Microbunching and Coherent Synchrotron Radiation in Linear Free Electron Lasers

Alexander Brynes
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
more » ... nding, 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 [...]
doi:10.17638/03127188 fatcat:pmx2pkbzkfdybi6hqycrkklt6u