Linear theory of multistage forward-wave amplifiers

A small-signal theory describing multistage gyro-traveling-wave tubes (gyro-TWTs) is developed. Multistage configurations of gyro-TWTs as well as conventional TWTs seem to be attractive because of their principal ability to operate stably with a high gain, since the stability can be provided by a shortening of each stage while the gain increases with the number of stages. Two regimes of operation, far from cutoff and near cutoff, are considered. In the former case, the equations can be reduced to those describing conventional TWTs and free electron laser amplifiers. Thus a part of the results obtained is also valid for these devices. For two-stage configurations (two waveguides separated by a drift region) the effect of the difference in waveguide cutoff frequencies on the gain and bandwidth is studied. This effect has a common nature with stagger-tuning of cavities, which is widely used in conventional klystrons and gyroklystrons for bandwidth enlargement. The trade-off in gain and bandwidth of such “stagger-tuned,” two-stage gyro-TWTs and TWTs is analyzed. Also, the theory of two-stage gyro-TWTs with tapered waveguides and external magnetic field is discussed.