THE TESLA CRYOGENIC ACCELERATOR MODULES
C. Pagani1, R. Bandelmann2, D. Barni1, M. Bonezzi1, G. Grygiel2, K. Jensch2, R. Lange2,A. Matheisen2, W.-D. Moeller2, H.-B. Peters2, B. Petersen2, P. Pierini1, J. Sekutowicz2,
D. Sellmann2, S. Wolff2, K. Zapfe2
1

INFN Milano-LASA, Milano, Italy
2
DESY, Hamburg, Germany

Abstract
The Tera-eV Energy Superconducting Linear
Accelerator (TESLA), a 33 km long superconducting
linear electron/positron collider of 500 GeV (upgradeableto 800 GeV) centre of mass energy, presently in theplanning phase at DESY, will consist of about 21000
superconducting RF 9-cell cavities of pure Niobium. Eachcavity of about 1 m length has to be cooled in a 2.0 K
helium bath and operated at 1.3 GHz with 5 Hz beam
repetition rate. The cavities, equipped with theirindividual helium vessels made from titanium, RF input
couplers , tuners and magnetic shielding, will beassembled in groups of 12 in about 16 m long cryostats(cryomodules) with thermal radiation shielding at 5-8 Kand 40-80 K. Part of the cryomodules will also beequipped with superconducting quadrupoles and steeringcoils. About 10% of the cryomodules will containcryogenic valves and instrumentation required for cooling
purposes.
The paper presents design, construction and assembly
details of the cryomodules, which are based on theexperience with somewhat shorter cryomodules, alreadyin use at the TESLA Test Facility (TTF) at DESY.

2 CRYOSTAT
Figure 1 shows a cross section of the cryostat. The
cryostat outer vacuum vessel is constructed from carbon
steel and has a standard diameter of 38’’ (0.965 m).
Adjacent vacuum vessels are connected to each other bymeans of a cylindrical sleeve with bellows, which iswelded to the vessels during installation.
Pumping stations every 250 m will provide the insulatingvacuum.
The following helium lines are integrated in thecryomodule:· 2 K forward line of single phase pressurized helium,· 2 K two phases supply line,· 2 K Gas Return Pipe (GRP) which returns the gas
pumped off the saturated He II,· 5-8 K forward and return lines for cooling the inner
thermal shield and for heat flow intercept of the maincoupler,· 40-80 K forward and return lines for cooling thesuperconducting current leads and the outer thermal
shield [3].
·1 INTRODUCTION
The design of the cryomodule for the TESLA project[1] has been essentially driven by the cost cutting withrespect to the existing superconducting systems. Themodule design is based on the 3rd generation of theTESLA test facility (TTF) one [2]; the length, which has
been increased to 16 m (17 m for the modules with aquadrupole package), has been the major change. The
driving criterion followed in order to decrease theproduction cost has been to significantly reduce thenumber of cryogenic components (especially cryogenic
supply boxes and cold-warm transition). This is obtainedby having a single long continuous string of about
2.5 km, called cryogenic unit [1]. Each long module
contains twelve 9-cell cavities (and a cold quadrupole
package in approximately 1/3 of the modules). The
modules are directly connected to each other in acryogenic unit.

Fig. 1: Cryostat cross section.

Fig.2: Longitudinal cross section of a 17 m long cryostat with 12 cavities and a central quadrupole. The GRP behaves as
the supporting structure and it is suspended to the vacuum vessel through three fiberglass posts.

