Measurement of nanometer electron beam sizes with laser interference using IPBSM

An e- beam size monitor, called the “Shintake Monitor” (or “IPBSM”), is installed at ATF2’s virtual interaction point (“IP”). It plays a crucial role in achieving ATF2’s Goal 1 of focusing the vertical e- beam size (σ_y*) down to a design value of 37 nm, using an ingenious technique of colliding the e- beam against a target of laser interference fringes. σ_y* is derived from the modulation depth of the resulting Compton signal photons measured by a downstream γ detector. IPBSM is the only existing device capable of measuring σy* as small as 20 nm with better than 10% resolution, and can accommodate a wide range of σ_y* up to a few μm by switching between laser crossing angles θ = 174°, 30°, and 2° - 8° according to beam tuning status. The effects of several major hardware upgrades have been confirmed during beam time, such as through suppressed signal jitters, improved resolution and stable measurements of σ_y* down to about 150 nm by Feb 2012. The aims of the extensive 2012 summer reform implemented upon the laser optics are higher reliability and reproducibility in alignment. Our goal for the autumn 2012 run is to stably measure σ_y* < 50 nm. This paper describes the system’s design, role in beam tuning, and various efforts to further improve its performance.