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Title: Study of RF breakdown in muon cooling cavities
Author: Zarrebini-Esfahani, Arash
ISNI:       0000 0004 5922 8341
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2016
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Particle accelerators are devices that are capable of forcing charged particles to very high energy levels. Modern particle accelerators are required to produce conditions of extremely high electromagnetic fields in the Radio Frequency (RF) range and produce extremely high accelerating gradients. This has led to various practical issues, one of the most significant being the phenomenon of RF breakdown in the accelerating cavities. When exposed to an intense electromagnetic field, a conducting surface can emit electrons. In the case of an accelerating cavity, these electrons are further accelerated by the RF field. Such emissions are capable of inflicting irreversible damage on the cavity surface and need to be avoided. Among the factors responsible for initiating such emission, the quality of the cavity surface in combination with the operating conditions has been identified as the main one. The mechanisms involved in the initiation of RF breakdown need to be better understood and related to the cavity design criteria, such that they can lead to correctly specified and reproducible designs. The cavity surface quality may be characterised in terms of surface finish and its chemical composition, both of which are strongly affected by the manufacturing processes. Unlike previous studies, this research has focused on the analysis of the effects of fabrication procedures on surface quality. The work involved manufacture and surface characterisation of button shaped samples, which were produced using relevant metal forming and polishing techniques, in preparation for future experimentation in a test system at Fermilab, USA. Although RF breakdown may be initiated locally at the metallic surface, its effects propagate globally across the entire cavity. Surface defects and impurities act as emission sites by inducing local field enhancements. Simulation methods were developed in order to simultaneously study electron emission due to local field enhancement and electron propagation across the cavity, with the benefit of being able to perform tracking in 3D and to verify enhancement factors obtained by theoretical measurements using the surface properties observed on the buttons. This research was conducted in close collaboration with Daresbury Laboratory and Lancaster University in UK as well as the MTA group Fermilab USA.
Supervisor: Ristic, Mihailo ; Long, Kenneth Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral