Low emittance operation of the SPring-8 storage ring by damping partition control

The SPring-8 storage ring is an 8-GeV light source and its emittance at normal operation is 6 nm with double bend achromat optics. To reduce the emittance further, we tested the control of damping partition number by changing the RF acceleration frequency to enhance the radiation damping of horizontal betatron motion and reduced the emittance to 2 nm. The emittance was estimated by several methods and the energy damping time was measured by the spectrum of the synchrotron oscillation. The measured values show good agreement with the result of the code MAD8.