Design of an Electro-Optic Beam Position Monitor for Unbunched Beams based on Frequency Domain DC Field Measurements

  • Antonio Cristiano

Student thesis: Doctoral Thesis


Continuous and accurate monitoring of the transverse position of a charged particle beam in an accelerator plays a crucial role in ensuring efficient operation of the accelerator and the success of the conducted physics experiments. This is typically achieved with Beam Position Monitors (BPMs) which are, however, insensitive to DC, or unbunched, beams. In such cases, other beam diagnostic tools are used, such as intercepting instruments which alter the properties of the passing beam. This feature poses a challenge for some experiments, e.g.Fixed Target Experiments at CERN, which rely on beams without any temporal structure. This thesis addresses the lack of DC-sensitive BPMs by exploring the use of Electro-Optic (EO) crystals as an alternative to traditional BPMs electrodes. The proposed technique requires four optical chains arranged symmetrically around the vacuum chamber. Each chains acts as an electrostatic field sensor composed of two EO crystals providing two different functionalities. One crystal, placed inside the vacuum chamber, encodes the intensity of the electrostatic field carried by the particle beam onto the polarisation state of the laser beam crossing the optical chain. The other EO crystal is installed outside the vacuum chamber and is modulated with a sinusoidal electric field. This allows the output signal to be analysed in the frequency domain, as well as setting a DC bias to control the system’s working point and to compensate for environmental changes of the crystal’s optical properties. A detailed study of low-frequency effects on the EO materials was carried out to evaluate the measurement error due to the collection of the space charge and the variation of the refractive indices due to temperature fluctuations. Laboratory measurements of the developed electrostatic field sensor, representing one BPM electrode, proved the feasibility of the proposed technique, comparing it to the analytical predictions obtained with a mathematical model of the setup. These measurements provided valuable insights into the performance of the system and further development and optimisation opportunities. The developed technology is not limited to particle accelerators, but can also find use in any application requiring DC field sensing in harsh environments without interfering with the measured field.
Date of Award18 Mar 2024
Original languageEnglish
SupervisorRichard Hill (Main Supervisor) & Minsi Chen (Co-Supervisor)

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