Measurement of nanometer electron beam sizes using laser interference by Shintake Monitor

The Shintake Monitor is an essential beam tuning device installed at the interaction point of ATF2, the final focus test beam line for ILC, to measure its O(10) nm order vertical e^- beam sizes (σ_y). The e-beam collides with a target of laser interference fringes, and σ_y is derived from the modulation depth of the resulting Compton signal photons measured by a downstream photon detector. The laser optics is designed to accommodate a wide range of σ_y from 25 nm to a few μm with better than 10% accuracy. Being the only existing device capable of measuring σ_y < 100 nm, the Shintake Monitor is crucial for verifying ATF2's Goal 1 of focusing σ_y down to the design value of 37 nm. It has demonstrated measurement stability of about 5%. Major improvements in hardware and measurement schemes contributed to suppressing error sources. This paper describes the design concepts and performance of Shintake Monitor, focusing on an extensive study of systematic errors with the aim of precisely extracting σ_y from the measured modulation.