THE TESLA CRYOGENIC DISTRIBUTION SYSTEM
S. Wolff, H. Lierl, B. Petersen, DESY, Notkestr. 85, 22607 Hamburg, Germany
H. Quack, TU Dresden, 01062 Dresden, Germany

Abstract
The construction of the 33 km long 500 GeV centreof-mass energy (upgradeable to 800 GeV) superconducting linear collider TESLA at DESY with an
integrated X-ray FEL facility of 0.1 nm wavelength has
been proposed by an international collaboration. The
collider will consist of more than 21000 superconducting
9-cell 1.3 GHz RF cavities assembled in about 1800
cryomodules containing also a superconducting magnet
package with a quadrupole and dipole steering coils.
The paper describes the cryogenic distribution system
necessary for cooling to 2 K, mainly consisting of twelve
2.5 km long cooling chains (cryogenic units) of
cryomodules, and gives information on the heat load
budget, on operating modes as well as on the energy
upgrade philosophy.

cryogenic units consisting mainly of cryomodules
housing the cavities and magnet packages. The symmetryis broken only at the first 5 km of the electron linac
where the 2 first refrigerators (one on the DESY site, theother 4.7 km away) serve only one unit, each. The reason
is a very crowded urban area near DESY with difficulties
to find a proper space for a refrigeration hall and the factthat higher heat loads occurs here from the additional 5
Hz for the FEL operation. Unit 1 and partly unit 2 are
declined by -8 mrad until the tunnel follows a line
levelled with respect to gravity.

1 INTRODUCTION
The 33 km long 500 GeV centre-of mass energy e+elinear collider TESLA (Tera-eV Energy Superconducting
Linear Accelerator), proposed by an international
collaboration [1] consists of two 15 km long linear
accelerators of 250 GeV each. The linacs pointing to eachother and operated at 5 Hz beam pulse repetition rate areplaced in a 5.2 m diameter underground tunnel startingfrom the DESY site in north-northwest direction. A highenergy experimental area is placed in the centre of thelinacs. The first 3 km of the electron linac are also used
for accelerating electrons for an X-ray Free Electron
Laser (FEL) with wave lengths down to 0.1 nm.
The accelerating structures consist of 1.3 GHz
superconducting 9-cell cavities of pure niobium. There
are more than 21000 cavities to be cooled to 2 K. The
required accelerating gradient for 500 GeV centre-ofmass energy is 23.4 MV/m at a quality factor Q0 = 1010.
The cavities are assembled in about 1800 cryostats(cryomodules) [2] forming a 15 km long cryogenic chain
in each linac. In addition to the main linacs there are 3 einjectors of 500 MeV ( one for standard high energy
beam, one for polarised electrons, one for the FEL beam)and 5 GeV pre-accelerators for e-and e+.

2 CRYOGENIC DISTRIBUTION
The cooling of the collider is performed from 7
refrigerators [3] in above-ground service halls placedalong the linacs at distances of about 5 km in each linac.
Generally each refrigerator supplies two 2.5 km long

Fig. 1: Cryogenic unit (one out of 12)
A cryogenic unit (Fig. 1) consists of about 150
cryomodules, one feed-box, one end-box and vacuum
barriers at distances of 500 m for limiting the pressure
increase in the cryogenic pipes in case of an accidental
insulation-vacuum break [4] and for better leak checking.
In order to limit the flow of liquid in the 2-phase helium
filling tube the 2.5 km cryomodule section is separated
into up to 16 cryogenic strings of about 170 m length.
The cryomodules house all helium tubing necessary forthe cryogenic process [2]. Each standard 16 m longcryomodule contains 12 cavities, housed in titanium
helium vessels, connected with the 2-phase helium fillingtube and supported at the 2 K gas return pipe (GRP)
which also acts as a girder. This cold mass surrounded byinsulation vacuum with thermal shields at 5-8 K and 4080 K is supported at the vacuum vessel by three glassfibre support posts. The cavities are equipped with HOM
couplers, RF antennas, mechanical motor- and piezoelectric-driven tuners [5] as well as main RF input
couplers connecting the cavity end with the outer vacuum
vessel wall. More than 1/3 of the cryomodules also
contain a superconducting magnet package withquadrupole magnets for beam focussing and one or two
dipole magnets for beam steering. A beam position
monitor (BPM) is attached to each magnet helium vessel.
These cryomodules are 17 m long.

