Author: White, G.R.
Paper Title Page
The Collimation System for LCLS-II*  
  • J.J. Welch, E. Marín, T.O. Raubenheimer, M. Santana-Leitner, G.R. White
    SLAC, Menlo Park, California, USA
  • C.E. Mayes
    Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
  Funding: * Work supported by U.S. Department of Energy contract DE-AC02-76SF00515.
Minimizing beam loss is particularly important for LCLS-II because of very high average power beams, radiation sensitive undulators, and the high cost of adding shielding to existing accelerator enclosures. For example, for acceptable undulator magnet lifetime, dark current originating at the cathode must be attenuated by approximately a factor of 10-7 in a single pass before it reaches the undulator. Multi-stage, high-efficiency collimation is necessary. The system is described in this paper. A model of beam halo and dark current is developed that includes sources due to Touschek, intra-beam, and beam-gas scattering, as well as field emission from superconducting cavities, photo-emission from stray light on the cathode, and cathode field emission. The location and gap of the collimators is optimized using tracking analysis and other tools developed and validated at Cornell. Collimator efficiency is estimated by tracking secondaries using a modified version of Lucretia which call GEANT4. Finally collimator jaw design is optimized to produce a minimum leakage using FLUKA. These topics are discussed in this paper.