Cherenkov Fibers for Beam Loss Monitoring at the CLIC Two Beam Module

Abstract

The Compact Linear Collider (CLIC) study is a feasibility study aiming at a nominal center of mass energy of 3TeV and is based on normal conducting travelling-wave accelerating structures, operating at very high field gradients of 100 MV/m. Such high fields require high peak power and hence a novel power source, the CLIC two beam system, has been developed, in which a high intensity, low energy drive beam (DB) supplies energy to a high energy, low intensity main beam (MB). At the Two Beam Modules (TBM), which compose the 2x21km long CLIC main linac, a protection against beam losses resulting from badly controlled beams is necessary and particularly challenging, since the beam power of both main beam (14 MW) and drive beam (70 MW) is impressive. To avoid operational downtimes and severe damages to machine components, a general Machine Protection System (MPS) scheme has been developed. The Beam Loss Monitoring (BLM) system is a key element of the CLIC machine protection system. Its main role will be to detect potentially dangerous beam instabilities and prevent subsequent injection into the main beam linac and drive beam decelerators. In terms of cost and the required number of channels, Cherenkov fibers are a promising option for the BLM system along the CLIC main linac. Cherenkov fibers are based on the idea of using optical fibers as a Cherenkov light radiator. After its production, a fraction of the Cherenkov light is trapped and guided to a photodetector at one end of the fiber by means of total reflection. This thesis presents a detailed study on the features of the technology of Cherenkov fibers and relates them to the requirements of the BLMs for the CLIC TBMs. The work of pre-existing studies [28, 46, 48] was extended to an analytical model that provides a tool to calculate the number of photons reaching the photodetector and their propagation velocities along the fiber as a function of various properties of the fiber and the crossing particle. Measurements to verify the model, using a multimode optical fiber irradiated at a high momentum proton test beam have been carried out. Light yields as a function of fiber diameter, and the angle between the beam and the fiber axes are presented. Using the Monte Carlo code FLUKA [3, 4], simulations on decisive beam loss scenarios for the CLIC TBM were performed. The simulated particle showers were used as an input for the developed model in order to study the sensitivity, the required dynamic range, the achievable longitudinal resolution of the loss location, and the ability to distinguish losses originating from either beam of the TBMs. Finally the limitations on the attainable longitudinal resolution of the loss location with respect to multi-bunch trains as used in CLIC are discussed