S1-G Report: Cryogenics - KEK

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8. Cryogenics



8
-
1
Two
-
Kelvin

(2K) Cryogenic System at STF

The operation temperature of superconducting RF cavities for the ILC is 2K. The simplest way to
produce the temperature of 2K is to evacuate liquid helium. This way is called “forced boiling.” Fig
ure
1 shows the phase diagram of helium. Helium has two liquid phases. One is an ordinary liquid phase,
which is designated as He I. Another is a superfluid liquid phase, designated as He II. The He I
domain is located at higher temperature side of liquid
phase as shown in Figure
8
-
1, while that of He
II at lower temperature side. These two liquid phases are separated by the “λ
-
line” (a green line in the
figure). This

λ
-
line
meets the saturated vapor pressure
curve

(saturation curve)
at 2.17 K. This point
i
n the phase diagram is called
the “
(lower)

λ
-
point.” Generally speaking liquid helium is transferred
from a liquefied helium storage vessel to cryostats of superconducting RF cavities by the pressure
difference. If the pressure difference is 0.0
3

MPa, the
saturation temperature of helium is 4.5 K
(absolute pressure of 0.13 MPa in this case). If we start to evacuate this liquid helium with any kinds
of vacuum pumps, the thermodynamic state of helium varies along the saturation curve (a red curve
in the figur
e), as shown with a blue arrow in the figure. If the evacuation capacity of the vacuum
pumps is enough, we can get superfluid helium at 2 K or lower finally.

A 2K cryogenic system for the S1
-
G is schematically illustrated in Figure 8
-
2. The system
consists

of various devices: a helium liquefier/refrigerator, a liquefied helium storage vessel, a 2K
refrigerator cold box, a superconducting RF cavity cryomodule, a helium gas pumping system and
high
-
performance transfer lines. Liquid helium is produced with a h
elium liquefier/refrigerator and
once stored in a liquefied helium storage vessel. The 2K refrigerator cold box contains a He I pot, a
He II pot, a heat exchange and a Joule
-
Thomson (J
-
T) valve. Liquid helium is transferred from the
storage vessel to the H
e I pot of the 2K refrigerator cold box through the high
-
performance transfer
lines. This transfer lines are designed and fabricated to reduce the heat load from room
-
temperature
environment as light as possible, whose cross section is shown in Figure 8
-
3.

Since the main transfer
line is employed to transfer not only liquid helium but also cold helium return gas and liquid nitrogen,
the transfer line is called also as a “multi
-
channel transfer line.” Liquid helium in the He I pot of 2K
refrigerator cold box

is distributed into the He II pot, precooling line of the superconducting cavity and
the 5K thermal shield of the cryomodule. As there are no valves in the cryomodules, the liquid helium
level in the cryomodules is the same as in the He II pot of the 2K r
efrigerator cold box. After filling up
the He II pot and the cryomodule with liquid helium, we start to evacuate gradually through a gas
return pipe (GRP) and the heat exchanger with several oil rotary vacuum pumps and mechanical
booster pumps until 3.2 kP
a (the saturation pressure of helium at 2K). Once 2K superfluid helium is
produced in the He II pot and the cryomodule, liquid helium in the He I pot is sent to the He II pot
through the heat exchanger, where liquid helium is cooled with the helium gas eva
porated from 2K
superfluid helium. The cooled liquid helium is then expanded through the Joule
-
Thomson valve to be
converted into superfluid helium at 2 K.

A helium liquefier/refrigerator had been moved to the STF to supply liquid helium and liquid
nitroge
n to the superconducting RF cavity cryomodules as a piece of ILC R&D at KEK. A prototype of
2K refrigerator cold box and high
-
performance transfer lines were designed and fabricated for the
tests of the superconducting RF cavity cryomodules for the ILC som
e years ago at the STF. Figure 4
shows the layout of the cryogenic system at the STF. The STF building has two stories, one is the
ground level and another the tunnel level, the downstairs of the ground level. The helium
liquefier/refrigerator, the liquefi
ed helium storage vessel and the helium gas pumping system (oil
rotary vacuum pumps and mechanical booster pumps) are located on the ground level. The 2K
refrigerator cold box and cryomodules of superconducting RF cavities are on the tunnel level. The
mult
i
-
channel high
-
performance transfer line is laid down from the ground level to the tunnel level
through a shaft between the two levels.