2.1 GRP and cryogenic supports
The GRP acts as a reference beam to which cavities andquadrupoles are attached. Three cryogenic supports
connect the GRP to the external vacuum vessel, thecentral one is fixed while the two laterals can slide tocompensate for the thermal differential shrinkage. Eachcryogenic support includes, for thermal insulation, afiberglass pipe, the post, that is terminated by two shrinkfit stainless steel flanges. Two other intermediate
aluminum flanges are also integrated in the post design, tosupport the thermal shields. The diameter of the fiberglass
pipe is larger than what is strictly required, in order toraise the mechanical eigenfrequencies of the system.
During cooldown the two edges of the ~ 17 m long GRP
move towards the center of the module by up to 26 mm.
To keep the cold input coupler head of each cavity
longitudinally fixed within an accuracy of 1 mm, eachcavity is anchored to the GRP via a sliding fixture, while
its longitudinal reference is given by an independent
system of invar and stainless steel rods [2]. The
longitudinal position of the two external posts has been
set to minimize the GRP top-down deformation, whose
value is affecting the fabrication cost. Finally, because themagnet package position tolerances are stricter than thoseof the cavities, the package has been placed just below thefixed central post. In this position the GRP deformations,
induced by the unavoidable forces generated by thebellows at the welded connection between two
consecutive cryostats [3], are minimal.

2.2 Thermal Shields
The cryogenic optimization of TESLA [1,3] requires twothermal shields at 5-8 K and at 40-80 K, respectively.
Both shields are manufactured from aluminum sheets(1050 alloy) and include a stiff upper part (divided in twosections) that supports 12 lower panels, one in front ofeach cavity (13 in the modules with quadrupoles). An
aluminum pipe, finger-welded to the shield roof and to
the panels [4] cools each shield. Bimetallic junctions areused to connect the aluminum pipe to the stainless steeltransition between cryomodules. The shield geometry andpartitioning have been checked by a finite element code
to test for cooldown deformations and thermal inertia. As
a reference, a 12-hour fast linear cooldown induces

deformations limited to less than 10 mm, which arecompatible with the geometric free space in the section.
In this extreme case, due to the asymmetric cooling, eachshield is bent laterally ("banana" effect), while all stresses
are below 30 MPa.
The shield panels are also used to indirectly cool othercomponents of the cryomodule as: the fiberglass postintermediate plates and the body of the power couplers.
The post plates, which support the cold mass and theshield roofs, need to be kept at different temperatures. In
particular, the lateral post plates are connected to theshields with sliding supports that do not assure a goodheat exchange. To achieve the post cooling, short andvery flexible copper braids have been developed andused. The same solution has been applied for the coupler
heat intercepts (called cones for their original shape). Thissolution has been already qualified during the cooldownof the second cryomodule. Data from the temperature
sensors showed that all behaviors are as predicted.
Blankets of multi-layer insulation (MLI) are placed on theoutside of the 5-8 K and the 40-80 K shields. A 10 layer
blanket is sufficient on the lower temperature shield whilea 30 layer one is required on the other. In addition 5
layers of MLI are wrapped on the cavity and quadrupole
helium vessels, the gas return pipe and the 5-8 K pipes toreduce heat transfer in the event of a vacuum failure.
The heat loads of a 17 m long cryostat with 12 cavities
and one magnet package have been calculated [1] making
use of the experience gained from other large cryogenic
systems (in particular HERA and TTF). Table 1
summarizes the quoted results. It is worthwhile to note
that, even at the chosen RF duty cycle of 0.5%, the cavity
Q being as high as 1·1010, the total cryogenic losses are
dominated by the RF dynamic losses at 2 K.

Table 1. Heat load of the 17 m long cryostat (12 cavities
and a quadrupole).
Temp
2K
5-8 K
40-80 K

Static [W]
1.74
11.32
90.13

Dynamic [W]
7.31
4.62
92.89

2.4 Assembly and alignment

Fig 3. The cavity sliding system. The rod defines thelongitudinal position. The low friction support frees thelongitudinal motion during cooldown-warmup cycles.

