The LINAC Coherent Light Source and Radiological Issues During the Commissioning Page: 1 of 1

                                
SLAC-PU B-13776

The Linac Coherent Light Source and Radiological Issues during the Commissioning
X. S. MAO, M. SANTANA LEITNER, J. VOLLAIRE
SLAC National Accelerator Laboratory, Sand Hill Road 2575, Menlo Park, CA 94022, USA, mao@slac.stanford.edu
The Linac Coherent Light Source (LCLS) at SLAC National Accelerator Laboratory is the world's first X-ray free
electron laser (XFEL). Pulses of x-ray laser light from LCLS will be many orders of magnitude brighter and
several orders of magnitude shorter than what can be produced by other x-ray sources available in the world.
These characteristics will enable frontier new science in many areas. This paper describes the LCLS beam
parameters and lay-out. The general radiological issues during commissioning are presented, such as radiation
dose rates and integrated doses outside the enclosure. Also, specific radiological issues related to X-ray free
electron lasers are discussed. XFEL with high peak power will burn through high-Z materials. The X-ray beam
needs to be blocked by stoppers when the downstream areas are occupied. LCLS stoppers feature a piece of boron
carbide (B4C), 10 mm thick. B4C is one of the best materials since it has a low absorption coefficient for X-rays
and a high melting temperature. Theoretical calculations indicate that the unfocused fluence of the LCLS XFEL
beam should be about one order of magnitude below the damage threshold for bulk B4C, for 830 eV FEL
radiation. However, these calculations have not been tested experimentally and cannot be validated until LCLS
begins providing 830 eV XFEL pulses. This paper describes the test plan for using the initial LCLS radiation to
evaluate the survivability of B4C and reports the preliminary results. Another major issue for LCLS is the
potential radiation damage to the LCLS undulator magnets during operation. TLD dosimeters were installed along
the LCLS undulators for each period of two or three weeks. This paper reports the integrated doses along the
undulators with and without XFEL generation.

Work supported in part by US Department of Energy contract DE-AC02-76SF00515.