125 MHz cavity for NAR

The author describes the design of an RF cavity installed in Nippon Telegraph and Telephone´s Normal-conducting Accelerating Ring (NAR), considering both the beam loading effect just after high current injection and higher-order-mode (HOM) transverse impedances. It was found using the 3-D program MAFIA that transverse HOM impedances, except for the TM₁₁₀ mode, were of a negligible order of less than 10^-9 MΩ in the case of an acceleration gap width of 2 cm. In the NAR, multiturn single cycle injection was applied because of the extremely long damping time of the 13-MeV injection beam. To estimate the induced RF voltage by the injected beam and the energy spread by bunch rotation, the behavior of injected electrons on the energy RF-phase space was simulated. The reentrant-type 125-MHz cavity with a gap of 2 cm and a detuning frequency of 1 MHz was fabricated