THE TESLA CRYO-PLANTS
H. Quack, M. Kauschke, C. Haberstroh, TU Dresden, 01062 Dresden, Germany
H. Lierl, B. Petersen, S. Wolff, DESY, Notkestr. 85, 22607 Hamburg, Germany

Abstract
The Tera-eV Energy Superconducting LinearAccelerator (TESLA) is a 32 km long superconducting
linear electron/positron collider of 500 GeV centre ofmass energy (upgradeable to 800 GeV), presently in theplanning phase at DESY [1,2]. About 21000
superconducting RF 9-cell cavities have to be cooled withsuperfluid helium at 2 K. The cavities are assembled ingroups of 12 into about 16 m long cryo-modules. The
roughly 1800 cryomodules are arranged in anunderground accelerator tunnel in 12 about 2.5 km longcryo-units. Except for the 2 first cryo-units every 2 cryounits will be supplied from one refrigerator in arefrigerator hall above ground. The first two cryo-units
have individual refrigerators in individual refrigerator
halls.
The design goals for the cryoplants are reliability,
simplicity and low installation and operating cost.

1 RELIABILITY, REDUNDANCY AND
COST
The refrigeration system needs to have an availability ofover 95%. Because the refrigerators in all 7 refrigerationhalls have to be working at the same time, each single hall
has to have an availability of at least (0.95)1 7 = 99.3% .
So availability has highest priority in all design
considerations.
The first questions is, whether in each refrigerator hall
one should install one large refrigerator or multiple
refrigerators with e. g. 2 * 100 % or 3 * 50 % capacity
including redundancy.
The sources of unavailability in existing cryogenic
refrigeration systems have been investigated to find out,
whether the installation of multiple refrigerators would
increase the availability. Table 1 shows the main causesof unavailability in order of the frequency of theiroccurrence.
It turns out that concerning the four most frequent sourcesof unavailability the effect of multiple plants is either
small, negligible or even detrimental. Only in case of acatastrophic component failure like loss of vacuum or oil
spill into the cold box piping, there would be a clear
advantage of multiple refrigerators, but these effects haveoccurred very seldom in the latest generation of helium
refrigerators.
So after consultancy with CERN, which was confronted
with a similar question, when a decision had to be madefor the LHC cryogenic system, and with refrigerator

manufacturers, we decided to go for just one single large
refrigerator at each of the seven refrigerator halls withsome built-in component redundancy and a clear strategy
to fight impurities.
Table 1:Main Sources of Unavailability in Existing
Refrigerators

12
3

4

5

Source ofExample
Multiple
Unavailability
refrigerators ...
External utility Electrical power, would bring nofailurescooling
water, advantageinstrument air
Blockage
by Air and/or water provide a littlefrozen impurities vapor
larger tolerance
OperationalControls,
are detrimental
problems
instrumentation, due to higher
operators
complexity
SingleMotor burnout, would bring nocomponent
compr. bearings, advantage overfailure
not leaking oil pump part redundancy
leading to total seal,
turbine within a single
plant shutdown
bearing trouble
refrigerator
Catastrophic
Loss insulation would have acomponent
vacuum, rupture positive effectfailure leading to heat exchanger,
oil spill into CB
plant shutdown

Further discussions with manufacturers showed that therequired size of the refrigerator is about 20 % larger thanthe largest existing LHC refrigerator, but the coldcomponents still fit into one single horizontal cold box,
which is transportable across the road.

2 REFRIGERATOR SIZE
Table 2 shows the cooling requirements of the 7 halls.
Table 2: Design Heat Loads [kW]
T2K
5K
40 K

Hall 1 Hall 2 Hall 3 Hall 4 Hall 5 Hall 6 Hall 7

3.70
4.73
44.6

3.07
4.60
53.5

4.23
7.42
80.7

4.21
7.39
79.6

3.87
6.88
74.4

5.13
8.25
78.6

4.22
7.36
80.5The design refrigeration loads in halls 3, 4, 5, and 7 areabout equal, whereas the requirements in halls 1 and 2 aresomewhat smaller and in hall 6 somewhat larger. It issuggested to install seven refrigerators, which are
identical in those components, which are difficult toreplace or to modify (coldbox, heat exchangers, expander
and cold compressor housings, adsorbers and piping).
Adjustment to the special needs, like the lower design

loads of the refrigerators in halls 1 and 2, can be coveredby a different number of compressors and special turbine
and cold compressor nozzles and wheels.
As usual for refrigerators of this size, the plants will be
designed for very high energy efficiency at the designpoint, i. e. the highest expected refrigeration load.
The TESLA refrigerators would also be designed for avery good part load efficiency. So if e. g. the accelerator
would be operated at part load, the electrical power
consumption would be reduced proportionally down toabout 60 % of the highest refrigeration load.

