• J.C. Dooling, W. Berg, A.F. Pietryla, B.X. Yang
  ANL, Argonne
• H.-D. Nuhn
  SLAC, Menlo Park, California

Funding: Work supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under contract number DE-AC02-06CH11357.

Simulations of the beam loss monitor (BLM) system built at the Advanced Photon Source (APS) for the Linear Coherent Light Source (LCLS) have been carried out using the Monte Carlo particle tracking code MARS. Cerenkov radiation generated by fast electrons in the quartz radiator of the BLM produces the signal used to estimate beam loss and dose in the LCLS undulator magnets. The calibration of the BLM signal with radiation components that cause undulator damage is the goal of the simulation effort. Beam loss has been simulated for several scenarios including undulator magnets in the normal operating position, “rolled-out” 80 mm from the beamline, and absent altogether. Beam loss is generated when an electron bunch strikes one of two targets: Al foil or carbon wire. In the former case, the foil is placed at OTR33, 85.8 m upstream of the FEL; in the latter, the first undulator beam finder wire (BFW01) position is used just upstream of the first magnet. The LCLS MARS model includes quadrupole focusing between OTR33 and the end of the FEL. The FODO lattice leads to complex loss patterns in the undulators consistent with betatron envelope maximums in both transverse planes.

• J.C. Dooling, W. Berg, L. Emery, B.X. Yang
  ANL, Argonne

Funding: Work supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under contract number DE-AC02-06CH11357.

An array of fused-silica, fiber optic bundles has been built to spatially monitor e-beam loss in the APS storage ring (SR). A prototype beam loss position monitor (BLPM) has been installed on unoccupied undulator straight sections. The BLPM allows for 6 fiber bundles, 3 above and 3 below the beam. The center bundles are aligned with the beam axis. Presently, 4 bundles are used, 3 above and one in the center position below the beam. Each bundle is 3 m in length and composed of 61 220-micron-diameter fibers for a total aperture of 2 mm. The first 30 cm of each bundle are aligned parallel to the beam in contact with the vacuum chamber. Light generated by fast electrons within the fibers is thought to come primarily from Cerenkov radiation. The rest of the fiber acts as a light pipe to transmit photons to shielded PMTs. Tests show good signal strength during stored-beam mode from Touschek scattering and deterministic losses that occur during top-up injection and beam dumps. Post-injection loss signals show spatial and temporal dynamics. Simulation work is expected to provide calibration for integrated losses that can be compared with progressive undulator demagnetization.

• A.H. Lumpkin
  Fermilab, Batavia
• W. Berg, Y.L. Li, S.J. Pasky, N. Sereno
  ANL, Argonne

Funding: Operated by Fermi Research Alliance, LLC under Contract No. DE-AC02-07CH11359 with the United States Department of Energy.

The challenge of mitigating the strong enhancements of the optical transition radiation (OTR) signal observed after bunch compression in the Advanced Photon Source (APS) linac chicane and at the Linac Coherent Light Source (LCLS) has recently been addressed. We have demonstrated a technique to mitigate the intensity of the coherent OTR (COTR) relative to the OTR signals on the APS beams at 325 MeV. Since the previously reported spectral content of the COTR at LCLS after the first compression stage is similar, the concepts should also apply to LCLS. We utilized the stronger violet content at 400 nm of the OTR compared to the observed gain factors of the COTR in the blue to NIR regime. We also demonstrated the use of an LSO:Ce scintillator that emits violet light to support lower-charge imaging. Spectral-dependence measurements of the COTR were done initially at the 325-MeV station using a series of band pass filters inserted before the CCD camera, but recent tests with an Oriel spectrometer with ICCD readout have extended those studies and confirmed the concepts. These techniques are complementary to the proposed use of a laser heater to mitigate the microbunching itself at LCLS.