Design and Performance of Coronagraph for Beam Halo Measurements in the LHC

The CERN Large Hadron Collider is equipped with two Beam Synchrotron Radiation systems (BSR), one per beam. These systems are used to monitor the transverse distribution of the beam, its longitudinal distribution and the abort gap population. During the 2015-2016 winter shut-down period, one of the two BSR systems was equipped with a prototype beam halo monitor, based on the Lyot coronagraph, classically used in astrophysics telescopes to observe the sun’s corona. The system design, as well as part of the optics, was taken from the coronagraph used in the KEK Photon Factory, adapted in order to satisfy the LHC BSR source constraints. This project is in the framework of the HL-LHC project, for which there is the requirement to monitor the beam halo at the level of $10^-6$ of the core intensity. This first prototype has been designed as a demonstrator system aimed at resolving a halo-core contrast in the $10^-3$ to $10^-4$ range. After illustrating the design of the LHC coronagraph and its technical implementation, this contribution presents the result of the first tests with beam and the planned system upgrades for 2017.