Design of Coded Aperture Optical Elements for SuperKEKB X-ray Beam Size Monitors

Abstract

We describe the design of coded aperture optical elements for the SuperKEKB x-ray beam size monitors. X-ray beam profile monitor are being installed in each ring of SuperKEKB (LER and HER) to provide high resolution bunch-by-bunch, turn-by-turn measurement capability for low emittance tuning, collision tuning and instability measurements. We use two types of optical elements, single-slit (pinhole) and multi-slit optical elements (coded apertures, CA). CA imaging offers greater open aperture than a single pinhole, for greater photon throughput and better statistical resolution for single-shot measurements. X-rays produced by a hard-bend magnet pass through a pinhole or CA optical element onto a detector. The resolution is obtained by calculating the differences between the images recorded by the detector for various simulated beam sizes, for a given number of photons. The CA elements that we have designed for use at SuperKEKB are estimated to provide 1.25-2.25 microns resolution for 10-25 microns of vertical beam sizes at 1 mA bunches. We present the design principle and optimizing process used to optimize the resolution at various beam sizes for SuperKEKB.