The operation of all conventional traveling wave tubes is fundamentally based on the interaction of an electron beam with the axial RF electric field of a traveling circuit wave; transverse electric fields play only a secondary role in the dynamics. It has long been known, however, that a TWT could be designed to exploit certain advantages inherent in the interaction of an electron beam with a transverse electric circuit wave. In 1960 Siegman [1] published a detailed small signal analysis of the interaction of a filamentary beam with a traveling transverse wave, and found that positive gain could indeed be obtained from this interaction, which produces no longitudinal bunching of the beam. In a transverse interaction, energy is extracted from the axial motion of the beam, as in the longitudinal interaction case, but in such a way that all particles lose the same amount of energy, independent of their location (phase) within an RF period. This result holds strictly speaking only for a filamentary beam. Briggs et.al. [2] were able to estimate the energy spread of a beam of finite radius and to use this estimate to bound the attainable efficiency of a transverse TWT. The Moscow State University group, in collaboration with Istok Corp. and Tory Corp. [3], has been successfully designing, developing, and marketing various types of microwave amplifiers and receiver protectors based on the transverse interaction. As of 2002, however, they were reporting only limited experimental success with transverse TWT's. This may be at least partly due to problems associated with the design and fabrication of a suitable slow wave circuit, which are not simple matters.