The transverse position of the charged particle beam circulating in an accelerator is one of the most basic yet crucial observables. Efficient and safe operation of every accelerator, as well as performance reach of many physic experiments, relies heavily on accurate beam position measurements. Such measurements are typically performed with Beam Position Monitors (BPM) – detectors equipped with electrodes coupling to the electromagnetic field of the beam. As the beam moves transversely, the amplitude of the electromagnetic field measured by the detector changes accordingly. The beam position can then be reconstructed by subtracting measurements of the opposite BPM electrodes placed around the beam.

Standard electrostatic or electromagnetic BPMs, however, are virtually insensitive to beams with no temporal structure, so-called DC beams. Therefore, accelerators using DC beams for scientific, medical and industrial applications, suffer from the lack of beam instrumentation available. At CERN, DC beams are already used on the Super Proton Synchrotron (SPS) for fixed-target experiments and proposed to be used even more intensively in the scope of the Physics Beyond Colliders study aiming at exploiting the full scientific potential of CERN’s vast accelerator infrastructure.

We propose to overcome the limitations of traditional BPMs by replacing classical conducting electrodes with state-of-the-art electro-optical crystals which are sensitive to DC electromagnetic fields. Such crystals, originally developed by the telecommunication industry as high-speed optical links, will encode the beam’s electromagnetic field onto a laser beam. We foresee to advance the state-of-the-art by using long crystals in order to get the best possible sensitivity as well as by using a highly sensitive photodetection technique.

Beyond accelerators, the proposed technology would suit applications in industry, energy, security or medicine: e.g. the measurement of electrical fields using non-metallic probes in high-voltage power station or compact high sensitivity electrical sensor for medical probes.