Stability Analysis of a Second Harmonic Coaxial-Waveguide Gyrotron Backward-Wave Oscillator
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Stability Analysis of a Second Harmonic Coaxial-Waveguide Gyrotron Backward-Wave Oscillator C. L. Hung & J. H. Hong
Received: 6 May 2012 / Accepted: 2 October 2012 / Published online: 7 October 2012 # Springer Science+Business Media New York 2012
Abstract This study analyzes the stability of a Ka-band second harmonic gyrotron backwardwave oscillator (gyro-BWO) with a coaxial interaction waveguide. All of the possible competing modes in the frequency tuning range are considered. To suppress various competing modes, the downstream part of the coaxial interaction waveguide is loaded with distributed losses. Although the competing modes have different kinds of transverse field distributions, simulation results show that the losses of the outer cylinder and those of the inner cylinder serve as complementary means of suppressing the competing modes. The losses can stabilize the competing modes while having minor effects on the start-oscillation current of the operating mode. Detailed investigations were performed involving the dependence of the start-oscillation currents on the parameters of the lossy inner cylinder and the lossy outer cylinder, including the resistivity and the length of the lossy section. Moreover, under stable operating conditions, the performances of the second harmonic coaxial gyro-BWO with different sets of circuit parameters are predicted and compared. Keywords Gyrotron backward-wave oscillator . Harmonic . Coaxial waveguide . Mode competition
1 Introduction Based on the electron cyclotron maser instability, the gyrotron backward-wave oscillator (gyro–BWO) is a gyrodevice in which the forward-propagating cyclotron harmonic beam mode interacts with a backward propagating waveguide mode. The electromagnetic wave is excited by the beam when the resonance condition w þ jkz jvz sΩc ffi 0
ð1Þ
is satisfied, where ω is the wave frequency, kz is the propagation constant (kz
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