Microbunching Instability Suppression for Improved FEL Performance

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Free Electron Lasers (FELs) are a fourth-generation light source capable of producing radiationover a broad range of wavelengths, while producing femtosecond (10-15 second) pulses. This allowsresearchers across several fields, from materials science to biology, to probe atomic scale structuresand dynamical processes with a high resolution. Development of brighter FELs with shorter pulsedurations allows these researchers to make much needed advancements in medicine, biology andmaterials science.As demand for brighter Free Electron Laser (FEL) X-rays increases, so does the need for highquality electron beams and higher peak currents, however there are various effects which makeachieving these requirements difficult. Microbunching instability (MBI) and Coherent SynchrotronRadiation (CSR) kicks are the effects which lead to increased emittance and decreased FELperformance. This project will investigate microbunching instability and distorted CSR kicksthrough a few aims, which will be summarised below.CSR is a phenomena that takes place in dipoles where relativistic particles, such as electrons,radiate coherently. When this radiation is absorbed by other particles in the bunch, there is aredistribution of energy along the bunch. This leads to longitudinal slices of the bunch being offsetin the transverse coordinates, and therefore increased projected emittance. Currently there are someCSR cancellation techniques used to minimise this effect, but these may be less effective if the CSRkicks are distorted by dipoles in non-dispersive sections. The conditions which lead to distortedCSR kicks will be investigated and the effectiveness of CSR cancellation techniques on distortedkicks will be studied.Microbunching instability arises from small initial energy modulations induced by CSR leading tocurrent density modulations (or microbunching) in the dispersive regions of a linear accelerator(linac). A type of singularity known as caustics can be used to mathematically describemicrobunching dynamics. This project aims to obtain the first experimental measurements ofparticle trajectory caustics in a linac, by designing an experiment to measure the evolution ofcaustic-induced current spikes at MAX-IV, in Lund, Sweden. An investigation into the role ofthreshold R56 on microbunching gain can also be made, as the relationship between the two is notcompletely understood. Better understanding of CSR and microbunching instability, will allow us tosuppress the undesirable effects and preserve beam quality and ultimately allow for brighter FELsources.