Beam commissioning and optimisation of the Swiss free electron laser in-vacuum undulators

Abstract

The Paul Scherrer Institute is currently commissioning a new research facility called SwissFEL. SwissFEL is an X-ray Free Electron Laser, which aims to produce X-ray pulses covering the wavelength range from 1 Å to 70 Å. Each pulse is just 1 fs to 60 fs long, allowing extremely fast reactions and processes to be investigated. This study aims to provide a framework for the online commissioning and optimisation of the light source on the SwissFEL hard X-ray beamline. The beamline consists of thirteen cascaded undulator units, each providing a variable magnetic field in which electrons are forced to emit X-ray radiation. To control and maximize the radiation, the individual units must operate as one. This requires stringent alignment tolerances, where the magnetic field is known at an accuracy of 10-4, and the radiation from all units adds up coherently. Due to accuracy limitations posed by the laboratory setup, this accuracy can only be achieved through an online procedure by characterising the undulators’ spontaneous radiation. Prior to the online procedure, a number of simulations have been performed to analyse and predict the effect of misalignment, magnetic field errors and radiation incoherence on the spontaneous radiation spectrum. The procedure to identify and decouple these errors during online commissioning was devised from this analysis, in addition to the models required to fit the data and determine the optimised operating conditions. This has been based on techniques used in X-ray FEL facilities that are already in operation, as well as innovative solutions as a result of the findings in this work, particularly in the coherence of the radiation. In this work, the models were tested with added electron beam energy jitter, which proved their efficacy under online measurement conditions. In this dissertation, the tools to interface with the equipment and perform the measurement procedures were also developed, and verified with beam measurements. The alignment procedure was verified with beam as part of this dissertation, resulting in significant FEL improvements. Magnetic field and radiation coherence have been optimised for one K value, for which minor adjustments were required. The procedure was confirmed to be effective and practical to be used for further measurements required to fully optimise the magnetic characterisation of the undulator and magnetic components on the undulator cell.