SLAC–PUB–12673 July 2007 LCLS Beam Dynamics Studies with the 3-D Parallel Impact-T Code * Y. Ding, Z. Huang, C. Limborg-Deprey Stanford Linear Accelerator Center, Menlo Park, CA 94025 J. Qiang Lawrence Berkeley National Laboratory, Berkeley, CA 94720 Abstract We present the beam dynamics studies with the 3-D Parallel Impact-T code on the Linac Coherent Light Source (LCLS) injector system. The physics model and recent additions of the Impact-T code are reviewed, and the space charge simulation studies are described. Simulation results show that for the normal design parameters of the LCLS with a smooth laser profile, the space charge effects above 135 MeV are negligible. However, the electron bunch can form a very-high current spike after the bunch compressor. In this case space charge still play an important role in affecting the beam quality in both longitudinal and transverse directions. Presented at the 22nd Particle Accelerator Conference, Albuquerque, New Mexico, U.S.A, June 25-29, 2007. * Work supported by Department of Energy contract DE–AC02–76SF00515.