LCLS Conceptual Design Report - Stanford Synchrotron Radiation ...

Photon Dose (photons/sec-cm)
L C L S C O N C E P T U A L D E S I G N R E P O R T
(x10 9 )
8
6
4
2
3-98
8360A3
0
0 20 40 60
Z (m)
Figure 8.29 Undulator photon flux profile.
The photo-desorption gas load is calculated as follows:
q p
( 2. 83x10−20
Torr − l / molecule)
80 100
= Nγ
η
(8.39)
where qp, photo-desorption gas load = Torr-liter/sec
η, photo-desorption rate = molecules/photon
Photo-desorption of copper and stainless steel beam tubes were investigated in the past.
Brookhaven National Laboratory conducted studies for the PEP-II Project, determining the values
of η with respect to flux [18]. The photon flux for the undulator vacuum system is in fact quite
low, so, realistically, photon scrubbing will not occur during the lifetime of the machine. From
the Brookhaven results, it has been determined that an η = 5 × 10 -3 molecules/photon is
appropriate for design purposes. Using this value, the photo-desorption profile is calculated and is
shown along with the calculated thermal desorption profile in Figure 8.30.
Using the desorption profiles, a vacuum pressure profile for the 100-m long undulator
vacuum system is calculated using VACCALC [19], a pipeline pressure computer code. The
entire undulator beam pipe is modeled using discrete pipeline segments. Each segment is defined
by its length (m), conductance (l/s), gas load (nTorr-l/s), and pumping speed (l/s). All values for
conductance, gas load and pumping speed are calculated for “hydrogen.” Figure 8.31 shows the
pressure profile along the length of the undulator vacuum system, with its average pressure being
1.06×10 -9 Torr (1 nTorr). This is well below the design requirement of 10 -7 Torr.
U N D U L A T O R ♦ 8-51