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 conv entional TWTs seem to be attractive because of their principal ability to o perate stably with a high. gain, since the stability can be provided by a s hortening 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 conv entional TWTs and free electron laser amplifiers. Thus a part of the result s obtained is also valid for these devices. For two-stage configurations (t wo waveguides separated by a drift region) the effect of the difference in waveguide cutoff frequencies on the gain and bandwidth is studied. This eff ect has a common nature with stagger-tuning of cavities, which is widely us ed in conventional klystrons and gyroklystrons for bandwidth enlargement. T he 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 tap ered waveguides;and external magnetic field is discussed. [S1063-651X(99)05 610-X].