Abstract
Measurements of the longitudinal phase-space distributions of electron bunches seeded with an external laser were done in order to study the impact of collective effects on seeded microbunches in free-electron lasers. When the collective effects of Coulomb forces in a drift space and coherent synchrotron radiation in a chicane are considered, velocity bunching of a seeded microbunch appears to be a viable alternative to compression with a magnetic chicane under high-gain harmonic generation seeding conditions. Measurements of these effects on seeded electron microbunches were performed with a rf deflecting structure and a dipole magnet which streak out the electron bunch for single-shot images of the longitudinal phase-space distribution. Particle tracking simulations in 3D predicted the compression dynamics of the seeded microbunches with collective effects.
Highlights
Free-electron lasers (FELs) driven by particle accelerators are in demand for experiments studying ultrafast processes in matter
When the temporal coherence of FEL light is determined by the shot-noise of an electron beam, as in self-amplified spontaneous emission (SASE), it is low [11,12,13], but if it is determined by an external seed laser, the FEL light takes on the temporal coherence, spectral and temporal shape, and stability of the external laser
For a seeded FEL using the high-gain harmonic generation (HGHG) scheme, microbunch trains with the periodicity of the seed laser wavelength will radiate at a harmonic of the microbunch repetition rate when they are sent through a FEL radiator tuned to that harmonic; shorter microbunches will have higher harmonic content [14,15]
Summary
Free-electron lasers (FELs) driven by particle accelerators are in demand for experiments studying ultrafast processes in matter. Since the harmonic content of a seeded beam is given by the Fourier transform of the longitudinal current distribution, a change in the microbunch length has a direct impact on the high harmonic content. We present simulations and measurements of the effect of CSR on the longitudinal phasespace distributions of seeded electron microbunches compressed in a chicane and we contrast it with the effect of Coulomb forces on seeded electron microbunches which are primarily compressed through velocity bunching in a drift space with quadrupole focusing optics. The investigation of these microbunch collective effects was done with an rf deflector and dipole spectrometer which streak out a 700 MeV electron bunch for single-shot measurements of the particle distribution in longitudinal phase-space. Quantitative agreement with simulations was observed within the error bars of the measurements and original physical interpretations are used to explain new effects discovered in the measurement method
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