Small-signal two-dimensional model of parasitic backward-wave oscillation in a monofilar-helix traveling-wave tube

We present a mathematical linear 2D model of the interaction of the synchronous forward and backward harmonics of the electromagnetic wave, existing in the slow-wave circuit, with the electron beam in a monofilar-helix traveling-wave tube (TWT) under periodic permanent magnetic (PPM) focusing. We accounted for the effect of the electron beam alternating rotation in the PPM focusing field, magnetic field amplitude and periodicity, and the specific coupling of the electron beam with the circuit wave. This coupling takes place in the monofilar-helix TWT only and decreases its stability with respect to the parasitic backward-wave oscillation in comparison to other types of helix TWT.