[en] A beam test for the feedback damping of a coherent instability at small-laser equipped storage ring (S-LSR) was carried out. The instability caused beam loss, which limited the stored beam current to be about 600 μA during beam stacking with electron cooling. A vertical feedback damping system (VFS) using an electro-static beam monitor (ESBPM) and a vertical kicker was applied to suppress this beam instability. The VFS realized not only suppressing the vertical coherent instability, but also revealing the characteristics of the feedback damping. (author)