Stability of the LCLS Undulator Hall

Stability of mechanical and electrical offsets of the bpm’s in the LCLS undulator section is critical to obtaining and maintaining stable FEL lasing. Simulations show that for the LCLS running at 1.5 Angstroms if the electron beam develops a 2 micron rms deviation from a perfectly straight line over a distance of about 10 meters, the FEL saturation length will increase by one gain length.[1] Nominally the feedback system will take changes in the electron beam trajectory, measured by the bpm’s, calculate and apply orbit corrections relatively easily. However, the efficacy of this technique relies on the ability of the bpm system to detect real electron beam trajectory changes at the level of 1 micron rms. One source of error in the determination of the electron beam trajectory is through changes in the mechanical or electrical offsets of the bpm’s. Such offset errors are erroneously imposed on the real beam trajectory by the feedback system. Bpm mechanical and electrical offsets can be determined by beam based alignment techniques using electron beams of different energies. However this measurement is time consuming and cannot be used during normal operation. Therefore it is of paramount importance to keep mechanical and electrical offsets as stable as possible — on the scale of a few microns over a period of at least a day. As part of the R&D for the NLC, studies were carried out in 1994 and 1996 in the FFTB tunnel where the LCLS undulator is to be housed, which measured magnet motion using a wire alignment syste