Recent Trends in Beam Size Measurements using the Spatial Coherence of Visible Synchrotron Radiation

The optical method of measuring the transverse beam profile and size using visible synchrotron radiation (SR) began with simple imaging systems. The resolution was limited by both diffraction and wavefront errors making it difficult to resolve beam sizes less than 50 m. Instead of imaging, an interferometric method for measuring the beam profile and size using spatial coherence was introduced. The method is based on Van Cittert-Zernike’s theorem, and can resolve 4-5 m beam sizes with an error of only 0.5 m. In this presentation, the principle of the measurement, the SR interferometer design, and some resent measurement results are reviewed. The incoherent field depth effect for the horizontal beam size measurement is also described with some recent results. Design study calculations for the SR interferometer at the LHC will be presented.