Royal Holloway, University of London, Surrey, United Kingdom
CERN, Geneva, Switzerland
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.
Royal Holloway, University of London, Surrey, United Kingdom
CERN, Geneva, Switzerland
A novel electro-optic beam position monitor capable of rapidly (<50ps) monitoring transverse intra-bunch perturbations is under development for the HL-LHC project. The EO-BPM relies on the fast optical response of two pairs of electro-optic crystals, whose birefringence is modified by the passing electric field of a 1ns proton bunch. Analytic models of the electric field are compared with electromagnetic simulations. A preliminary opto-mechanical design of the EO-BPM was manufactured and installed at the CERN SPS in 2016. The prototype is equipped with two pairs of 5mm cubic LiNbO3 crystals, mounted in the horizontal and vertical planes. A polarized CW 780nm laser in the counting room transmits light via 160m of PM fibre to the SPS, where delivery optics directs light through a pair of crystals in the accelerator vacuum. The input polarization state to the crystal can be remotely controlled. The modulated light after the crystal is analyzed, fibre-coupled and recorded by a fast photodetector in the counting room. Following the recent installation, we present the detailed setup and report the latest status on commissioning the device in-situ at the CERN SPS.