For assembly purpose the beam tube at both ends of thecryomodule is closed by hand valves. A higher ordermode (HOM) absorber is placed at the beam tube
interconnection of the modules.
Some special cryomodules with 8 cavities and one
magnet package, respectively 4 cavities and 4 magnet
packages are necessary in the e-/FEL injectors and in thepositron pre-accelerator. A list of main cryogenic
components is shown in Table 1.
A cryogenic string (Fig. 2) generally consists of 10
cryomodules, the last of which contains 3 control valves(two for cool-down and warm-up, one Joule-Thomson(JT)-valve for steady state operation), 3 flow sensors, aliquid helium separator and a small helium reservoir (bothwith level sensors, the reservoir with a heater). In addition
to the above mentioned 2 K helium 2-phase tube there is amanifold for feeding cool-down or warm-up helium to thebottom of the cavity and magnet helium vessels.
A 200 mm diameter warm gas tube connects therefrigerators of each linac in order to allow helium gas
exchange in case of a complete refrigerator break-down.
Table1: Main cryogenic components
typemodules (12 cavities, mo magnet package)
modules (12 cavities, 1 magnet package)
modules (8 cavities, 1 magnet package)
modules (4 cavities, 4 magnet packages)
vacuum barriers
feed-boxes
end-boxes
injector feed-boxes
standard distribution boxes
single distribution box
injector distribution box
transfer lines (m)number1004
742
82748
1919
3
6
1
1824

3 INSTRUMENTATION
For proper control of cooling, RF and beam operation a
lot of instrumentation is necessary. Because of the large
numbers of cavities and cryomodules involved theinstrumentation has to be kept to the necessary minimum.
In the string interconnection cryomodules (the one withthe valves) and in the feed- and end-boxes platinum
sensors as well as low temperature sensors (carbon-glass,CERNOX or TVO) will be installed in the cooldown/warm-up lines, in the 2 K supply line, at theentrance of the 2 K 2-phase helium tube and at the 2 Kgas return pipe for cooling purposes. The cooldown/warm-up lines as well as the 2 K helium entrance tothe string will be equipped with cold flow meters. Liquidlevel sensors will be installed in the liquid separator andin the small helium reservoir, heaters in the reservoir
only. In addition each cryomodule will be equipped withplatinum sensors at each RF main coupler (each cavity)

and at the 50 K intercept of the HTS current leads for themagnet packages. All temperature sensors are installed
redundant. For protection of the current leads voltage taps
are necessary. RF antennas are installed at each cavity
and at the beam position monitor. The mechanical tuner at
each cavity is controlled with a stepping motor and apiezo-electric quartz.
Table 2: Total numbers of instrumentation
typenumber
platinum sensors (Pt)45574
low temperature sensors (Lt)1322
superconducting level sensors (L)748
carbon resistor chains374
cold flow meters (F)580electrical heaters (H)374
HOM antennas
42400
pick-up antennas
21200
e- sensors
21200
BPM antennas
3108
voltage taps
8010
motor steering
21200
piezo-electric quartz steering
21200

Fig. 2: Cryogenic string (C: cavity, Q: quadrupole, Pt:
platinum sensor, Lt: low temperature sensor, F: flow
meter, JT: Joule-Thomson valve, FC: flow control, LC:level control, L: level sensor, H: heater, R: reservoir)

4 OPERATING MODES
During steady state operation, helium of 2.2 K and 1.2
bar is flowing from the distribution box of the refrigerator
via the feed-box through all cryomodules of one unit. At
each string interconnection cryomodule (and at the endbox) a fraction of the helium is diverted and expanded to31 mbar over the JT valve into a liquid helium separator.
From here it flows through the 76 mm diameter 2 K 2phase tube connecting all cavities. This tube is generally
filled to the half with liquid (except for the declined
section in units 1 and 2) thus keeping the helium vesselsof cavities and magnets filled. At the end of the string thehelium is filling a small reservoir equipped with a levelsensor and heater, both used for control of the helium
flow through this branch. At each cryomoduleinterconnection the 2-phase line is connected to the GRP

through which the evaporated helium returns to therefrigerator. The mass flow for nominal operation is 68
g/s (for 1.411 kW load) resulting in a pressure drop of0.33 mbar in the GRP over the unit.
Detailed investigations of the 2-phase Helium II flow
have been made showing temperature, pressure and liquid
level distributions and thus the validity of the concept [6].
During cool-down helium gas of varying temperature is
fed from the distribution box of the refrigerator over thefeed-box through the cryomodules in the line mentioned
above (in the tube later used for one-phase 2 K helium).
At each cryogenic string interconnection the helium isthen conducted via flow meters and control valves into
the manifolds on both sides of this module from which itflows in narrow tubes to the bottom of the cavity andmagnet helium vessel, cooling the cavities and magnets
and returning through the GRP to the refrigerator. Thetime needed to cool-down the about 700 tons (2 units) to
2 K is about 10 days. In addition 2 ½ days are needed tofill the helium vessels with liquid. Similar times areneeded for warm-up where the helium flows in the sameway.

