In high beam-current storage rings like KEK B-factory (KEKB) [1], severe coupled-bunch longitudinal instability can be driven by the accelerating mode of significantly detuned RF cavities. The RF feedback with a comb filter, which can reduce the effective cavity impedance at the synchrotron sidebands of the revolution harmonics, is a possible way to damp the instability. This paper describes the results of the experimental RF feedback loop, which includes a N-path filter referred to as a parallel comb filter, a choke mode cavity, and a 1.2 MW klystron. With the feedback, the real part of the cavity impedance was reduced by 16 dB to 24 dB, at the five synchrotron sidebands 100 kHz apart each other.
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