Current Status of Nanometer Beam Size Monitor for ATF2

The Accelerator Test Facility 2 (ATF2) is an extension of the ATF beamline extraction featuring an ILC-type final focus system. Among the project's major purposes are establishment of hardware and beam handling technologies aimed at transverse focusing of ATF's electron beam to below 40 nm in the vertical. A laser-interferometer type high resolution beam size monitor named the "Shintake Monitor" is installed at ATF2's virtual interaction point, and plays a crucial role in achieving this aim. A laser interference fringe is formed by crossing two coherent laser rays. This functions as an interaction target for probing the electron beam. Beam size sensitivity of the monitor depends on the pitch of the interference fringe, and maximizes at about one fifth of the pitch. The Shintake Monitor at ATF2 is designed to be capable of measuring beam sizes ranging from 6 μm down to 20 nm in the vertical. A vertical beam size of approximately 300 nm has been measured during the run of May 2010. For this running period, ATF2 operated under a special optics configuration with ten times the nominal IP beta function. For the most recent run, due to switching beam optics back to nominal, BG levels rose about 10 times from May. Shintake Monitor had been proven in May to fulfill expectations provided BG is low. However with high BG, its accuracy decreased, which makes low S/N a major concern. In this paper, we describe the design and current status of the Shintake Monitor.