  SuperKEKB is an asymmetric electron-positron circular collider based on
nano-beam scheme at interaction region and large beam current. Large beam
current makes growth rates of longitudinal coupled-bunch instabilities (LCBI)
large. Especially some lowest modes near accelerating frequency are serious. On
the design parameter for SuperKEKB, $\mu$ = -1, -2, -3 mode of LCBI can be
destabilized. We developed new LCBI damper to suppress newly arisen LCBI modes
($\mu$ = -1, -2, -3) in SuperKEKB. The new damper will be installed in Low
Level RF control system. The new LCBI damper is independent of main LLRF
control components. In the test bench measurement, our new LCBI damper has good
performance and satisfied required specifications. For preparation of using
LCBI damper, we produced the simulation of beam oscillation damped by RF
feedback. The results of this simulation shows that we need more dampers than
them we prepared. We report profile of new damper and results of test bench
measurement and feedback simulation in this paper.
