Abstract
A self-consistent large signal theory is developed for the gyrotron travelling wave amplifier with the sever. The theory allows for tapering of both the magnetic field and the waveguide cross-section along the axial direction. A computer code for the large signal theory has been tested for the rectangular gyrotron amplifier circuit with the sever where the RF wave is completely terminated. For a circuit with uniform cross-section, we find 50% optimum efficiency and 52 dB gain when there is no axial velocity spread. At 10% axial velocity spread, the efficiency becomes 37%. The tapered two-stage gyrotron travelling wave tube (gyro-TWT) considered in this paper gives rise to 20% bandwidth and 40 dB total gain for zero axial velocity spread. The tapered case is much more sensitive to the beam velocity spread than the uniform guide.

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