• A. Novokhatski
  SLAC, Menlo Park, California

An ultra short bunch is used in the LCLS operation. The bunch is prepared, compressed and accelerated in the main part of the machine, which includes an injector, two bunch compressors and a linac. A feedback system precisely controls the bunch energy before it enters a 300 m long beam transport beam line (LTU). In this line and later in the undulator section (132 m long) the bunch may loss energy due to wake field radiation. Additionally, wake fields may add a significant linear energy spread to the bunch which has to be compensated at an early stage in the linac. The energy loss has to be compensated by varying the K-parameter of the undulators. This means that a precise knowledge of the wake fields in this part of the machine is very important. Resistive wake fields are known and well calculated. We discuss an additional part of the wake fields, which comes from the different vacuum elements like bellows, BPMs, transitions, vacuum ports, vacuum valves and other elements. We use the code NOVO and include analytical estimations for the wake potential calculations. We show that the bunch energy losses due to these elements are about 20% of the total wake field losses.