Side effects of bunch-by-bunch feedback system on SuperKEKB collider

The bunch-by-bunch feedback system is now an key function in high-current, multi-bunch storage rings to suppress coupled-bunch instability and/or to reduce the effects of injection vibration. In high-luminosity e+e- colliders such as SuperKEKB, strong beam-beam interactions occur due to collisions, which usually introduce very wide frequency response on the transverse bunch motion far out of the betatron tune. In vertical plane it may cause increase of the beam size which has a large impact on the luminosity. In this presentation, we will present the principle and configuration of our bunch feedback system, the cause of the side effects of the bunch feedback system, and several trials to overcome them.