The x-ray free-electron laser has established itself as the brightest available source of x-rays, extending the coherence and brilliance properties of conventional atomic lasers down to the sub-Angstrom level. The high-brightness electron beams that are used to drive the free-electron laser process, undergo a number of collective instabilities that can generate complex phase-space structures and induce the emission of coherent radiation, an effect that is generally termed microbunching instability.
The main subject of this dissertation is the collective evolution of beam mi- crobunching under the effect of longitudinal space-charge forces. We develop a three-dimensional kinetic theory of space-charge effects leading to collective suppression and amplification of beam microbunching. This model gives, for the first time, a fully self-consistent description of the space-charge instability, with the inclusion of three-dimensional and thermal effects. After establishing a self-consistent theoretical foundation for space-charge effects, we present two experiments related to the space-charge instability. The generation of broad- band coherent undulator radiation with a longitudinal space-charge amplifier is demonstrated experimentally for the first time. This experiment extends the capabilities of free-electron laser facilities by allowing the generation of coherent broadband radiation pulses, thus accessing regimes of operation currently unavailable for fourth generation light sources. Finally a coherent diffraction imaging technique for the reconstruction of beam microbunching is designed and experimentally tested. This technique is based on the application of an oversampling phase-retrieval method to the far-field coherent transition radiation emitted by a microbunched electron beam and has applications in the diagnostic of compressed electron beams and free-electron lasers.
While the microbunching instability is generally regarded as a detrimental effect, this work shows that the coherent effects associated with the induced microbunching can be optimized and used to our advantage for the development of new coherent radiation sources and advanced beam diagnostics.