Large signal simulations of helix TWTs with varying beam tunnel radius

It is common to have an axially varying beam tunnel radius in helix TWTs. For example, severs separating different sections of helix circuits have radii larger than those of the adjacent helix circuits. Similarly, the beam tunnel expands at the end of the helix circuits where the electron beam emerges from the helix and enters a drift section or collector region. As the beam experiences a change in wall radius, its associated DC space charge potential changes as well, inducing changes in its kinetic energy. It is important to account for such effects in obtaining energy distribution of the spent beam as the beam tunnel opens up to a drift section or collector at the end of the interaction space. This is especially true when the spent beam distribution is transferred to a gun code for the ensuing collector simulation. In such a case, the energy distribution of the spent beam may vary significantly depending on the axial location of the transfer point. The effect on beam energy due to changing tunnel size has been modeled in the large signal code CHRISTINE 3D. Specifically, a fully two-dimensional (R-Z) Poisson solve is applied to the entire beam tunnel region, including the transition from helix circuits to the enlarged drift/collector sections. The resulting DC space charge fields are included in the equations for the beam dynamics. An iterative scheme similar to that employed by steady-state electrostatic gun codes is required to bring convergence between the beam trajectories and the DC fields. We use the parameters from an existing C band TWT as illustration.