Overview of Laserwire Beam Profile and Emittance Measurements for High Power Proton Accelerators

Laserwires were originally developed to measure micron-sized electron beams via Compton scattering, where traditional wire scanners are at the limit of their resolution. Laserwires have since been applied to larger beam-size, high power H− ion beams, where the non-invasive method can probe beam densities that would damage traditional diagnostics. While photo-detachment of H− ions is now routine to measure beam profiles, extending the technique to transverse and longitudinal emittance measurements is a key aim of the laserwire emittance scanner under construction at the Front End Test Stand (FETS) at the RAL. A pulsed, 30kHz, 8kW peak power laser is fibre-coupled to motorized collimating optics, which controls the position and thickness of the laserwire delivered to the H− interaction chamber. The laserwire slices out a beamlet of neutralized particles, which propagate to a downstream scintillator and camera. The emittance is reconstructed from 2D images as the laserwire position is scanned. Results from the delivery optics, scintillator tests and particle tracking simulations of the full system are reviewed. Plans to deploy the FETS laser system at the Linac4 at CERN are outlined.