Wakefield and Impedance Studies with Ultra-Relativistic Beams for CLARA and UK FEL

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The CLARA accelerator at Daresbury is designed to accelerate very short bunches of electrons, with a charge of 250 pC, to an energy of 250 MeV. These bunches, femto second in length, are sensitive to wakefields generated by transitions in various components in the overall accelerator -such as bellows, bends, and other transitions and obstacles. The Fourier transform of this wakefields constitute the beam impedance. This, and the corresponding wakefield excited by ultra-relativistic beams, will be measured with bench-top measurement techniques, and with beam-based measurements at VELA/CLARA in the UK and on CALFIFES at CERN. Sophisticated analytical techniques and a suite of simulations will also be employed to characterise both the wakefield and the beam dynamics. The purpose of this research will be to analyse a multitude of components in the CLARA accelerator, characterise the wakefield both from analytical models, where appropriate, numerical simulation, and bench top measurements. Several novel analytical methods will be employed to gain an understanding of the general behaviour of wakefields associated with these accelerator components. This will have direct application to the CLARA facility, and UK-FEL. This project is a crossover between physics and engineering. The physics aspect involves studying the fundamental wakefield aspects of the CLARA accelerator -and their interaction with the beam through advanced beam dynamics simulations. The engineering component is concerned with assessing the wakefield and impedance associated with each component and suggesting amelioration techniques where appropriate. We anticipate collaboration with CERN colleagues on the CALIFES facility in particular. A similar characterisation has been applied to components in the LHC accelerator -resulting in an impedance budget which allowed the upgrade phase to proceed. This project will necessarily entail a strong coupling between rf and beam dynamics -in which the beam impedance of components will be characterised and the implications on the beam assessed with analytical models and intense beam dynamics simulations.