8
-
2 Upgrade of Cryogenic System for S1
-
G

The cavity configuration

of

the S1
-
G was 8 cavities in 2 cryomodules, i.e. 4
cavities in each
cryomodule. The number of the cavities was the largest at that time. Even the 2K cryogenic system
was designed to produce 30W cooling capacity at 2K, the stable operation of the cryogenic system
depended on the static heat load from the cr
yomodules and on the dynamic heat load from the
superconducting RF cavities. To prepare the increment of the heat load from the more cavities than
before, we had employed additional vacuum pumps to increase the cooling capacity of the cryogenic
system. A P
C
-
aided small
-
scale control and data acquisition system was introduced into the
cryogenic system in advance of the S1
-
G tests for long
-
term operation of the cryogenic system. This
control and data acquisition system can control valve openings in the helium

gas pumping system
with signals from pressure transducers to realize the stable operation of the cryogenic system, i.e. the
maintenance of constant temperature of superfluid helium and to acquire data of temperatures,
pressures, liquid helium levels of th
e cryogenic system. The acquired data are sent to the main PC of
the STF for data storage through the Ethernet. This system consists of commercially available
devices: a data
-
logger unit, various terminal units, a control valve, and a laptop PC. Since ther
e is no
special device in this system, we can easily extend, replace and modify this control and data
acquisition system by ourselves in case of modification of the cryogenic system.



8
-
3 Operation of Cryogenic System

Since the production and consumption
of liquefied gas, such as liquid helium and liquid nitrogen,
are subject to the High Pressure Gas Safety Act in Japan, a supervisory safety worker, who has an
authorized license (a high pressure gas production safety management certificate), should be on d
uty
whenever the cryogenic system (mainly the helium circulation compressors) is in operation. We
considered that the S1
-
G test was not a full
-
scale operation of the accelerator (i.e. a short term test or
experiment of some scientific instruments in an ope
n cryogenic system) in the legal sense of the High
Pressure Gas Safety Act. Then the operation of the cryogenic system was restricted on a day
-
by
-
day
basis. We started to cool down the cryomodules in the mornings and stopped in the evenings. On
Saturdays a
nd Sundays the cryogenic system was not operated. A supply of liquefied nitrogen to the
80K thermal shields is also considered legally as an operation of the cryogenic system. It was not
allowed to supply liquid nitrogen to the 80K thermal shield line in t
he absence of a supervisory safety
worker during nights and weekends.

The principal cold mass of the cryomodule is the GRPs, not the superconducting RF cavities.
This is because the pipe is made of stainless steel with thick wall, long length and large dia
meter. The
cavities could be readily cooled down, since liquid helium and superfluid helium touch them directly,
while the GRPs could be cooled down only with cold helium gas from the helium vessels of the
superconducting RF cavities. Then during nights an
d weekends when the cool
-
down was suspended,
the cavities and their helium vessels were gradually warmed up, while the GRPs cooled down. The
inversed phenomena could be observed when the cryomodules were warmed up after the tests.

Since
a rapid cool
-
down m
ay cause any vacuum leakage of superconducting RF cavities and/or
excess deformation of cryomodule structure, the cool
-
down was intentionally carried out at slow rate.
It was estimated that it would take almost 2 weeks (10 workdays) to complete cool
-
down o
f the 8
superconducting RF cavities from room temperature to 2 K. From the room temperature to about 200
K, we supplied cold helium gas, which was cooled with liquid nitrogen, to the precooling line, the 2K
line and also to the 5K thermal shield line. The
80K thermal shield line was supplied with liquid
nitrogen. It took 3 workdays until the temperature of the helium vessels of the cavities was about 200
K. From the 4th workday, liquid helium was supplied to the 2K line and the 5K thermal shield line.
Liqui
d nitrogen was supplied to the 80K thermal shield line. On the 9th workday liquid helium came to
be filled up the helium vessel of the first cryomodule, and all helium vessels came to be filled up with
liquid helium on the 10th workday. On the day the heli
um gas pumping system was activated for
hours to decrease the temperature of the GRPs and the cavities. The next Monday we resupplied
liquid helium to the 2K refrigerator cold box and the cryomodules, and evacuated it until the
temperature decreased to 2 K
.

