Drive-Induced Backward-Wave Oscillations in Helix Traveling-Wave Tubes—A Case Study

The output section of helix traveling-wave tubes (TWTs) usually features a tapered delay line to enhance the amplifier performance. This alters the phase velocity of the circuit waves, which, in particular, disturbs the narrowband synchronism between the backward wave (BW) and the electron beam. As a result, the excitation threshold of BW oscillations (BWOs) increases. Large-signal operation significantly alters beam-wave synchronism, as the electrons loose kinetic energy during the amplification process, i.e., toward the TWT output. As a result, beam and BW may interact over a larger distance, thus lowering the threshold for the onset of BWOs. These would thus be drive-induced in contrast to the zero-drive BWOs mentioned above. The following investigates the occurrence of such drive-induced BWOs as a case study of several different TWT models and proposes a theory for the large-signal instability. As the simulation process is quite involved and time-consuming, a heuristic approach for the fast prediction of the oscillation is presented as well.