Analysis of BWO start oscillation condition in a helix TWT with attenuator under magnetic focusing

In a practical helix traveling-wave tube (TWT), there are always magnetic focusing for constraining the electron beam as it passes through the interaction region and attenuator or sever for suppressing the oscillations, including backward-wave oscillation (BWO) and improving the output power. In the event that the attenuator (sever) and magnetic focusing were considered, a linear theory is employed to analyze BWO start oscillation condition. Numerical results show that the start oscillation length of the TWT with attenuator is much larger than that of the TWT without attenuator, that it is possible to increase the start oscillation length by tailoring the parameters of the magnetic focusing. So, in order to predict the BWO accurately, we should take into the attenuator (sever) and magnetic focusing account.