NONLINEAR-THEORY OF COLLECTIVE EFFECTS IN HELIX TRAVELING-WAVE TUBES

A time-dependent collective nonlinear analysis of a helix traveling wa ve tube including fluctuating (ac) space-charge effects is presented f or a configuration where an electron beam propagates through a sheath helix surrounded by a conducting wall. The effects of dielectric and v ane loading of the helix are included, as is efficiency enhancement by tapering the helix pitch, and external focusing by means of either a uniform solenoidal magnetic field or a periodic field produced by a pe riodic permanent magnet stack. Dielectric loading is described under t he assumption that the gap between the helix and the wall is uniformly filled by a dielectric material. Vane loading describes the insertion of an arbitrary number of vanes running the length of the helix. The electromagnetic field is represented as a superposition of azimuthally symmetric waves in a vacuum sheath helix. The propagation of each wav e in vacuo, as well as the interaction of each wave with the electron beam, is included by allowing the amplitudes of the waves to vary in z and t. The dynamical equation for the field is solved in conjunction with the three-dimensional Lorentz force equations for an ensemble of electrons. Collective effects from the fluctuating rf beam space-charg e waves are also included in the analysis by means of a superposition of solutions of the Helmholtz equation. The simulation is compared wit h a linear theory of the interaction, and an example is described corr esponding to a tube built at Northrop-Grumman Corp. (C) 1997 American Institute of Physics.