The nanometer beam size monitor (Shintake Monitor) at ATF2

摘要

My presentation focuses on the Shintake (Beamsize) Monitor which can measure nanometer electron beam sizes. The Shintake Monitor is installed in the Accelerator Test Facility 2 (ATF2) at KEK, Japan. ATF2 is a realistic scaled down model of the final focus system for the International Linear Collider. The final focusing scheme named the Local Chromaticity Correction will be tested for the first time in the world. The vertical design beam size at the focal point (virtual interaction point) is 37 nm. Shintake monitor has been designed to measure a beamsize down to 20 nm. It employs the interference pattern made by splitting laser beams and crossing them at the focal point of the electron beam. In their intersecting region, the electromagnetic fields of the two laser beams form a standing wave (interference fringe). The probability of the Compton scattering varies according to the phase of the standing wave where the electrons pass through. Then the total energy of photons from the Compton scattering is measured in a multi-layered ganma ray detector located downstream from the interaction point. This scheme was originally proposed by T. Shintake whose team measured a beamsize of approximately 65 nm with 10 percent resolution at FFTB, SLAC, a former test facility for the ILC. We upgraded this monitor to measure the even smaller beam sizes to be available at ATF2. The laser wavelength has been modified from 1064 nm to 532 nm using a second harmonics generator. The laser optics was newly designed and constructed by implementing a laser wire scheme to measure a larger horizontal beam size, and by enabling different crossing angles of split laser beams to measure a wide (diverse) range of vertical beam sizes. The gamma detector for Shintake monitor has also been newly developed. We evaluated the performance of Shintake monitor with a beam of several microns in size and confirmed its consistency with wire scanner measurements. The expected performance of the Shintake monitor and the current status of the electron beam at ATF, achieved a record in beam size history and near future plan for 37 nm beam size measurement will be mentioned.