A non-invasive, compact laserwire system has been developed to measure thetransverse emittance of an H- beam and has been demonstrated at the new LINAC4injector for the LHC at CERN. Light from a low power, pulsed laser source isconveyed via fibre to collide with the H- beam, a fraction of which isneutralized and then intercepted by a downstream diamond detector. Scanning thefocused laser across the H- beam and measuring the distribution of thephoto-neutralized particles enables the transverse emittance to bereconstructed. The vertical phase-space distribution of a 3 MeV beam duringLINAC4 commissioning has been measured by the laserwire and verified with aconventional slit and grid method.
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