A novel rectangular-ring slow-wave structure for interaction with a sheet electron beam

Summary form only given. Conventional round electron beams have a practical current limit beyond which space-charge forces will de-bunch the beam and prevent further rf power amplification. Alternatively, rectangular cross-section sheet electron beams can circumvent this limitation by distributing the current over a larger area by stretching the width of the beam. This topology can enable a >5× increase in current over round beam systems while maintaining relatively low operating voltages. Another advantage of the sheet beam is that the coupling impedance has a relatively small dependence on beam thickness.We describe a novel slow-wave structure formed by a periodic array of rectangular rings interconnected by parallel diagonal conductors forming two zigzag-lines ("rectangularring slow-wave structure"). The zigzags form the inductive part of the slow-wave structure while the rectangular rings are largely capacitive and, with proper design, can slow the phase velocity (ω/β) of the wave to be synchronous with the longitudinally streaming electron beam. The period of the structure can be much less than the condition for backwardwave oscillation to occur. Compared with a rectangular helix,1"3 this new structure should be less prone to backwardwave oscillation. We will present measurements and simulations of the cold characteristics of a test structure and analyses of mode competition and bandwidth optimization for a Kaband rectangular-ring structure.