Detection of a DC Electric Field Using Electro-Optical Crystals

Standard beam position monitors (BPM) are intrinsically insensitive to beams with no temporal structure, so-called DC beams, which many CERN experiments rely on. We therefore propose a novel detection technique in which the usual BPM electrodes are replaced with electro-optic (EO) crystals. When exposed to an electric field, such crystals change their optical properties. This can be exploited to encode the electric field magnitude onto the polarisation state of a laser beam crossing the crystal. An additional EO crystal, placed outside the vacuum chamber, can be used to control the system’s working point and to introduce a sinusoidal modulation, allowing DC measurements to be performed in the frequency domain. This contribution presents the working principle of this measurement technique, its known limitations, and possible solutions to further increase the system’s performance. Analytical results and simulations for a double-crystal optical chain are benchmarked against the experimental data taken on a laboratory test bench.