3d and nonlinear simulation of helix traveling wave tube and investigation of the effect of space harmonics in output power

in this paper, the operation of helix traveling wave tube in interaction region is investigated by using a three dimensional large signal lagrangian model in frequency domain. the complete simulation of these tubes is essential for optimum design and minimum manufacturing cost. this 3d code can compute interception current (or current losses ), beam radius changes in interaction region and the exact path of motion of particles in the beam. moreover, this model can describe the behavior of the tube in the presence of non-ideal beam. in the helix twt, the fields in the slow wave circuit are expressed in terms of space harmonics. the expansion coefficients are determined by both theoretical approach and by numerical simulation using cst software package. they are used in the 3d code for investigating the effect of different harmonics in beam-wave interaction.