Microbunching and Coherent Synchrotron Radiation in Linear Free Electron Lasers

The optimal performance of short-wavelength free-electron lasers (FELs), driven by high-energy bunches of electrons, is limited by collective interactions that occur due to the self-fields of particles within the bunch. An understanding of these collective effects is therefore crucial for current and future machines. In particular, it is important when designing and operating such a machine that these effects are understood, and mitigated as much as possible. In order to achieve such an understanding, a correspondence between the theory of the impact of these collective effects, their calculation using computer-based simulation codes, and experimental measurements of the effects, is essential. This thesis presents a study of two such collective effects: coherent synchrotron radiation (CSR) and the microbunching instability. An extension to the 1D theory of CSR is derived, which correctly takes account of effects arising due to the electron bunch entering and exiting a bending magnet. Theoretical predictions of these CSR transient effects are then compared with results from simulation codes. The CSR-induced emittance growth is then studied experimentally in the FERMI FEL across a range of electron bunch parameters, showing good agreement between theory, simulation and experiment in most cases, and some divergence during more extreme bunch compression scenarios. In addition, the microbunching instability in the FERMI FEL has been studied extensively. A new method of characterising the instability using 2D Fourier analysis has been developed, which uncovers previously unseen parameters, and demonstrates the necessity of performing a thorough analysis in order to understand fully this effect. The microbunching instability has also been induced, by imposing periodic modulations on electron bunches across a number of accelerator lattice configurations. Comparisons between theory, simulation and experiment are also shown in this case, demonstrating an improved understanding of the development of these collective effects

Start-to-end simulations of the CLARA FEL test facility

CLARA is a new FEL test facility being developed at STFC Daresbury Laboratory in the UK, aiming to deliver advanced FEL capabilities including few-cycle pulse generation and Fourier transform limited output. Commissioning is underway on the front-end (photo-injector and first linac) while the later stages are being procured and assembled. Start-to-end (S2E) simulations of the full facility are presented, including optimisation of the accelerator setup to deliver the required properties of one of the electron beam modes specified for FEL operation. FEL simulations are performed using the Genesis 1.3 and Puffin codes and the results are compared

Nanoscale transient polarization gratings

We present the generation of transient polarization gratings at the nanoscale, achieved using a tailored accelerator configuration of the FERMI free electron laser. We demonstrate the capabilities of such a transient polarization grating by comparing its induced dynamics with the ones triggered by a more conventional intensity grating on a thin film ferrimagnetic alloy. While the signal of the intensity grating is dominated by the thermoelastic response of the system, such a contribution is suppressed in the case of the polarization grating. This exposes helicity-dependent magnetization dynamics that have so-far remained hidden under the large thermally driven response. We anticipate nanoscale transient polarization gratings to become useful for the study of any physical, chemical and biological systems possessing chiral symmetry

Femtosecond polarization shaping of free-electron laser pulses

We demonstrate the generation of extreme-ultraviolet (XUV) free-electron laser (FEL) pulses with time-dependent polarization. To achieve polarization modulation on a femtosecond timescale, we combine two mutually delayed counterrotating circularly polarized subpulses from two cross-polarized undulators. The polarization profile of the pulses is probed by angle-resolved photoemission and above-threshold ionization of helium; the results agree with solutions of the time-dependent Schrödinger equation. The stability limit of the scheme is mainly set by electron-beam energy fluctuations, however, at a level that will not compromise experiments in the XUV. Our results demonstrate the potential to improve the resolution and element selectivity of methods based on polarization shaping and may lead to the development of new coherent control schemes for probing and manipulating core electrons in matter

Optical Beam Loss Monitors Based on Fibres for the CLARA Phase 1 Beam-Line

Fibre based Optical Beam Loss Monitors (oBLMs) are on-line devices used in-situ to measure losses along a beam-line. The technology is based on the detection of Cherenkov radiation, produced inside quartz fibres placed alongside the beampipe, from the interaction of secondary showers generated from losses hitting the vacuum pipe. This contribution presents ongoing developments of an oBLM system installed along the Compact Linear Accelerator for Research and Applications (CLARA). The oBLM system consists of 4 channels which allows for sub-metre loss resolution with two dimensional coverage along the entirety of the beam line, as opposed to conventional localised BLM systems. The system was first commissioned to measure dark current from the injector. The ability of the system to locate longitudinal positions of known beam loss locations has also been measured and has shown excellent agreement. We present measurements acquired from the detector during regular operation and during dedicated beam tests. We also discuss the incorporation of the monitor into the accelerator diagnostics system and its use in assisting accelerator characterisation and performance

Optical Beam Loss Monitor for RF Cavity Characterisation

Beam Loss Monitors (BLMs) based on optical fibres have been under development for many years as an alternative solution to commonly used methods, such as ionisation chambers. Optical BLMs (oBLMs) maintain standard BLM functionality but can also be used for machine and personal protection. They can be implemented over the entire beam line providing excellent position and time resolution, while being insensitive to radiation induced damage. This contribution describes how oBLMs can also assist in the characterisation of RF cavities during commissioning and operation. It first presents the design principle of highly compact monitors and the underpinning theory for particle loss detection, before discussing data obtained in experimental tests at the electron accelerator CLARA. It then shows how a 4-channel oBLM can be applied for efficient cavity monitoring. Finally, the results are put into a broader context underlying the application potential in accelerators and light sources