LCLS Conceptual Design Report - Stanford Synchrotron Radiation ...

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
Assuming a pumping speed of 1 l/s, the average pressure is 3.59 × 10 -8 Torr. If the pumping
speed is increased to 10 l/s, the average pressure decreases to 3.34 × 10 -8 Torr (only a 7.6%
improvement). From this, it can be seen that minimizing the gas load has a much greater effect on
beam pipe pressure than does pumping speed. The gas load within the undulator vacuum system
comes from two processes, thermal desorption and photo-desorption. Thermal desorption is
common to all types of vacuum systems; it is the heat-stimulated release of gas constituents
adsorbed on the walls of the system. Photo-desorption is the outgassing that occurs due to
synchrotron radiation hitting the walls of the beam pipe and desorbing gas molecules. Good
thermal desorption data exists for UHV processed copper plated stainless steel from the PEP-II
project [16,17]. Typically, qt = 5 × 10 -13 Torr-liter/sec-sq cm (@ T = 20 o C) after a 200 o C bake for
4 hours. Since the undulator vacuum system is expected to operate at 20 °C, this value will be
used.
Photodesorption is a little harder to estimate. The undulator produces 90 GW of total peak
power. Photon flux is estimated using 100 GW of power hitting the walls of the vacuum system
using the following formula:
where Nγ, photon flux = photons/sec
N
γ
15 ( 6.242 10 keV/J)
PSR t f x
= (8.38)
E
photon
PSR, synchrotron radiation power = 100×10 9 Watts
t, pulse length = 100 × 10 -15 sec
f, pulse frequency = 120 sec -1
Ephoton, average photon energy = 200 keV/photon
Nγ for the undulator is 3.74 × 10 13 photons/sec. The distribution of the spontaneous photon flux is
assumed to increase linearly along the length of the vacuum system as shown in Figure 8.29.
8-50 ♦ U N D U L A T O R