THE CRYOGENIC SYSTEM OF TESLA
S. Wolff, DESY, Notkestr. 85, 22607 Hamburg, Germany
for the TESLA collaboration

Abstract
TESLA, a 33 km long 500 GeV centre-of-mass energy
superconducting linear collider (upgradeable to 800GeV) with an integrated X-ray FEL facility of 0.1 nm
wave length, has been proposed by an international
collaboration to be built at DESY. The backbone of thiscollider are more than 21000 superconducting 9-cell RFcavities, operated at 1.3 GHz with 5 Hz beam repetition
rate, and about 800 superconducting magnet packages
with quadrupoles and steering dipoles. The paper gives anoverview of the cryogenic system necessary to cool theseelements to 2 K. The system mainly consists of about
1800 cryomodules assembled in twelve 2.5 km longcryogenic units, supplied from 7 refrigerators.

1 INTRODUCTION
The 33 km long e+e- linear collider TESLA (Tera eVEnergy Superconductiong Linear Accelerator) with 500
GeV centre-of-mass energy and an incorporated X-ray
FEL facility has been proposed by the international
TESLA Colaboration [1]. The collider consists of 2 about
15 km long linear accelerators pointing to each other, one
for electrons, the other for positrons. Other major
components are 500 MeV e- injectors for the standard
high energy beam, for a polarized beam and for the FEL
beam, a 250 MeV e+ injector and 5 GeV pre-acceleratorsand 17 km long damping rings for electrons andpositrons. There is a high energy physics experimental
area at the centre and close to it the X-ray FEL laboratory.
The special feature of TESLA are superconducting
accelerating structures. The energy can be upgraded to800 GeV centre-of-mass. A Technical Design Report(TDR) including costs has been worked out under theassumption that TESLA will be built north-northwest ofDESY. The TDR has been submitted to the German
Scientific Council at the German federal government.

2 CRYOMODULES
2.1 Cavities
The elementary acceleration structure is a 9-cell1.038 m long (effective length, Fig. 1) superconducting
cavity of 1.3 GHz made of pure 2.8 mm thick niobium tobe cooled to 2 K. The nominal acceleration gradient at
500 GeV centre-of-mass energy is 23.4 MV/m reached ata quality factor Q0 = 1010. The cavities are housed inhelium vessels made from titanium.

The availability of this technology has been
demonstrated at the TESLA Test Facility at DESY [2].
There will be more than 21000 cavities of this kind inTESLA.

Fig. 1: Superconducting 9-cell cavity

2.2 Magnet Packages
Beam focussing is achieved by cos2Θ superconducting
quadrupoles of 0.52 m field length and 60 T/m gradient at
100 A (Fig. 2). The quadrupoles are housed in helium
vessels which also contain vertical and horizontal (inevery second package) superconducting steering dipolesof 0.074 T field at 40 A. These magnet packages arecooled to 2 K, same as the cavities. The coils are powered
through high temperature superconducting (HTS) current
leads without gas cooling. Attached to the magnet
package is a cavity type beam position monitor (BPM).

Fig. 2: Superconducting magnet package
There will be about 800 magnet packages in thecollider.

2.3 Cryostat
The active superconducting elements are assembled in
16-17 m long cryostats (Fig. 3) mostly all of which house
12 cavities in the so called compressed 9-cell structure(space between end cells of neighboured cavities: 283mm) [3]. Up to 150 GeV every second cryomodule andbetween 150 and 250 GeV every third cryomodule also
houses a superconducting magnet package. Attached tothe cavities are input couplers, higher order mode (HOM)
couplers, tuners, bellows and flanges. The active elements(cavities and magnets) are supported by a 300 mm
diameter girder tube acting as gas return pipe (GRP)
which itself is supported at the outer vacuum vessel
through 3 fibre glass support posts. The “coldFig. 4: Cryogenic unit (consisting of 16 cryogenic strings)
mass” is surrounded by two thermal radiation shields (oneat 5-8 K, the other at 40-80 K) covered with multi-layer
insulation (MLI). The cryomodule also contains thenecessary helium process tubes for cavity cooling (2 K),shield cooling and cool-down/warm-up.

