First Beam Tests at the CERN SPS of an Electro-Optic Beam Position Monitor for the HL-LHC

An Electro-Optic Beam Position Monitor is being developed for the HL-LHC, aimed at the detection of high order proton bunch instabilities and as a diagnostic for crabbed bunch rotation. A prototype EO-BPM was installed in the CERN SPS during 2016 and recent first beam tests of the EO pick-up are presented. The tested system comprises two opposing pick-ups, each equipped with 5mm cubic LiNbO3 crystals in vacuum, illuminated by polarized light from a fibre-coupled CW 780 nm laser. The 1 ns proton bunch induces a temporal modulation in the polarization state of light emerging from each birefringent crystal, by the Pockels effect. The modulation is analyzed, then recorded by a fibre-coupled fast photodetector in the counting room. The very first experimental signals obtained by the EO-pickups of a passing proton bunch are reported as a proof of concept of the idea. Moreover, the expected response of the beam signal is measured with respect to remotely controlled changes in the polarizer and analyzer orientations. The data are compared with analytical and electromagnetic simulations. Following the first detection, we report the latest status of the prototype design and future prospects.