Studies of breakdown and pre-breakdown phenomena in high-gradient accelerating structures

Thesis

Vacuum breakdown is a complex process and an important limiting factor of the performance of normal-conducting high-gradient particle accelerators, and can result in loss of luminosity in particle collider applications, as well as damage to accelerating structures. The work presented here was done in the context of the Compact Linear Collider (CLIC) study, but is also relevant to a variety of applications such as medical linear accelerators or high-electric-field vacuum electronics. The aspects of vacuum breakdown discussed in this thesis together provide the theoretical basis for important technological parameters for the design of normal-conducting high-gradient devices: the influence of radio-frequency design and material properties on the achievable field, and the mechanism of conditioning. The first part of this thesis discusses the development of an improved quantitative limit which determines the maximum accelerating gradient at which a given structure geometry could operate, with the intention of guiding the design of improved accelerating structures. It models the coupling of radio-frequency power to a breakdown, giving a value for surface electric field when loaded by a nascent breakdown. Calculations were performed on various cases that were tested experimentally, showing excellent consistency and the potential to become a very general model of vacuum breakdowns. The second part presents an experimental study of dislocation dynamics in copper surfaces subject to high electric fields, to better understand the mechanism of the nucleation of breakdowns. This is believed to involve stochastic deformation of microscopic features under high-electric-field stress. Field-emitted current from radio-frequency structures, as well as electrodes subject to a static electric field were precisely measured, revealing small fluctuations in the latter case. The dependence of the rate of events on the surface field and the distribution of time intervals between events was found to match prior theoretical predictions. The rate of fluctuations was also found to decrease with the cumulative number of voltage pulses applied, supporting the idea of work hardening due to stress from pulsed electric fields as a mechanism for conditioning.

Authors: Paszkiewicz, J

• DOI: 10.5287/ora-1zgrpxb90
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: fluctuation; compact linear collider; nucleation; high gradient; field emission; linear accelerator; dark current; radiofrequency; RF; CLIC; dislocation; vacuum breakdown
• Subjects: radiofrequency engineering; accelerator physics; breakdown science; materials science