2.4 Cryomodule Heat Loads
Heat loads have been calculated for static operation(without RF and magnet current), 5 Hz dynamic operation
at 23.4 MV/m with quality factor Q0=1010, and for
additional 5 Hz FEL operation at 21.1 MV/m (only in thefirst part of the electron linac) with the same Q0. Theresults are shown in Table 1. As can be seen the losses are
dominated by the RF losses ( 81 % at 2 K).

3 CRYOGENIC DISTRIBUTION
3.1 Cryogenic Units

Fig. 3: Cryomodule cross section
Table 1: Heat loads [W] of a 17 m long cryomodulewith quadrupoleTemp.
staticdynamic, high additionalenergy beam
dynamic,
FEL beam
2K
1.74
7.31
5.47
5-8 K
11.32
4.62
4.18
40-80 K
90.13
92.89
70.18

About every 150 cryomodules are assembled to a 2.5
km long cryogenic section called cryogenic unit (Fig. 4),
supplied cryogenically from a refrigerator from one sidethrough a feed box [4]. Vacuum barriers are installed
every 500 m in order to limit the pressure increase in thecryogenic lines in case of an accidental break of theinsulation vacuum [5] and for better leak checking. There
are 12 units in total, 6 for each linac. Except for the first 3km (mainly unit no. 1) which are declined by - 8 mrad theunits are levelled with respect to gravity.

3.2 Cryogenic Strings
Inside each cryogenic unit cryogenic strings of 10 cryomodules form the smallest cooling circuit (Fig. 5). They
are about 170 m long. A special cryomodule containing 3

Fig. 5: Cryogenic string ( consisting of 10 cryomodules)

Fig. 6: Cryogenic system (consisting of 12 cryogenic units supplied from 7 refrigeration halls)

control valves (one of them a Joule-Thomson (JT) valve)is necessary at the end of each cryogenic string.
During steady state operation, 2.2 K one phase heliumof 1.2 bar coming from the feed-box is expanded in the JTvalve into a small liquid helium separator. From here theliquid is flowing through the 76 mm diameter 2 phasetube into the cavity and magnet helium vessels.
Evaporating helium is returned through tube connections
at each cryomodule interconnection to the GRP andfurther to the refrigerator.
Cool-down and warm-up is performed by helium gas ofsuitable temperature directed from the supply line (sameas the 2.2 K one phase line) though a control valve and agas header supplying all helium vessels from the bottom.

3.3 Unit Heat Loads
Heat loads have been calculated for all cryogenic units
from all components like modules, feed- and end-boxes,
vacuum barriers, distribution boxes and transfer lines.
Table 2: Design heat loads [kW] at refrigeration halls
Thall1 hall2 hall3 hall4 hall5 hall6 hall7
2 K 3.70 3.07 4.23 4.21 3.87 5.13 4.22
5 K 4.73 4.60 7.42 7.39 6.88 8.25 7.36
40 K 44.6 53.5 80.7 79.6 74.4 78.6 80.5
Also the heat loads of 3 e- injectors on the DESY site,of the superconducting RF for the 2 damping rings andfor the bunch compressors, and of the additional 5 Hz

operation for the FEL in the first part of electron linac
have been taken into account. The design refrigeration
capacity at the position of the 7 cryogenic halls (see next
chapter and Fig. 6) has been derived by multiplying theseloads with the factor 1.5 for uncertainties in thecalculation and cryomodule performance as well as forcontrol reserve (Table 2).

4 REFRIGERATION PLANTS
4.1 General Concept
Except for the first 2 cryogenic units which have theirown refrigeration plant, one refrigeration plant will serve
2 cryogenic units (Fig. 6). As the size of these plants will
be only 20 % larger than the ones of the Large Hadron
Collider (LHC) at CERN, they will be based on a wellestablished technology. They will have a highthermodynamic efficiency (compressors 55 %, cycle
49 %, total Carnot 27 %, which is equivalent to 245
Wel/W(4.5 K)), a good part load efficiency, highreliability and will be completely remote controlled.
There are no turbines above 80 K [6].

