Higher order modes and instabilities in coupled-cavity TWT´s

Laboratory records available to the authors, dating back to 1975, report several test CCTWT\´s troubled by an uncommon "trapped oscillation," explained as a tube section operating as a multigap monotron or extended-interaction klystron (EIK) involving a higher order mode of the interaction circuit. Antisymmetric RF field distributions for these modes tend to make the section a resonator with little external coupling at multiple close-spaced frequencies. All varieties of CCTWT interaction circuit are equally able to support such modes and the list of problem tubes inculpates no type or feature of the circuit per se (as opposed to beam conductance, which was relatively prominent in all). RF fields and propagation characteristics for these modes are discussed in three categories of interaction circuit. When the tube is designed for a given performance as a TWT (based on the conventional mode), parameters such as beam conductance and circuit length and loss combine to determine gain through the well-known TWT relationships. The fact that these parameters combine under very different relationships as regards potential oscillation as an EIK involving a higher order mode, however, helps explain why fairly similar tubes are not all troubled by the instability in question. Since conventional one-dimensional analytic approaches are inadequate for modeling beam interaction with antisymmetrically distributed RF fields, fully three-dimensional modeling capability is looked to as the analytic tool needed to evaluate CCTWT designs (and corrective changes when necessary) relative to the instability discussed.