Lechner, C. Ackermann, S. Azima, A. Bödewadt, J. Brenner, G. Dohlus, M. Drescher, M. Ekanayake, N. Golz, T. Hacker, K.E. Khan, S. Laarmann, T. Lazzarino, L.L. Limberg, T. Maltezopoulos, Th. Miltchev, V. Molo, R. Plath, T. Rönsch-Schulenburg, J. Roßbach, J. Schneidmiller, E. Stojanovic, N. Yurkov, M.V. TUA02 electron laser FEL undulator photon 2015 2015-12 Collective effects and instabilities due to longitudinal space charge and coherent synchrotron radiation can degrade the quality of the ultra-relativistic, high-brightness electron bunches driving free-electron lasers (FELs). In this contribution, we demonstrate suppression of FEL lasing induced by a laser-triggered microbunching instability at the free-electron laser FLASH. The interaction between the electron bunches and the 800-nm laser pulses takes place in an undulator upstream of the FEL undulators. A significant decrease of XUV photon pulse energies has been observed in coincidence with the laser-electron overlap in the modulator. We discuss the underlying mechanisms based on longitudinal space charge amplification (LSCA) [E.A. Schneidmiller and M.V. Yurkov, Phys. Rev. ST Accel. Beams 13, 110701 (2010)] and present measurements.