Three-dimensional simulation of helix travelling wave tubes

Summary form only given. We present a three-dimensional nonlinear formulation and simulation of a helix travelling wave tube (TWT) using a sheath helix model. The simulation is capable of treating both DC and pulsed electron beams as well as single-frequency or multi-tone operation. The model relies upon a spectral decomposition of the electromagnetic fields in terms of the vacuum sheath helix polarizations. A Poynting-like equation describes the energy balance. The electron orbits are treated using the complete Lorentz force equations. The field equations are integrated on a grid and advanced in time using a MacCormack predictor-corrector scheme, and the electron orbit equations are integrated using a fourth order Runge-Kutta algorithm. Charge is accumulated on the grid and the field is interpolated to the particle location by a linear map. Several numerical cases are considered. Simulation of the injection, of a DC beam and a signal at a single frequency is compared with a linear field theory of the helix TWT interaction, and good agreement is found. Simulation of a prebunched beam is also discussed.