Observation of longitudinal phase space fragmentation at the TESLA test facility free-electron laser

It has been reproducibly observed that the energy distribution of the beam, when fully longitudinally compressed for SASE operation, breaks up into several peaks. In this paper a description of the experimental setup, beam operating conditions, and observations is presented to enable further theoretical studies of this effect. (C) 2001 Elsevier Science B.V. All rights reserved.