Calculations and design of a third harmonic cavity for increasing the beam lifetime

Higher harmonic cavities are used widely in light sources to increase the bunch length and decrease the charge density. This approach may be adopted to increase the beam lifetime in HLS-II. The beam dynamics and bunch lengthening with a passive harmonic cavity are presented in this paper. The result shows the charge density is decreased greatly. A third harmonic cavity similar to the KEK-PF RF cavity for HLS-II is designed, in which the higher order modes are absorbed by SiC ducts and three couplers with rodshaped antenna.