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-- AlbertoArteche - 16 Mar 2015

• Library of talks/papers

Electro-Optical Beam Position Monitors

Introduction

A new initiative to upgrade the CERN Head-Tail monitors for the SPS and LHC has stimulated a collaboration between the CERN Beam Instrumentation group and Royal Holloway, University of London (RHUL). Head-tail (HT) monitors are the main instruments to visualise and study beam instabilities as they occur. The present head-tail monitors are based on strip-line beam position monitors and fast sampling oscilloscopes. However they only offer a bandwidth up to few GHz limited both by the pick-up, the cables and the acquisition system. The novel pick-ups based on electro-optical crystal and laser pulses have already demonstrated fast time response in the picosecond range, making this technique a promising candidate to achieve higher resolutions to improve the head-tail monitors capacity to solve bunch shapes and instabilities.

An ambitious upgrade of the LHC accelerator called the High-Luminosity LHC will enable searches for rare particle decay processes, by increasing the LHC luminosity by a factor of 10. A critical technique in achieving the luminosity goals is the addition of crab cavities on either side of the interaction regions that rotate the bunches so that they collide head on in the experimental detectors, thus improving the spatial overlap and luminosity. Secondary crab cavities downstream of the detector must then take out the rotation. At RHUL, a novel beam diagnostic to measure the bunch rotation is under development, based on electro-optical crystals, which have sufficient time resolution to monitor intra- bunch perturbations. The group is developing of a prototype electro-optic Beam Position Monitor, in collaboration with CERN beam instrumentation group, that will be initially tested to monitor intra-bunch instabilities in the CERN SPS. Success would validate their use as a future diagnostic tool for the HL-LHC that could have direct impact on the physics performance at the machine detector interface.

Reference material on electro-optical BPMs

• Radiation hardness testing
• Lithium Niobate Modulator
• James Doherty report
• Other...

Crystal characterisation test stand

• General description.
• Talk for CERN...

Simulations

• List of key parameters for nominal and pilot LHC bunches.
  • 25ns LHC bunch spacing, 2808 bunches per beam.
  • Protons per bunch:
    • 1e9 Pilot??
    • 1.15e11 Nominal LHC bunch, at 25ns? (check this)
    • 2.2e11 HL-LHC at 25ns
    • 3.5e11 max?
  • 1ns FWHM bunch
  • 10 - 20 ps time resolution
  • 10 micron transverse spatial resolution

• Simple static field model.
• Electromagnetic simulations of moving field in BPM.

Mechanical Design

• CERN BPM body.