Experimental Results of a Compact Laserwire System for Non-Invasive H⁻ Beam Profile Measurements at CERN's Linac4

A non-invasive laserwire system is being developed for quasi-continuous monitoring of the transverse profile and emittance of the final 160 MeV beam at CERN's LINAC4. As part of these developments, a compact laser-based profile monitor was recently tested during LINAC4 commissioning at beam energies of 50 MeV, 80 MeV and 10⁷ MeV. A laser with a tunable pulse width (1-300 ns) and ~200 W peak power in a surface hutch delivers light via a 75 m LMA transport fibre to the accelerator. Automated scanning optics deliver a free space <150 micron width laserwire to the interaction chamber, where a transverse slice of the hydrogen ion beam is neutralised via photo-detachment. The liberated electrons are deflected by a low field dipole and captured by a sCVD diamond detector, that can be scanned in synchronisation with the laserwire position. The laserwire profile of the LINAC4 beam has been measured at all commissioning energies and is found in very good agreement with interpolated profiles from conventional SEM-grid and wire scanner measurements, positioned up and downstream of the laserwire setup. Improvements based on these prototype tests for the design of the final system are presented.