Linearized field theory of a dielectric-loaded helix traveling wave tube amplifier

A linearized relativistic field theory of a helix traveling wave tube (TWT) is presented for a configuration where either a thin annular beam or a solid beam propagates through a sheath helix enclosed within a loss-free wall in which the gap between the helix and the outer wall is filled with a dielectric. A linear analysis of the interaction is solved subject to the boundary conditions imposed by the beam, helix, and wall. In the case of the annular beam, the electrons are assumed to be strongly magnetized. In contrast, the effect of variations in the axial magnetic field are included in the electron dynamics for the solid beam analysis. Determinantal dispersion equations are obtained for the azimuthally symmetric modes which implicitly includes beam space-charge effects without recourse to a heuristic model of the space-charge field. Numerical solutions of the dispersion equations are discussed and compared with experiments.