Variation of the ATF damping ring circumference

The circumference change by the thermal expansion of the accelerator hall was observed at the KEK-ATF damping ring (DR) since the commissioning. The previous measurement showed 6 mm of the circumference change during a year. After introduced the air cooling of the center area of the ring, the circumference change reduced to 4 mm. Recently, the ATF2 project using the extracted beam from the DR is in progress and the precise beam tuning is needed to make the ultimate small beam size at the virtual focus point. The circumference change is a serious problem for the ATF2 beam tuning, which is caused by the energy change, the orbit change and the beta function change for the extracted beam. This paper describes the measurement results and the compensation scheme. (author)