LCLS Conceptual Design Report - Stanford Synchrotron Radiation ...

X-rays
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
4
Gas influx
X
Xe
2
1
p = 152 Torr
Symmetry plane
X = 27 cm
L 1 = 10 cm
L 2 = 30 cm
To pump, 20 L / S
5
3 M N
6
L 1
Gas flow
8
9
p = 10 -2 Torr
Figure 9.6 The conceptual design for the gas cell attenuator
To pump, 200 L / S
To pump, 5×10 3 L / S
7
L 2
10
12
Diverging
molecular
stream
p = 3×10 -6
Torr
11
X-rays
Gas outflux
Q = 1.4×10 12
atoms/s
The operation of the gas cell in the weak-field (linear) regime using xenon as an absorber has
been calculated for reference. In Figure 9.7 the absolute attenuation of x-rays through xenon for
four given pressure, tg, [Torr-cm] products is plotted from 800 to 25000 eV. The curves indicate
that a 2000 Torr Xe gas jet with tg=1 cm would provide at least two orders of magnitude of
attenuation over the low-energy range of LCLS (800–4000 eV). Note that this absorption
calculation assumes that the absorption mechanisms are essentially uni-molecular and linear.
With suitable design and a sufficiently low repetition (pulse) rate the loading of the vacuum
system by the required amount of gas should be maintainable at acceptable levels.
X - R A Y B E A M T R A N S P O R T A N D D I A G N O S T I C S ♦ 9-11