  The J-PARC Main Ring (MR) has achieved the delivery of the 30 GeV proton beam
with the beam power of 500 kW to the neutrino experiment in May 2018. The
longitudinal coupled-bunch instabilities have been observed in the MR for the
beam power beyond 470 kW. Since more significant coupled-bunch oscillation was
observed in higher beam power, the mitigation of the coupled-bunch
instabilities is necessary for the stable beam delivery with the beam power
beyond 500 kW. A new feedback system was developed to suppress the
coupled-bunch instabilities. The feedback system consists of a wall current
monitor, an FPGA-based feedback processor, RF power amplifiers, and an RF
cavity as a longitudinal kicker. The synchrotron sideband components of the
beam signal picked up by the wall current monitor are detected by the feedback
processor and used for the feedback control. The single-sideband filtering
technique is employed in the feedback processor to control each coupled-bunch
mode separately. To accommodate the change of the synchrotron frequency during
the acceleration, a synchrotron frequency tracking CIC filter is used as a low
pass filter in the single-sideband filter. We present the preliminary beam test
results to suppress the beam oscillation with the developed feedback system.
