Theoretical Study And Numerical Simulation Of The Gyrotron Traveling Wave Amplifier

High power and broad bandwidth capability of gyrotron traveling wave amplifier(gyro-TWA) makes it an attractive source of coherent electromagnetic radiation inmillimeter and sub-millimeter wavelength ranges. Gyro-TWAs could be applied verybroadly, especially in radar and communication systems. The dissertation is focusedon the theory and simulation of beam-wave interaction in the gyro-TWA.The linear theory of gyro-TWA operating in the TE mode is extended to studythe effect of electron beam misalignment. The concrete example of a gyro-TWA withthe TE₀₁ mode at the fundamental harmonic shows that the effect is greatly dependenton the guiding-center radius of the beam. The physical mechanism of the effect is alsoanalyzed.A linear theory is developed to study the TM mode gyrotron backward waveoscillator (gyro-BWO). It is shown that, for the TM mode gyro-BWO interaction, theelectron can simultaneously losses its transverse and axial energy, and the modestarting current will not reduce obviously with the reduce of the electron velocity ratioTherefore, the TM mode may be appropriate for gyro-BWO.An analytic formula of propagation constant for any mode in a lossy circularwaveguide is presented, which includes the effect of the thickness of the lossy layeron the waveguide. The analytic calculation results are in good agreement with thoseof an accurate dispersion equation. It has been shown that large attenuation forcompeting mode can be realized while that of operating mode is small byappropriately choosing the thickness and conductivity of the lossy layer.An analytic theory describing the bunching of the electron beam in gyro-TWTwith two-stage interaction circuits is developed. The analytic formulas of thetraveling-wave bunching parameter, ballistic bunching parameter, and the harmoniccurrent of the bunching beam are obtained. The theory is helpful for us to understandthe bunching procedures corresponding to different harmonic operations. The preliminary design of a harmonic-doubling gyro-TWA has been carried out.The input stage of the amplifier operates in the TE₀₁ mode at fundamental cyclotronharmonic. Its output stage operates in the TE₀₂ mode at second cyclotron harmonic.The stabilities of both the input stage and the output stage are analyzed respectively.The effects of various operating parameters are also studied. Numerical calculationsshows that a peak power of 150kW with an efficiency of 14.3%, a gain of 30dB, and a3dB bandwidth of 2.8% can be obtained when driven by 70kV, 15A electron beamwith a velocity ratio of 1.2 and a velocity spread of 5%. With a tapered magnetic field,the peak power, efficiency and gain can be increased to 240kW, 22.8% and 36dBrespectively, but the bandwidth is reduced to 0.7GHz (2.18%).Based on the above research work, a set of numerical simulation programs ofbam-wave interaction in gyro-TWT have been formed, which includes three parts:linear analysis programs, stability analysis programs, and nonlinear analysis programs.The effect of various factors, such as distributed losses, guiding-center drifting andthe magnetic field taper, can be considered in the nonlinear programs, which are alsousable for multi-stage circuit and harmonic-multiplying operating mode. The resultsobtained with our programs are in good agreement with those presented in somearticles published in some international journals.