  A non-invasive, compact laserwire system has been developed to measure the
transverse emittance of an H- beam and has been demonstrated at the new LINAC4
injector for the LHC at CERN. Light from a low power, pulsed laser source is
conveyed via fibre to collide with the H- beam, a fraction of which is
neutralized and then intercepted by a downstream diamond detector. Scanning the
focused laser across the H- beam and measuring the distribution of the
photo-neutralized particles enables the transverse emittance to be
reconstructed. The vertical phase-space distribution of a 3 MeV beam during
LINAC4 commissioning has been measured by the laserwire and verified with a
conventional slit and grid method.