2.3 Sliding cavities supports
The experience gained during the assembly of the first
three TTF cryomodules suggested to improve the cavity
fixture design [2,5] in terms of maintaining the cavity
alignment [3]. Due to the system design, it was necessary
to follow a complicate procedure to keep cavities andmagnet positions within the alignment requirements. Inorder to develop a fixture system compatible withsuperstructure [1] and semi-fixed couplers [2] wecompletely redesigned the cavity supports. The solutionthat has been studied and implemented keeps the cavity
transverse position fixed, while leaving the cavity
longitudinal position independent from the GRP thermal
contraction and extension during the cooldown-warmup
cycles. These objectives have been reached with a set of
low friction sliding supports [5]. These supports, whose
section is presented in Fig. 3, consist of a C-shaped
stainless steel element that clamps a titanium pad, weldedon the cavity helium tank. The connection is mediated bya sequence of rolling needles, runners and reference
screws. In each constrained direction (vertical and lateral)a reference screw defines the cold position of the cavity
axis, and a spring washer package, loaded at 800 N, keeps
the pad into contact. To qualify the new fixture scheme,
the friction coefficient, between the cavity pad and thesliding fixture, has been measured reproducing all thepossible extreme operational conditions [5]. The staticand dynamic force required to move the system wasalways below 6 N. This low friction value results in agood decoupling of the cavity longitudinal position withrespect to the supporting GRP. Moreover, In order to becompatible with semi-rigid couplers and superstructures[1] the longitudinal position has to be fixed. As for theTTF-Phase2 cryomodules [2], the six more internal
cavities are clamped to an Invar rod, that is fixed at thecenter post position and runs along the module.
Conversely, to compensate for the excessive contractionof the Invar rod in the case of a 17 m cryomodule, themost external cavities are indirectly connected to theInvar rod via a stainless steel rod, to limit the overalllongitudinal displacement of the cavity positions to lessthan ±1mm, as required.

The cryomodule design, including fabrication procedures
and tolerances, is the key factor to obtaining the requiredalignment precision for cavities and magnets, while at thesame time limiting the costs. The alignment of theindividual active components, cavities and quadrupoles, isperformed once they are anchored to the supporting GRP.
The alignment of the GRP, via three Taylor-Hobson
spheres, is used as a reference for the axis of the activecomponents. Assembly and cool down do not affect thisexternal reference, except for a predictable andreproducible parallel vertical motion.
A crucial step in cutting the production cost was to relax
the general tolerances of the GRP and of the vacuum
vessel, while using a long milling machine to preciselyreference (after welding of ancillary components) all theimportant axes and planes.
The GRP is now fabricated according to the standard
high-quality welded pipe tolerances (5 mm), includingstraightness. After welding of the cavity supports, lower
post flanges and end flanges, the axis is defined on themilling machine according to the end flange centers andtransferred to the lower post flanges. The interconnection
bellows, properly fastened, are already included tominimize the inter-module lateral forces. The pipe is thenturned and the cavity supports are referenced to thedefined GRP axis. A similar procedure is also applied to
the vacuum vessel to reference the coupler ports andupper flanges to the vessel axis. After welding and before
milling, a stress relief is performed on the GRP to avoid
permanent deformations induced by thermal cycling.
The module assembly is performed in three major steps:
1. The cavity string is prepared in the class 10 cleanroom, pre-aligned and closed;
2. The string is then transferred to the assembly area
where it is attached to the aligned GRP;
3. Each cavity and quadrupole is aligned with respect to
the GRP using the three optical targets on the posts(Taylor-Hobson spheres) and then fixed.
The module can now be moved and the alignment can berecovered through the external targets. The last step isperformed in a third assembly station where, after theassembly of the thermal shields, a long fixed cantilever
and a movable cart are used to insert the cold mass into
the vacuum vessel. Module alignment in the linac is madeusing three optical targets placed on the vacuum vessel
and referenced to those on the GRP.

6 REFERENCES[1] TESLA Technical Design Report, DESY-2001-000,
ECFA-2001-209, TESLA-2001-000, FEL-2001-000,
March 2001.
[2] C. Pagani, D. Barni, M. Bonezzi and J. G. Weisend
II, Adv. in Cryo. Eng., Vol. 45A, 2000, p. 939.
[3] S. Wolff, in Proceedings of this conference.
[4] C. Pagani, D. Barni, M. Bonezzi, J. G. Weisend II,
Adv. in Cryo. Eng., Vol. 43A, 1998, p. 307.
[5] D. Barni, M. Castelnuovo , M. Fusetti, C. Pagani.
G. Varisco, Adv. in Cryo. Eng., Vol. 45A, 2000,
p. 905.