Development of a Prototype Electro-Optic Beam Position Monitor at the CERN SPS

A novel electro-optic beam position monitor capable of rapidly (<50ps) monitoring transverse intra-bunch perturbations is under development for the HL-LHC project. The EO-BPM relies on the fast optical response of two pairs of electro-optic crystals, whose birefringence is modified by the passing electric field of a 1ns proton bunch. Analytic models of the electric field are compared with electromagnetic simulations. A preliminary opto-mechanical design of the EO-BPM was manufactured and installed at the CERN SPS in 2016. The prototype is equipped with two pairs of 5mm cubic LiNbO3 crystals, mounted in the horizontal and vertical planes. A polarized CW 780nm laser in the counting room transmits light via 160m of PM fibre to the SPS, where delivery optics directs light through a pair of crystals in the accelerator vacuum. The input polarization state to the crystal can be remotely controlled. The modulated light after the crystal is analyzed, fibre-coupled and recorded by a fast photodetector in the counting room. Following the recent installation, we present the detailed setup and report the latest status on commissioning the device in-situ at the CERN SPS.