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

The Shintake Monitor is an essential beam tuning device installed at the interaction point (IP) of ATF2 [1], the final focus test beam line of the Accelerator Test Facility (ATF) to measure its nanometer 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. By switching between several laser crossing angle modes, it is designed to accommodate a wide range of σ_y^* from 20 nm to a few micrometers with better than 10% accuracy. Owing to this ingenious technique, Shintake Monitor¹ [2,3] is the only existing device capable of measuring σ_y^*<100 nm, and is crucial for verifying ATF2's Goal 1 of focusing σ_y ^* down to the design value of 37 nm. Shintake Monitor has demonstrated stable σ_y^* measurement with 5-10% stability. Major improvements in hardware and measurement schemes contributed to the suppression of error sources. This paper describes the design concepts and beam time performance of Shintake Monitor, as well as an extensive study of systematic errors with the aim of precisely extracting σ_y ^* from the measured modulation.