Bandwidth Broadening in RF-Structure

Broad bandwidth represents one of the features the most sought after in electron tubes. In linear accelerators, it ensures the insensitiveness of the RF-structure to mechanical error or beam loading. This is commonly achieved by increasing the coupling between accelerating cells. To go further however, coalescence between two passbands or more is being used. The postcoupled Alvarez structure, the side-coupled or resonant slot-coupled cavity chain, as well as the dish-and-washer structure, among others, are well known examples. More generally, coalescence can take place in complex RF-structures such as multiperiodic systems. Presently, the use of mode coalescence is extended to certain types of RF-structures in TWTs in order to avoid band-edge oscillations. The purpose of this paper is to derive rules that mode coalescence obeys. Examples will be given for illustrations.