development, gradients of above 35 MV/m may bereached with slightly reduced Q0 (5*109) [7]. This would
allow to operate TESLA at energies up to 800 GeV,
especially if the newly proposed 2-nine-cellsuperstructure version [8] for the cryomodules is used. Asthe dynamic heat load in the cavities scales with thesquare of the gradient, the refrigeration capacity must beincreased considerably. The proposed solution is to
double the refrigeration capacity in the cryogenic halls[3]. An operation at 4 Hz beam pulse repetition rateshould be possible.

7 SUMMARY AND OUTLOOK
A cryogenic distribution system has been proposed forTESLA consisting of 12 cryogenic units of 2.5 km length,
each, with up to 16 about 170 m long cryogenic strings as
the smallest cooling loops. Steady state and transient
operation modes have been investigated. The calculated
heat loads have been used to define design refrigeration
capacities for the refrigeration halls. The proposed
scheme allows an energy upgrade to 800 GeV if therefrigeration capacity is doubled.

5 HEAT LOAD BUDGET

8 REFERENCES

Heat loads have been calculated for all individual
cryogenic components in the cryogenic units starting withthe cryomodules and including vacuum barriers, feed- andend-boxes and transferlines. Assumed is an acceleration
gradient of 23.4 MV/m at Q0=1*1010. Some of the units
contain additional equipment for the injectors and preaccelerators (units 1 and 7), for the superconducting RF
used in the damping rings (units 1 and 10) and for 5
cryomodules used in each bunch compressor (units 1 and12). In units 1 and 2 (partly) the operation of the X-ray
FEL leads to additional heat loads (acceleration gradient21.1 MV/m at Q0=1*1010) . A factor of 1.5 has been
applied to the calculated heat loads for establishing thedesign refrigeration power of the cryogenic plants which
generally serve 2 units except hall 1 (only unit 1) and hall
2 (only unit 2) (Table 3).

[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 C. Pagani, R. Bandelmann, D. Barni, M. Bonezzi, G.
Grygiel, K. Jensch, R. Lange, A. Matheisen, W.-D.
Moeller, H.-B. Peters, B. Petersen, P. Pierini, J.
Sekutowicz, D. Sellmann, S. Wolff, K. Zapfe, TheTESLA
CryogenicAccelerator
Modules,
Proceedings of this conference.
[3] H. Quack, H. Lierl, B. Petersen, S. Wolff, TheTESLA Cryo-Plants, Proceedings of this conference.
[4] B. Petersen, S. Wolff, Numerical Simulations ofPossible Fault Conditions in the Operation of the
TTF/FEL and TESLA Linear Accelerators,
International
Cryogenic
Proceedings
18thEngineering Conference ICEC18, Mumbai, India,
Narosa Publ., p. 67, 2000.
[5] M. Liepe, W. D. Moeller, S. N. Simrock, Dynamic
Lorentz Force Compensation with a Fast
Piezoelectric Tuner, DESY TESLA-00-39, 2000.
[6] Y. Xiang, B. Petersen, S. W. Van Sciver, J. G.
Weisend II, S. Wolff, Numerical Study of Two-Phase
Helium II Stratified Channel Flow, Proceedings ofthe Cryogenic Engineering Conference, July 12-15,
1999 Montreal, Canada, Advances in Cryogenic
Engineering, Vol 45, 1001, 2000.
[7] L. Lilje, Cavity R&D for TESLA, Proceedings of thisconference.
[8] J.
Sekutowicz,M.
Ferrario,C.
Tang,
Superconducting Superstructure for the TESLACollider: A Concepì, Physical Review Special
Topics: Accelerators and Beams, 2:062001, 1999.

Table 3: Design heat loads [kW] at refrigeration halls
Thall1 hall2 hall3 hall4 hall5 hall6 hall72 K 3.70 3.07 4.23 4.21 3.87 5.13 4.225 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

6 ENERGY UPGRADE PHILOSOPHY
The centre of mass energy of the proposed TESLA is500 GeV. Seeing the progress achieved in the cavity