4.2 Refrigerators

For helium storage there are a dewar for 120 m3 liquidand generally 5 gas tanks of 250 m3 (20 bar) at each cryohall. A 200 mm diameter helium warm gas tube connects
the helium plants of each linac to allow helium exchange
and recovery in case of a complete refrigerator break
down. This tube is also used to collect helium from safetyvalves at the 5-8 K shield cooling line in case of an
accidental break of the insulation vacuum. For moredetails on safety aspects see reference [5]. The main
cryogenic components of the whole system are listed inTable 3.
The refrigerator requirements and performances arebeing studied theoretically in a model refrigerator (Fig. 7)[6].

4.3 Plant Operation
The refrigeration plants will be operated fully remotecontrolled and unattended. Personnel will be available
only on call (like for LHC at CERN). Special care must
be taken for impurity handling. Cool-down and warm-up
of ~ 700 tons (2 cryogenic units) can be done within 10days without liquid N2 assistance. The filling with liquid
helium will then require 2 ½ more days. No heaters arerequired for warm-up of the returning gas.

The main components of each refrigerator are 10 screw
compressors in two stages (one redundant compressor ineach stage) with 24 bar maximum pressure, a horizontal
cold-box with 9 turbines, and two 3-4 stage cold
compressor trains (one redundant) in order to reach 2 K.
Apart from the compressors there are also redundant oilpumps, oil coolers and helium after-coolers. Inside thecold-box there are double 80 K and 20 K adsorbers which
are switchable. A distribution box in the 15 m diametertunnel shaft allows to distribute the refrigeration power tothe different consumers (cryogenic units, injectors etc.).
Table 3: Main cryogenic components
type
cryomodules (several types)
vacuum barriers
feed- and end-boxes
cryo-units (2.5 km long)
injector feed-boxes
distribution boxes
injector distribution box
transfer lines
cryo-plants (cold-boxes)
warm screw compressors
cold compressor
liquid helium dewars
warm gas storage vessels

number1781
48
38
123
7
1824 m768
56
731
Fig. 7: Model refrigerator

The electrical power consumption in each cryogenic
hall is shown in Table 4.
Table 4: Required el. power at refrigeration halls [MW]hall1 hall2 hall3 hall4 hall5 hall6 hall7500
GeV 2.40
5 Hz

2.33

3.40

3.38

3.13

3.74

3.39800
GeV 3.424 Hz

3.88

6.35

6.33

5.65

6.62

6.28

DESY. A concept for the cryogenic distribution through2.5 km long cryogenic units supplied by a total of seven
refrigerators has been found. The refrigerators are basedon a sound technology similar to the one used at LHC. Anupgrade for operation up to 800 GeV centre-of-massenergy is foreseen. Costing of all components has been
done and has been submitted to authorities for approval.
More detailed work is necessary, however.

6 REFERENCES

4.4 Upgrade Philosophy
For operation at 800 GeV centre-of-mass, the cavities will
have to be operated at about 37.4 MV/m or only
35 MV/m provided that the new superstructure scheme[7] presently in development at DESY for a morecompact assembly of cavities in the cryomodules is used.
The refrigeration capacity has to be doubled in eachcryogenic hall in that case. For this purpose a second
cold-box must be installed in the refrigeration halls andthe halls must be extended to give space for a second setof warm compressors (Fig. 8). A third cold compressor
chain is also necessary for redundancy. The distribution
box is already prepared for the energy upgrade. Making
use of safety margin for the primary installed plants (for500 GeV centre-of-mass energy operation), a beam pulse
repetition rate of ~ 4 Hz will be possible (see Table 4).

5 SUMMARY AND OUTLOOKThe planning of the cryogenic system for TESLA is welladvanced. The system is based on the well establisheddesign of superconducting 1.3 GHz cavities andcryomodules developed for the TESLA Test Facility atFig. 8: Cryogenic hall for 800 GeV operation[1] TESLA The Superconducting Electron PositronLinear 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] TESLA Collaboration, ed. D. A. Edwards, TESLATest Facility Linac – Design Report, DESY TESLA95-01, 1995.
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Grygiel, K. Jensch, R. Lange, A. Matheisen, W.-D.
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CryogenicAccelerator
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Proceedings of this conference.
[4] S. Wolff, H.Lierl, H. Quack, B. Petersen, TheTESLA Cryogenic Distribution System, Proceedingsof this conference.
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Proceedings
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Engineering Conference ICEC18, Mumbai, India,Narosa Publ., p. 67, 2000.
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