Theoretical investigation of some closed delay structures for high-power travelling-wave tubes

Four types of all-metal delay structures with rectangular cross-section are described¿the slit-coupled line, the labyrinth slit line, the zero-mode line and the folded-waveguide line. Their phase curves are derived, and their merits as delay structures for high-power travelling-wave tubes are discussed and compared with those of other types. The first three lines have positive dispersion in fundamental operation and a maximum hot bandwidth of about 15%. They can be used for electron velocities greater than one-third of the velocity of light. The folded waveguide line has negative dispersion, up to 40% bandwidth, and it has in space-harmonic operation a longitudinal impedance higher than the first two lines, but lower than the zero-mode line, when compared at the same bandwidth. The velocity region is somewhat larger.