title: Studies towards an enhanced understanding of electron beams and their diagnostics
Creators: name: Devlin, LJ
Publisher: name: University of Liverpool Repository
Year: 2016

description: A large part of current research and innovation capacity depends on high quality electron beams. Electron accelerators are found in many applications from cancer therapy, cargo scanners, high energy particle colliders, synchrotron light sources to free electron lasers. Electron beams also offer exciting opportunities for developments at the cutting edge of science, for example on novel accelerating schemes which promise accelerating gradients several orders of magnitude higher than what can be realised with conventional radiofrequency accelerators. Whilst R&amp;D into the optimisation of electron accelerators has been performed over many decades, further improvements are still required so their potential can be fully exploited. This includes studies into advanced instrumentation to yield more precise information about the beam itself, novel simulation tools that model the physics of emission and interaction processes, as well as improved beam generation and shaping schemes to enhance the achievable beam brightness. In the frame of this PhD work, different areas of electron beams and associated technologies have been studied whilst still pertaining to the same question: How can we better understand the generation, control and use of electron beams? The thesis is split into three main sections: After a general introduction to the subject, studies into electron emission and initial beam shaping are covered in chapter 2, before the results from investigations into novel beam loss detection techniques are presented in chapter 3. In any accelerator based light source, maintaining the beam quality is crucial. The electron source itself is a key element that determines the achievable beam quality. A variety of source materials are currently under test across many institutes to better understand emission characteristics and to identify the best materials and optimum preparation methods. Here, one experimental set-up at the Cockcroft Institute has been considered. The Transverse Energy Spread Spectrometer (TESS) apparatus uses electrostatic elements to measure both the transverse and longitudinal energy properties of electron beams generated from photocathodes. This then provides detailed information about the electron emission process and allows optimisation of beam generation schemes. To enhance the understanding of the experimental data, a dedicated particle tracking code has been developed which uses accurate maps obtained from simulation or experiment to represent all electromagnetic fields of the set-up. This code has then been used to study two different techniques to measure the longitudinal energy spread of electrons emitted from gallium arsenide. The first technique was to use a wire mesh as an energy filter which the electrons must pass through, the second was to generate a potential difference between the cathode source and detector system. Experimental results are presented for both techniques and the analysis is supplemented with simulations. Monte Carlo results are in good agreement with experimental data, however, through this analysis it is found that using the meshes as a wire filter may not be an efficient method of measuring the longitudinal energy distribution curve as a result of the potential distribution surrounding the wire meshes. The second method is sufficient to the task however the resolution of the technique may be affected by focusing effects around the meshes. Measurement and detection of beam loss is important for any accelerator as unwanted losses yield reduced beam transmission and cause higher background noise. It is crucial in high energy accelerators where unwanted loss particles could easily damage parts of the accelerator and the experiments. Currently used techniques, however, are often limited in their dynamic range, spatial and time resolution, radiation hardness and can be expensive. A detector based on optical fibres with photodetector readout has been studied as part of this work as a detector for a future electron-positron collider. It can cover large distances at the expense of many smaller localised detectors. The ultimate performance of this detector is strongly affected by the light sensor at the fibre end. This in turn limits the performance to locate beam loss intensity and position and hence ultimately limits the understanding of beam loses along an accelerator. To this end Silicon Photomultipliers have been studied as an advanced light sensor that shows great promise for beam loss applications. Preliminary models are compared to experimental data and it is shown that the detector performance in this application is limited by the finite number of pixels available to fire along with pixel recovery/dead time. Measurements carried out at the CLIC Test Facility at CERN using an optical fibre systems are presented and used to assess the system’s performance to specific beam structures. Several techniques are used to analyses signals recorded by multiple detectors however it is found that the layout of optical fibres, which could not be changed, limited full understanding, however, these results are the basis of later experiments which fully measure the effect of a fibre optic beam loss monitor to different beam structures and additional changes to the set-up, which may aid further tests, are covered.