Vertical beam size measurement in the CESR-TA storage ring using x-rays from synchrotron radiation

We describe the construction and operation of an X-ray beam size monitor (xBSM), a device measuring e + and e − beam sizes in the CESR-TA storage ring using synchrotron radiation. The device can measure vertical beam sizes of 10 – 100 μ m on a turn-by-turn, bunch-by-bunch basis at e ± beam energies of ~ 2 GeV . At such beam energies the xBSM images X-rays of ϵ ≈ 1 – 10 keV ( λ ≈ 0.1 – 1 nm ) that emerge from a hard-bend magnet through a single- or multiple-slit (coded aperture) optical element onto an array of 32 InGaAs photodiodes with 50 μ m pitch. Beamlines and detectors are entirely in-vacuum, enabling single-shot beam size measurement down to below 0.1 mA ( 2.5 × 10 9 particles) per bunch and inter-bunch spacing of as little as 4 ns. At E b = 2.1 GeV , systematic precision of ~ 1 μ m is achieved for a beam size of ~ 12 μ m ; this is expected to scale as ∝ 1 / σ b and ∝ 1 / E b . 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.