Analysis of an azimuthally periodic vane-loaded cylindrical waveguide for a gyro-travelling-wave tube

A cylindrical waveguide, provided with wedge-shaped metal vanes projecting radially inward from the wall of the guide, excited in the transverse elect ric (TE) mode, was analysed. The analysis was carried out considering the a ngular harmonics generated by the angular positioning of the vanes. A set o f equations was generated in the Fourier amplitudes of field constants. The condition for nontrivial solutions for the field constants gave the disper sion relation of the structure. From the expression of power flow down the structure, its interaction impedance was also estimated. The shape of the d ispersion characteristics and the value of the cutoff frequency as well as the interaction impedance characteristics of the waveguide were found to de pend on the vane parameters-their number as well as their radial and angula r dimensions. The optimum vane parameters were obtained corresponding to th e minimum variation of the slope of the omega-beta dispersion plot, such pa rameters being useful from the standpoint of the bandwidth of a gyro-travel ling-wave tube (gyro-TWT) using a vane-loaded cylindrical waveguide as the interaction structure. The dispersion and impedance characteristics, which were found typically for the TE01 mode as defined for the structure, taking four vanes, were more sensitive to the number and angular width of the van es than to their radial depth. The value of the interaction impedance, calc ulated at the potential beam position, was found to be higher for a loaded waveguide than for an unloaded one, and it depended on the frequency of ope ration relative to the cutoff. The interaction impedance also depended on t he position of the beam relative to the waveguide wall where it was estimat ed, and hence the optimum beam position corresponding to the maximum intera ction impedance was found. The theory was validated against the dispersion characteristics reported elsewhere typically for four-vane magnetron-like s tructures excited in the 2 pi mode. Although the present study was restrict ed to 'cold' analysis of the structure in the absence of the electron beam, it could provide important feedback for analysing a gyro-TWT, using a vane -loaded cylindrical waveguide, and hence for predicting the structure param eters for the wide-band performance of the device.