A: The Pohang Light Source-II (PLS-II) is currently providing a top-up mode user-service operation with maximum available beam current of 400 mA and a beam emittance of below 10 nmrad. The dimension of the beam bunch shortened to accomplish a low beam emittance of below 10 nm-rad from a high beam current of 400 mA increases the bunch charge density. As a result, the electron beam lifetime is significantly degraded and a high gradient of power is lost in the vacuum components of the storage ring. A study on how to reduce the bunch charge density without degrading beam emittance found that reducing the RF accelerating voltage (V acc ) can lower the bunch charge density by lengthening the bunch in the longitudinal direction. In addition, the V acc required for stable operation with beam current of 400 mA can be reduced by lowering the external cavity quality factors (Q ext values) of the superconducting cavities (SCs). To control the Q ext values of SCs gradually without accessing the accelerator tunnel, a remote control motorized three-probetuner was installed in the transmission line of each SC. The optimum installation position of the three-probe-tuner was determined by using a finite-difference time-domain (FDTD) simulation and by experimenting on various installation positions of the three-probe-tuner. The Q ext values of all the SCs were lowered to 1.40 × 10 5 , and then, the V acc required to store the beam current of 400 mA was decreased from 4.8 MV to 4.2 MV, which corresponds to 10% lengthening of the beam bunches. The stable operation with the reduced V acc was confirmed during a 400 mA ten-day top-up mode user-service. Currently, the RF system of the PLS-II storage ring delivers the user-service operation with lowered Q ext values to reduce the power loss at the vacuum components as well as the cryogenic heat load of SCs, and no significant problems have been found. This method of reducing the V acc may also be applied in other synchrotron facilities. K: Instrumentation for synchrotron radiation accelerators; Instrumentation for particle accelerators and storage rings -high energy (linear accelerators, synchrotrons) 1Corresponding author.