A 3D large signal model for helix TWT

In a helix traveling wave tube, the interaction between the electron beam and the backward wave can lead to instabilities as BWO (backward wave oscillation) if the beam current is large enough. As the efficient part of the circuit fields varies in the angular direction, a 3D tool is needed to model such a phenomena. The numerical model is presented, with some examples of BWO starting conditions research. Close to the π mode, both forward and backward interact with the beam and any coupling of both waves has a strong effect. Consequently the threshold current is lowered. Several causes of coupling have been studied.