The X-ray beam size monitor, (xBSM) is an instrument for measuring the sizes of the electron and positron beams using synchrotron radiation. The device can measure vertical beam sizes of 10–100 μm on a turn-by-turn, bunch-by-bunch basis at beam energies of ~2GeV. The xBSM images X-rays that emerge from a hard-bend magnet through a single- or multiple-slit (coded aperture) optical element onto an array of 32 InGaAs photodiodes. Beamlines and detectors are entirely in-vacuum, enabling single-shot beam size measurement down to below 0.1 mA (2.5 billion particles) per bunch and inter-bunch spacing of as little as 4 ns. A systematic precision of ~1 micron is achieved for a beam size of ~12 microns. Achieving this precision requires comprehensive alignment and calibration of the detector, optical elements, and X-ray beam. Data from the xBSM have been used to extract characteristics of beam oscillations on long and short timescales, and to make detailed studies of low-emittance tuning, intra-beam scattering, electron cloud effects, and multi-bunch instabilities.
X-ray beam size monitor optics (left) and photo-diode pixel detector (right). Source to optics L~4m. Optics to detector L'~10m.