Origin of second harmonic signals in octave bandwidth traveling-wave tubes

摘要

In a helix traveling-wave tube (TWT) with a bandwidth exceeding one octave, the second harmonic of an input signal near the low end of the band will experience exponential growth. In such a case, we have found that the non-linear electron orbits in the beam, as opposed to the orbital crowding exhibited in the linear electron orbits, are the main source for second harmonic generation. This unexpected result is due to the synchronous amplification of the second harmonic. We demonstrate this phenomenon from the solution of the non-linear equations that we have formulated that govern evolution of the second harmonic field. Good agreement between theory and simulation using the CHRISTINE code is found.