Hacker, K.E. Ackermann, S. Bödewadt, J. Dohlus, M. Ekanayake, N. Khan, S. Laarmann, T. Lazzarino, L.L. Lechner, C. Maltezopoulos, Th. Molo, R. Plath, T. Roßbach, J. Schlarb, H. WEP031 electron simulation laser FEL bunching 2015 2015-12 Measurements of the longitudinal phase-space distribution 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. 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 when the collective effects of Coulomb forces in a drift space and coherent synchrotron radiation in a chicane are considered. Measurements of these effects on seeded electron microbunches were performed with an 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.