A typical temperature evolution of 8 helium vessels of the cavities in 2 cryomodules during
cooling
-
down is shown in Figure 8
-
5. That of 2 GRPs is shown in Figure 8
-
6. Date is described as
year/month/day. Gray strips in these figures indicate weekends. S
ince the C
-
1 cavity (and then the
helium vessel of C
-
1) was located at the upstream most, that is, the closest to the 2K refrigerator cold
box, its temperature came to go down first. According to the order of position from the 2K refrigerator
cold box, so
did temperatures of individual helium vessels. Because of the day
-
by
-
day operation of
the cryogenic system, the temperature evolution in the figure shows a zigzag shape. Once after the
cryomodules were cooled down to 2 K, their temperature rose a little du
ring the nights and rather
more during weekends.

Hence, on the Mondays it t
ook

almost
one day to resupply liquid helium to
each helium vessels and to evacuate liquid helium
in the vessels
until
the

temperature reached to 2 K.
On other weekdays the supercon
ducting cavities became stable at 2 K by the noon, and tests on the
cavities and RF systems could be carried out in the afternoons. Cold helium gas flew from the
downstream end of the GRP of the cryomodule A (CM
-
A) to the upstream end of the GRP of the
cry
omodule C (CM
-
C), since the 2K helium supply line connected to the downstream end of the GRP
of CM
-
A. Hence the downstream end of the GRP of CM
-
A was cooled down first, as shown in Figure
8
-
6.

The temperature evolution during the free warm
-
up of the helium

vessels is shown in Figure 8
-
7.
On the contrary of the cool
-
down case, the temperature evolution of helium vessels is not so simple.
In CM
-
A the A
-
4 helium vessel, which was located at the downstream most, came to warm up first.
Then A
-
3, A
-
2 and A
-
1 foll
owed in order of position in the module. In CM
-
C, however, C
-
1, the
upstream most, did first, followed by C
-
2, C
-
3 and C
-
4 in order of position. This means that there are
two heat flow paths to the superconducting RF cavities and their helium vessels, i.e.

heat flows into
the cavity string from both ends of the string. The temperature evolution of the GRPs, shown in Figure
8
-
8, is rather uniform during the free warm
-
up. This is because the GRPs are the largest cold mass in
the cryomodules and then their hea
t capacity is also large.



8
-
4 Summary

From June 2010 to February 2011 the cryogenic tests of the S1
-
G superconducting RF cavities
had carried out intermittently. The 2K cryogenic system did work well and stably with a help of the
PC
-
aided small
-
scale

con
trol and data acquisition system. It took two weeks (10 workdays) to cool 8
superconducting RF cavities in 2 cryomodules down to 2 K, as planned for slow cool
-
down. It is
confirmed that the cooling capacity of the cryogenic system with the buildup helium g
as pumping
system was satisfied its specification to maintain the 8 cavities in a superconducting state at 2 K.


Figures


Figure 8
-
1 Phase diagram of helium.





Figure 8
-
2 Schematic diagram of 2K cryogenic system for S1
-
G.



Figure 8
-
3 Cross section o
f high
-
performance transfer line (MLI not shown).




Figure 8
-
4 Cryogenic system at STF.



Figure 8
-
5 Temperature evolution of 8 liquid helium vessels of S1
-
G during cool
-
down.




Figure 8
-
6 Temperature evolution of 2 gas return pipes (GRPs) of S1
-
G dur
ing cool
-
down.


Figure 8
-
7 Temperature evolution of 8 liquid helium vessels of S1
-
G during warm
-
up.




Figure 8
-
8 Temperature evolution of 2 gas return pipes (GRPs) of S1
-
G during warm
-
up.