We have developed an ultra high position resolution cavity BPM for the final focus system of ATF2. ATF2 is a test beam line for ILC final focus system starting its operation in 2008, and is going to demonstrate 37 nm beam size and nano-meter beam orbit stability at virtual IP. For these purposes, a few nano-meter position resolution is required for this special cavity BPM, which is called the IP-BPM.

ATF2 is an extension of ATF, the Accelerator Test Facility, located at KEK, Tsukuba, Japan. ATF has achieved ultra low beam emittance required for ILC, and development of various beam diagnosis devices is taking place at existing ATF beam line.

We have fabricated 2 blocks of IP-BPM, consisting of two cavities in each block. IP-BPM can measure beam position in vertical and horizontal independently with a single cavity. The IP-BPM blocks are installed at ATF extraction line, and its position resolution was measured. As a result, position resolution of IP-BPM was proved to be 8.72 ± 0.28 ± 0.35 nm, in case of single bunch operation. This is the best record ever achieved in the world today.

Also under multiple operation mode, IP-BPM could measure beam position of each bunch with sub-micron resolution, and is expected to achieve nano-meter resolution in ideal condition.