Synchrotron radiation interferometry for beam size measurement at low current and in large dynamic range

The Synchrotron Radiation Interferometry (SRI) has become a widely used technique to measure the small transverse size of the electron beam in the storage ring. In a typical SRI system for the routine storage ring operation, synchrotron radiation from a dipole magnet is used to illuminate a double slit with a small slit opening and a relatively large slit separation to form a large number of interference fringes on the observation plane. However, a different type of SRI is needed for intrabeam scattering (IBS) research to measure the beam size at ultralow currents and in a wide dynamic range. Such a system requires a double slit with a large slit opening to increase the light input while having the capability of accurately measuring the beam size with a range of visibility. By examining the impact of the nonuniform wave amplitude of synchrotron radiation and that of the varying visibility (due to a changing beam size) on the beam size measurement, we propose a new physics model for this type of SRI. This new model is validated using simulation, showing significantly improved results when compared with the conventional model. Based on this new model, we have developed and tested an SRI system dedicated to the IBS study on the Duke storage ring. This system has been used successfully to measure electron beams with about 10 mu A of current and with a higher current but a variable size. This new physics model can also improve the measurement accuracy and consistency of the conventional SRIs, especially at low visibility.