Development of a Laser-based Emittance Monitor for Negative Hydrogen Beams

High energy particle accelerators are designed to collide charged particle beams and thus study the collision products. Maximising the collision rate, to generate sufficient statistics for precise measurements of rare processes, is one of the key parameters for optimising the overall collider performance. The CERN Large Hadron Collider (LHC) Injectors Upgrade (LIU) includes the construction of LINAC4, a completely new machine working as a first linear acceleration stage for the LHC beam. By accelerating a negative hydrogen beam (H-) instead of protons, it aims to double the beam brightness via a more efficient transfer to the first circular accelerator and subsequently boost the LHC collision rate. To achieve this, a precise knowledge of the transverse beam characteristics in terms of beam emittance is essential. This thesis work covers the development of a laser-based monitor meant to measure non-destructively the LINAC4 beam transverse profile and emittance. This included the implementation of different prototypes that were successfully tested at various beam energies during the LINAC4 commissioning and the design of the final system to be installed at the LINAC4 top energy. The laser emittance meter is based on the photo-detachment effect which describes the liberation of an electron from a negative hydrogen ion. By scanning a focused laser-beam through the H- beam, the profile can be reconstructed by counting the amount of the detached electrons. The transverse emittance can be obtained by separating the H0 from the H- beam by a dipole magnet and recording the H0 profiles downstream the dipole for each laser position. The thesis work introduces the basic principles of transverse beam dynamics and the techniques typically used to determine the transverse beam characteristics. Hereafter a bibliographic research is presented about the state of the art in the field of beam diagnostics based on photo-detachment. Then, an extensive simulation campaign of the photo-detachment process led to the conclusion that the laser-system can be based on a low-power laser-source and a fibre-optic transfer of the laser beam to the interaction point with the H- beam. To distinguish the H0 created by the laser interaction from back-ground radiation, a fast, sensitive and radiation-hard diamond strip-detector was implemented. First tests of a prototype were conducted at a 3 MeV and 12 MeV H- beam and the results were compared with conventional techniques, which resulted to agree within +- 3% in terms of emittance. In the following LINAC4 beam commissioning periods at 50, 80 and 107 MeV a second prototype of the instrument based on monitoring the detached electrons has been validated. The setup, consisting of a laser system with 73 m long fibre-optic transfer line, electron deflector magnet and single-crystal diamond detector, has been fully characterised and the obtained profiles were compared with conventional techniques, where an agreement within +- 2% has been found. All these prototype tests were essential for the design of permanent installations at the LINAC4 top energy of 160 MeV in the transfer-line towards the injection into the successive machine (PS-Booster). The design combines electron and H0 detection and provides a simultaneous emittance and profile measurement in the horizontal and vertical planes, with an expected resolution of < 75 μm and < 100 μrad. The instruments shall be routinely operated for automated on-line monitoring of the transverse beam parameters to optimise the PS-Booster injection and subsequently avoid losses and the consequent activation of the environment. Due to the non-destructive measurement method, the instrument can gather an extensive amount of beam characteristics data without leading to downtimes of the machine.