Improved three-dimensional model for the nonlinear analysis of helix travelling-wave tubes

Based upon Tien's disc model, a plastic three-dimensional (3D) super-partic le model is proposed to improve the nonlinear analysis of helix travelling- wave tubes. The 3D space-charge forces are calculated, and 3D large-signal working equations are obtained. Numerical results employing a periodic magn etic focusing field are presented. The effects of the space-charge forces w ith respect to the different directions are investigated and a comparison o f travelling wave amplifier (TWA) solutions obtained with the 3D model and the general two-dimensional (2D) large-signal programs is given. The 3D ele ctron motions are also analysed. Some useful conclusions, on both theory an d experimental researches on TWAs, are obtained: (1) the 3D numerical resul ts are obviously different from the 2D ones, in which the saturation gain d ecreases while the interaction length increases; (2) because the angular el ectron bunching results from axial spacial bunching in the helical 3D elect ron motions, there exists an angular space-charge force; despite the RF pot entials and magnetic focusing fields being symmetrical around the axis; (3) the angular space-charge forces can be safely ignored whereas the radial a nd the axial space-charge forces must be included in the 3D calculation.