A low-power laserwire profile monitor for H$^{-}$ beams: Design and experimental results

An instrument to non-destructively measure the transverse profile of an H$^{-}$ beam has been developed and tested at CERN’s new LINAC4. The vertical profile of the H$^{-}$ beam has been measured, at beam commissioning energies of 50, 80 and 107 MeV. The instrument consists of a laser-source which is coupled to an optical fibre, delivering laser pulses with hundreds Watts of peak power to the accelerator tunnel, where the laser beam is focused horizontally onto the H- beam. Due to photo-detachment, electrons are liberated from the ions and can be deflected by a weak dipole field onto a diamond detector. The beam profile is obtained by scanning the laser beam vertically across the H$^{-}$ beam and recording for each position the number of arriving electrons. The design, implementation and characterisation of the novel instrument shall be described and the measured beam profiles compared to profiles obtained with SEM-grids and wirescanners.