THEORY OF HELIX TRAVELING-WAVE TUBES WITH DIELECTRIC AND VANE LOADING

A time-dependent nonlinear analysis of a helix traveling wave tube (TW T) is presented for a configuration where an electron beam propagates through a sheath helix surrounded by a conducting wall. The effects of dielectric and vane loading are included in the formulation as is eff iciency enhancement by tapering the helix pitch. Dielectric loading is described under the assumption that the gap between the helix and the wall is uniformly filled by a dielectric material. The vane-loading m odel describes the insertion of an arbitrary number of vanes running t he length of the helix, and the polarization of the field between the vanes is assumed to be an azimuthally symmetric transverse-electric mo de. The field is represented as a superposition of azimuthally symmetr ic waves in a vacuum sheath helix. An overall explicit sinusoidal vari ation of the form exp(ikz - i omega t) is assumed (where omega denotes the angular frequency corresponding to the wave number k in the vacuu m sheath helix), and the polarization and radial variation of each wav e is determined by the boundary conditions in a vacuum sheath helix. T he propagation of each wave in vacuo as well as the interaction of eac h wave with the electron beam is included by allowing the amplitudes o f the waves to vary in z and t. A dynamical equation for the field amp litudes is derived analogously to Poynting's equation, and solved in c onjunction with the three-dimensional Lorentz force equations for an e nsemble of electrons. Electron beams with a both a continuous and emis sion-gated pulse format are analyzed, and the model is compared with l inear theory of the interaction as well as with the performance of a T WTs operated at the Naval Research Laboratory and at Northrop-Grumman Corporation. (C) 1996 American Institute of Physics.