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 system with an inherent resolution of 100 nm. The reference wires extended in four sections for a total length of about 440 feet starting near magnet QA2 near the muon shielding in the beam switchyard and ending about 85 feet out into the research yard section of the FFTB. The planned location for the LCLS undulator section partially overlaps the area where the measurements were made. Two papers were written describing measurements made with the system: one by Assmann, Salsberg and Montag,[2] and another by K. Floettmann[3]. I will discuss the results from these papers and what they imply for the LCLS. As both efforts were mainly interested in the effect of magnet motion on the obtainable electron beam spot size and the stability implications to future linear accelerators, they concentrated on finding the most stable conditions that could be obtained, even if such conditions could only be obtained for a short time and over a limited area. In the LCLS case it will be necessary to have adequate stability for essentially year round operation over the full 120 m length of the undulator and there is no possibility of choosing an alternate site which might be quieter. Approximately 80% of the tunnel where the measurements were made were within the relatively stable underground (beam switchyard) portion of the FFTB. Unfortunately for the LCLS, about 80% of the undulator will be located in the less stable above ground (research yard) region of the FFTB tunnel.
- Research Organization:
- SLAC National Accelerator Lab., Menlo Park, CA (United States)
- Sponsoring Organization:
- USDOE Office of Science (SC)
- DOE Contract Number:
- AC02-76SF00515
- OSTI ID:
- 839684
- Report Number(s):
- SLAC-TN-05-024; TRN: US0503528
- Country of Publication:
- United States
- Language:
- English
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