  The Linac Coherent Light Source (LCLS) at Stanford Linear Accelerator Center
(SLAC) is in the process of being upgraded to a superconducting radio frequency
(SRF) accelerator and renamed LCLS-II. This upgrade requires thirty five 1.3
GHz SRF cryomodules (CM) and two 3.9 GHz CM. A cryogenic distribution system
(CDS) is in development by Fermi National Accelerator Laboratory to
interconnect the CM Linac with the cryogenic plant (CP). The CDS design
utilizes cryogenic helium to support the CM operations with a high temperature
thermal shield around 55 K, a low temperature thermal intercepts around 5 K,
and a SRF cavity liquid helium supply and subatmospheric vapor return both
around 2 K. Additionally the design must accommodate a Linac consisting of two
parallel cryogenic strings, supported by two independent CP utilizing CDS
components such as distribution boxes, transfer lines, feed caps and endcaps.
The paper describes the overall layout of the cryogenic distribution system and
the major thermodynamic factors which influence the CDS design including heat
loads, pressure drops, temperature profiles, and pressure relieving
requirements. In addition the paper describes how the models are created to
perform the analyses.
