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Transverse Emittance Blow-Up due to the Operation of Wire Scanners, Analytical Predictions and Measurements

Wire Scanner monitors are used in the CERN accelerators to measure the transverse beam size. In the SPS and the LHC they will serve as calibration devices for other emittance monitors. The PSB, PS and SPS are equipped with scanners which move a 30 µ m wire through the beam, with a speed that can vary between 0.4 to 20m/s. During each scan, the beam suffers an emittance blow up, due to multiple Coulomb scattering of the beam protons on the lattice nuclei of the wire material. The effect depends on the particles energy, the betatron function at the monitor location and on the monitor characteristics (wire material, diameter and speed). The comparison between the predicted emittance increase and a number of experimental measurements is presented. For the small LHC beams the relative emittance blow-up can exceed several 10-2in the SPS.