Simulation of time-dependent energy modulation by wake fields and its impact on gain in the VUV free electron laser of the TESLA Test Facility

For shorter bunches and narrower undulator gaps the interaction between the electrons in the bunch and the wake fields becomes so large that the FEL a mplification is affected. For a typical vacuum chamber of an X-ray or VUV F ree Electron Laser three major sources of wake fields exist: a resistance o f the beam pipe, a change in the geometric aperture and the surface roughne ss of the beam pipe. The generated wake fields, which move along with the e lectrons, change the electron energy and momentum, depending on the electro n longitudinal and transverse position. In particular, the accumulated ener gy modulation shifts the electrons away from the resonance condition. Based on an analytic model the energy loss by the wake fields has been incorpora ted into the time-dependent FEL simulation code GENESIS 1.3. For the parame ters of the TESLA Test Facility the influence of the bunch length, beam pip e diameter and surface roughness has been studied. The results are presente d in this paper. (C) 2000 Published by Elsevier Science B.V. All rights res erved.