3 TECHNOLOGY OF LARGE HELIUM
REFRIGERATORSOver the last forty years, accelerator physics has been
instrumental to push helium refrigerator technology into
the direction of higher reliability, better efficiency andlower cost. All major components of the TESLA helium
refrigerators have been used successfully in similarsystems before.
While the design of the mechanical components is already
fixed, continuous progress is going on concerning quality
assurance, instrumentation, diagnostics and computer
simulation of operational conditions [3].
The special challenge of the TESLA refrigeration systemis the remote operation of several such refrigerators froma central operating room without any service persons atsite. It is of advantage for TESLA, that the LHC
cryogenic system will be in operation a number of years
earlier. The TESLA refrigeration system has many
similarities with the LHC system, like number of plants,
distance between plants, superfluid helium cooling andthe helium inventory of about 100 t. The TESLA system
has about an eight times smaller cold mass, so cool-down
and warm-up will be simpler.

tunnel. In an extended standstill period, now and then oneof the plants on each side of the accelerator has to be
started to reliquefy the collected boil-off gas.
In case of an unplanned refrigerator shutdown and an
associated pressure rise in all cold systems, the helium
inventory of the portion of the system, which has a highdesign pressure, will be directed directly to the 20 bar
buffer.
The helium of the part of the system with lower designpressure, i. e. mainly the inventory of the cavity cryostats,will be allowed to rise in pressure and temperature by the„2 K static heat load“. Such a warm-up from 2 to 4,4 K
will take about 10 to 20 hours. If the pressure in thecryostat exceeds a pressure of about 1,2 bar, vapor isallowed to leave the cryostat, partially also through the 58 K shield line. So additional static heat load to thecryostat is avoided.

5 MODEL REFRIGERATOR
A model refrigerator has been designed by theTU Dresden and DESY, with valuable advice from CERN
[4,5,6] and from industry [7, 8]. The model refrigerator
allows to get information on component sizes, number ofcompressors, flow rates in different loops etc., to get
provisional data on power consumption for thespecification of utility systems and the operating budget
and to get the approximate size of components for thebuilding layout.
This model refrigerator was discussed extensively with
the industrial companies, which are presently building theLHC refrigerators. Industry confirmed, that the important
aspects of the model refrigerator are a correct basis for theplanning of TESLA and that the estimated cost isadequate.

4 HELIUM STORAGE
When in full operation the TESLA cryosystem willcontain about 100 t of helium. The commercial value ofthe helium is about 20 EU/kg, i.e. 2,2 MEU for the total
inventory. When the cryostats are warmed up, this helium
has to be handled outside the cryostats.
Whereas CERN has planned for LHC to provide storage
for only about 50 % of the helium, for the TESLA system
it is proposed to store all helium in a combination of
gaseous storage and liquid storage. Gaseous storage will
be provided for about 25 % of the inventory in 31 storage
vessels with a geometric volume of 250 m3 each, working
between 1 bar and 23 bar. On the other hand, nearly
100 % of the inventory can be stored in liquid form inseven 120 m3 liquid helium dewars.
Halls 1 and 2 will be equipped with one dewar and 3 gas
tanks each, whereas stations 3 to 7 will be equipped with
one dewar and 5 gas tanks each. All tanks and dewars will
be installed outside the refrigerator halls.
The ambient temperature helium systems of halls 1, 2, 3
and 4 on one side and 5, 6 and 7 on the other side areinterconnected by ambient temperature pipes through theFig. 1: Flow Diagram of Model Refrigerator
Helium is compressed at ambient temperature by a twostage screw compressor group to a pressure in the 20 bar

