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

摘要

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 um 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 urn pitch. Beamlines and detectors are entirely in-vacuum, enabling single-shot beam size measurement down to below 0.1 mA (2.5 x 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 μm is achieved for a beam size of ～ 12 um; this is expected to scale as x l/σ_b and x l/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.