Beam measurements at the cern sps using interferometric electo-optic pickups

Since 2016 a prototype electro-optic pickup has been installed on the SPS as part of the ongoing development of a high bandwidth electro-optic beam position monitor for the High Luminosity LHC. Following the success of initial beam signal observations with the prototype, improvements of the sensitivity and stability of the pickup have become the main focus of the project. A new concept has been developed which uses an interferometric technique to measure the image field of a passing bunch. One arm of an interferometer passes through an electro-optic lithium niobate crystal, embedded in a pickup, whereas the other arm bypasses. The recombination after the pickup results in an interference pattern that changes as a bunch passes by, due to the electro-optic response of the crystal to the image field. This technique enhances the sensitivity to the field and improves control of the working point. Results from high intensity beams at the SPS are presented. These include a comparison between two different interferometric configurations that were tested on different pickups with similar beam conditions. The stability is assessed by frequency scanning interferometry during beam operation. INTRODUCTION TO CONCEPT The High Luminosity Large Hadron Collider requires high-bandwidth diagnostics to monitor the crabbed rotation of the proton bunches and to detect rapid, high order bunch instabilities [1, 2]. The solution being developed in the HL-LHC-UK collaboration between Royal Holloway and CERN is an Electro-Optic Beam Position Monitor (EOBPM), which in essence is an electrostatic BPM that incorporates high bandwidth lithium niobate crystals placed between electrodes in the core of the pickup. As the relativistic proton bunch passes by an electro-optic pickup, the electric field is concentrated by the electrode to interact with the polarised light traversing the crystal by the Pockels effect. The analogue of the longitudinal bunch profile convoluted with the average transverse offset along the proton bunch is imprinted in the phase modulation of light passing through the crystal. In the SPS prototype results presented here, the phase of the modulation of light in the output fibre is transformed into an intensity modulation by combining with an optical path through a second fibre that bypasses the crystal, enabling the rapid beam signal to be recorded by a remote fast photodetector [3]. ∗ alberto.arteche@rhul.ac.uk This paper reviews various optical configurations that have been tested at the CERN SPS and presents new electromagnetic simulations based on an upgraded pickup design that is planned to be installed at the LHC for tests during Run-III. EO-BPM OPTICAL CONFIGURATIONS