range. After re-cooling to ambient temperature andcareful oil removal and drying from residual water vapor,the high pressure helium is cooled in heat exchanger
HX 1 to 80 K, where it is purified from residual air inswitchable adsorbers.
Then a part of the flow is split off and expanded inturbines T1, T2 and T3. After T1 a part of the stream issplit off and cooled in HX 3 to 40 K. Here one part of theflow leaves the coldbox and cools the 40/80 K shield in
the cryostats. It returns back, is cooled in HX 2 and enters
T2. A pressure drop of about 1 bar is foreseen for thisshield flow.
One part of the flow already expanded in T1 andprecooled in HX 3 is further cooled in HX 4 andexpanded in T4 and T5 to atmospheric pressure. The mainhigh pressure stream leaving the 80 K adsorbers is cooledin HX 2, 3, 4, 5 and 6 and in between purified from neonand hydrogen in the switchable „20 K adsorbers“. The
high pressure stream is expanded in two parallel turbines
T6 and T7 to an intermediate pressure of about 5 bar. Themajor part is further cooled in exchangers 8, 9 and 10 andis then used for the 5-8 K shield. After return from thisshield, the gas is expanded in T 8.
Another part of the 5 bar flow is expanded in T9 andsubcooled in HX 11 before flowing to the cavity
cryostats. After throttling to the 30 mbar pressure leveland evaporation in the cryostats, the low pressure helium
vapor returns to the refrigerator through the 300 mm linein the cryostats. After superheating in HX 11, the gas iscompressed in a three or four-stage cold compression
system to a pressure close to atmospheric pressure, but
not necessarily to above atmospheric pressure. Thisstream is separately warmed up to ambient in exchangers
4, 3, 2 and 1 and enters its own low pressure screw
compressor.
The design flow rates, pressures and power requirementsof the model refrigerator are presented in Table 3.
Table 3: Process Parameters of the Model Refrigerator
2 K Load4253 W

Mass flow
199.4 g/s

5 – 8 K
Shield40 – 80 K
Shield7465 W

249.8 g/s

80788 W

383.3 g/s

Outlet
1.1 bar
2.2 K
5.5 bar5.16 K
16.0 bar
40 KCompressors
LP I

199.4 g/s

LP II

1369 g/s

HP1568.4 g/s

24.0 bar
ElectricPower

Temp.
Level

Refrigeration

COP

2K
5–8K
40 – 80 K
Total

4.25 kW
7.46 kW80.78 kW588 W/W
168 W/W
17 W/W

Return27.5 mbar
2.0 K
5.0 bar
8.2 K
14.0 bar
80 K

0.92 bar

1.4 bar

2500 kW1254 kW1373 kW5147 kW% ofTotal
Power
49 %
24 %
27 %
100 %

The indicated power consumption of 5147 kW is thepower consumption of the biggest refrigerator for thedesign case. The power at smaller refrigerators or at part
load operation can be calculated by using the indicatedCOPs for the individual temperature levels.

6 UPGRADE PHILOSOPHY
For operation at 800 GeV centre-of-mass, therefrigeration capacity has to be about doubled in eachcryogenic hall. For this purpose a second cold-box can beinstalled in the refrigeration halls and the halls must be
extended to give space for a second set of warm
compressors (Fig. 2). A third cold compressor chain isalso necessary for redundancy. The distribution box isalready prepared for the energy upgrade.

Fig. 2: Layout of the refrigeration hall

8 REFERENCES[1] TESLA The Superconducting Electron Positron
Linear Collider with an integrated X-Ray Laser
Laboratory, Technical Design Report, DESY-2001000, ECFA-2001-209, TESLA-2001-000, TESLAFEL-2001-000, March 2001.
[2 S.Wolff, The Cryogenic System of TESLA,
Proceedings of this conference.
[3] M. Kauschke, Numerische Simulation grosser
kryotechnischer Anlagen, Dissertation, TU Dresden
2001[4] S. Claudet, Ph. Gayet, Ph. Lebrun, L. Tavian, U.
Wagner, Economics of Large Helium Cryogenic
Systems: Experience from Recent Projects at CERN,
Adv. Cryog. Eng. Vol. 45, 1301-1308, 2000
[5] M. Barranco-Luque, L. Tavian, Gaseous Helium
Storage and Management in the Cryogenic System
for LHC, Proc. ICEC 18, Mumbai, India 2000, 143146[6] S. Claudet, Ph. Lebrun, L. Taviant, Towards Cost-toPerformance Optimisation of Large Superfluid
Helium Refrigeration Systems, Proc. ICEC 18,
Mumbai, India 2000, 203-206[7] Preliminary Industrial Study on the TESLA
Refrigerators, Air Liquide, 1997[8] Study on the TESLA Cryogenic System, Linde
Kryotechnik, 1996