Fermilab SRF cryomodule operational experience Page: 3 of 7
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300
27C
240
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E
C)ield,t80 K Shield and
y Circuit Outlet Temperatu' 5 K Sh
Cav t
>-1 0 K / h r S t e p s
5K Shield, 80 K Shield and
CavityCnrcuit nlet Temperatures150
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90
60
30
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LC
20
30
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-4-OK Sr
Levity
- avity
K Shi
+ AK hiaicd Inlet
field Outlet
Inlet
Outlet
ld Outlet
eld Inlet40
50
Time [hours]
FIGURE 2. CM1 5K Shield, 80K Shield and Cavity Circuit Cooldown Temperatures.
The 1.3 GHz cryomodule design requires that its individual circuits be cooled at
predetermined rates in order to limit thermal stresses. One such circuit is the helium gas
return pipe (GRP) which is a 300 mm pipe that acts as the rigid support (strong back) from
which all of the cavities are suspended. Based on a computer simulation of the cryomodule
cooldown and operating experience with similar design modules at DESY, the maximum
vertical gradient along this circuit is limited to less than 15 degrees. The longitudinal
gradient is to be maintained at 50 degrees or less and lastly the overall cooldown rate is to
be 10 degrees per hour or less over the range of temperatures from approximately 300 K to
100 K. The thermal shields circuits have identical longitudinal cooldown rate constraints.
FIGURE 3 shows that the maximum longitudinal temperature gradient requirements
of the GRP and shield circuits were satisfied throughout the cooldown. F or each circuit,
the supply temperature was reduced by a maximum of 10 degrees each hour between the
temperature range of 300 K to 100 K while monitoring the longitudinal and vertical
temperature differences. In certain instances the step change was delayed until the
temperature difference stabilized.
The gas return pipe was instrumented with a set of three in-flow Cernox* temperature
sensors at each end of the GRP. The sensors were attached to a post in the center of the
pipe and spaced to measure the temperature of the helium flow at the top, middle and
bottom of the pipe. The difference between the top and bottom sensors determined the
vertical gradient in the GRP. Additional surface mounted sensors were installed at each
end to measure the surface temperature at the top, side and bottom of the pipe. These
sensors were monitored during cooldown to maintain the desired vertical gradients.
FIGURE 4 di splays the vertical gradient, difference between the top and bottom
temperatures, of the GRP during cooldown for both the in-flow and surface mounted
sensors. Both the Endcap and Feedcap vertical gradients satisfied the 15 degreeNll
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Martinez, A.; Klebaner, A. L.; Theilacker, J. C.; DeGraff, B. D.; White, M. & Johnson, G. S. Fermilab SRF cryomodule operational experience, article, June 1, 2011; Batavia, Illinois. (https://digital.library.unt.edu/ark:/67531/metadc833358/m1/3/: accessed April 23, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.