Beam Size Measurements Using Interferometry at LHC

During the long LHC shutdown 2013-2014, both the LHC and its injector chain underwent significant upgrades. The most important changes concerned increasing the maximum LHC beam energy from 4TeV to 6.5TeV and reducing the transverse emittance of the beam from the LHC injectors. These upgrades pose challenges to the measurement of the transverse beam size via Synchrotron Radiation (SR) imaging, as the radiation parameters approach the diffraction limit. Optical SR interferometry, widely used in synchrotron light facilities, was considered as an alternative method to measure the 150 'm rms beam size at top energy as it allows measurements below the diffraction limit. A system based on this technique was therefore implemented in the LHC, for the first time on a proton machine. This paper describes the design of the LHC interferometer and its two SR sources (a superconducting undulator at low energy and a bending dipole at high energy), along with the expected performance in terms of beam size measurement as compared to the imaging system. The world's first proton beam interferogram measured at the LHC will be shown and plans to make this an operational monitor will be presented.