A Three cavity system which suppresses the coupled bunch instability associated with the accelerating mode

Abstract

If the beam loading on a large electron or positron storage ring is extremely heavy, the accelerating mode of a normal-conducting cavity gives rise to a coupled-bunch synchrotron instability.This instability can be cured by increasing the stored energy of a cavity system, for example by attaching a storage cavity to the accelerating cavity.However, three problems should be solved in order to put the system into practical use.First, the stored energy in the storage cavity should be made significantly higher than that in the accelerating cavity.Second, the parasitic modes inherent to a coupled-cavity system causes another instability.Third, the accelerating field is unstable against beam loading if one uses a two-cavity system comprising only accelerating and storage cavities.In order to solve these problems simultaneously we propose the use of a three-cavity system with a coupling cavity in between the accelerating and storage cavities.In particular, a coupler installed to the coupling cavity can efficiently damp both the 0 and 1r modes, while having a minimal effect on the accelerating 1r/2 mode.Furthermore, the excitation effect of one of the 0 and 1r modes on coupled-bunch oscillation is approximately cancelled by the damping effect of the other mode.A theoretical analysis using the coupled-resonator model shows that the system is quite promising.