2D modeling of TWTs based on serpentine and folded waveguide structures

Abstract

Travelling Wave Tubes (TWTs) with slow-wave structures, based on a serpentine or folded waveguides, are promising devices for operation at sub-Terahertz and Terahertz frequency ranges. Development of such types of TWTs requires accurate and efficient computational tools suitable for their design, analysis and optimization. Recently we have developed the new model <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sup> , which allows us to extend an existed 2D large-signal code TESLA-CC <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> on the modeling of amplifiers based on serpentine or folded waveguide structures. To model properties of an external slow-wave structure, consisting of the piece of the serpentine or folded-waveguide, we have introduced the circuit model, based on the transmission line loaded by the shunts representing interaction gaps coupled into the beam-tunnel. Boundary conditions for the line are chosen to provide good match of the finite length structure at the given operating frequency. Represented such model system of equations can be formulated as an eigen-problem, which solution allows find eigen-frequencies and eigen-modes of the finite-length periodical structure. Computed from this problem eigenmodes then are used as a basis for representation of fields on the gaps along the slow-wave structure, which are selfconsistently coupled with the 2D axi-symmetric fields inside the beam-tunnel, computed as a full solution of Maxwell equations together with the relativistic equations of 3D motion of beam electrons. Used in the code time-advanced algorithm allows to find convergent solution of a selfconsistent large-signal beam-wave interaction. The first results of the 2D modeling by using the newly developed algorithm and their comparisons with the available to us results of the modeling by other codes and experimental data will be discussed as well.