| A brief description of the status of the high power microwave amplifiers is given. We have summarized the measures taken in some main amplifiers on widening the output bandwidth and pointed out that in the future high resolution radar, the high capacity intensive communication system and the electronic defense and counter measure appli- cations, the gyrotron traveling wave tube(gyro -TWT) amplifier has a bri]iiant future because of its high output frequency and high power capacity. We have made a detailed theoretical analysis on the beam-wave interaction in a gyro-TWT amplifier and derived a compact and simplified nonlinear equation set on the evolution of the radiation field. Electron beam bunching is analyzed theoretically and the formulas of the small signal gain is also derived. We find analytically that in some cases, axial and azimuthal bunching may be reinforced and at the entrance of the interaction region, there maybe exist a gain dip. We have also put forward and studied two models for widening the bandwidth of a gyro-TWT. The first one is a double stream gyro-TWT, in which two electron beams are injecting into the interaction region simultaneously. Simulations show that the band- width may surpass 20% and the peak gain is also high. The second one is a gyro-TWT with tapered guiding magnetic field, in which after properly choosing an initial value, the guiding field has a slightly linear decrease from a given position in the interaction region. High efficiency (>40%) and wideband (-20%) output are achieved by simulation. Further research on the frequency multiplied harmonic gyro-TWT is also carried on in this paper. We have obtained the optimal parameters of each section by simulation and achieved high frequency and broadband output by using an electron beam with a low velocity ratio in an uniform circuit.
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