band gyro-BWO experiment. A detailed time-frequency analysis of both the simulation and experimental results further suggests that the competitiveness of a specific mode depends more on its field structure than on its initial growth rate. This perspective is expected to add a new basis for the understanding of mode competition phenomena in the gyro-BWO

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Available from: Tsun-Hsu Chang, Aug 18, 2014
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    • "However, a gyro- BWO with a tapered interaction waveguide is susceptible to the problem of axial mode competition [11], [12]. In a tapered waveguide, a higher order axial mode ( > 1, which usually has a higher oscillation frequency) can penetrate deeper than a fundamental axial mode ( = 1); therefore, it may have a startoscillation current lower than the fundamental axial mode [11]. The application of distributed losses at the downstream end of the interaction waveguide is effective in suppressing the higher order axial mode [12], [13]. "
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    ABSTRACT: Mode competition is a severe problem in the development of a high-power gyrotron backward-wave oscillator (gyro-BWO) in terahertz (THz) region. To improve the stability of this device, this paper investigates the possibility of using a tapered coaxial waveguide as the interaction structure of a 0.3-THz gyro-BWO. The inner cylinder rarefies the mode density near the operating mode. Moreover, tapering the wall of the outer cylinder suppresses almost all the competing modes and shifts the optimum magnetic tuning range of the operating mode under stable operating conditions. The gyro-BWO is predicted to stably generate a peak power of 17.2 kW with 11.5% efficiency at 0.307 THz, and a continuous tuning bandwidth of 3.4 GHz $({approx}{1.1%})$ for a 30-kV, 5-A electron beam with $alpha=1.0$ and an axial velocity spread of 5%. Compared with the uniform case, the tapered coaxial interaction waveguide enhances both the stability and the frequency tunability of the THz gyro-BWO.
    IEEE Transactions on Electron Devices 06/2014; 61(6):1812-1817. DOI:10.1109/TED.2013.2296299 · 2.47 Impact Factor