Optimisation of Storage Rings and RF Accelerators via Advanced Optical-Fibre Based Detectors

In particle accelerators, diverse causes may lead to the unwanted deviation of the beam particles from the nominal beam orbit and their loss in the machine. To protect from danger- ous beam losses and provide valuable information on the accelerator operation, Beam Loss Monitors (BLMs) are being employed. Conventional BLMs are localised detectors, with a wide range of characteristics, depending on which, the appropriate monitor for each application is selected. The optical fibre BLM is an alternative type of detector, used and examined in various facilities for its ability to cover and effectively protect long parts (of the order of 100 m) of an accelerator. The Compact Linear Collider (CLIC) is a proposal for a future $e^− − e^+$ collider, based on the simultaneous operation of two parallel beam lines, aiming to reach a 3 TeV center of mass energy at approximately 48 km of machine length. In the present work an optical fibre based detector was developed, consisting of a quartz optical fibre coupled with an Silicon Photon Multiplier (SiPM), to be studied as a beam loss monitor for RF accelerators, such as CLIC, and storage rings. A dedicated housing was fabri- cated for the photosensor and the related electronics, which optimised the system in terms of sensitivity, low noise and robustness. Instead of the most commonly used, for such detectors, Photo Multiplier Tubes (PMTs), this study made use of the SiPMs taking advantage of their insensitivity to magnetic fields and their efficiency in terms of cost and required power. The developed system was characterised for its capabilities as a BLM in the CLIC Test Facility 3 (CTF3) and in the Australian Synchrotron Light Source (ASLS). A remarkable intrinsic time resolution of 260 ps was measured, and a discrimination in beam losses with a 25 cm spacing between them was demonstrated for single bunch beams. For 350 ns long electron beams, in order to distinguish simultaneous beam losses on different locations, a distance of 3 m between them was required. The ability of the detector to monitor steady state losses was validated, and a method to assess all beam loss locations utilising only one initially identified, was demonstrated. The limitations introduced to BLMs from the beam loss crosstalk effect in parallel beam lines was investigated, while the sensitivity limitations induced by the RF cavity electron field emission background were estimated as not significant. The optical fibre based detector, combined with appropriate localised detectors at high risk locations, was proven a promising system for the monitoring of beam losses, for both linear accelerators and storage rings. Finally, a modified, highly sensitive version of the detector was introduced as an advanced RF cavity diagnostics tool. This was proven able of monitoring RF breakdowns and field emitted electrons along with estimating the Fowler-Nordheim field enhancement factor. Additionally, both measurements and simulations confirmed the presence of high energy electrons in the radiation environment of accelerating structures due to electron field emission.