THE TESLA ACCELERATOR MODULE TEST FACILITY
B.Petersen,W.D.Moeller, B.Sparr, S.Wolff,DESY, Notkestrasse 85, D-22607 Hamburg, GermanyAbstract
The superconducting TESLA linear accelerator willconsist of about 1800 cryomodules. Before theirinstallation in the TESLA tunnel, the accelerator
cryomodules have to be qualified after the assembly inrandom tests. The qualification includes the check of thegeneral mechanical dimensions and the measurement of
the cryogenic performance of all systems, in particular,theperformance of the cavities.
In addition, all about 21000 single cavities have to betested before the assembly in the cryomodules at a rate ofabout 24 per day. The paper presents the layout of the testfacility.

1 INTRODUCTION
The Tera-eV Energy Superconducting Linear
Accelerator (TESLA) will consist of about 21000
superconducting RF 9-cell cavities of pure niobium,cooled in a 2.0 K helium bath. The cavities will beassembled in groups of 12 in about 16-17 m longcryostats (cryomodules). Among other things, thecryomodules will be equipped with thermal shields at 4080 K and 5-8 K temperature levels respectively.
Before their installation in the TESLA tunnel, the about
1800 accelerator cryomodules have to be qualified. The
qualification includes the check of the general mechanicaldimensions and the measurement of the cryogenic
performance of all systems, in particular, the performanceof the cavities. At the start of the cryomodule series
production the tests will cover 100 % of the cryomodules,
in order to check and adjust the fabrication. As soon as
the non-acceptance rate will decrease below a value in theorder of 1 %, only about 25 % of the cryomodules will betested.
These random tests can be applied with low risk for thecommissioning of the accelerator, because all main single
components, like cavities and magnet packages, will betested to an extent of 100 % before the assembly of thecryomodules. In addition and in contrast to the operationof superconducting high energy physics storage rings like
HERA and LHC, the low performance of single
cryomodules can be tolerated for the operation of the linac
to some extent.
For the time being, the performance of single
superconducting RF-cavities can only be monitored bycryogenic tests. Industrial studies [1] have shown that theeffort, needed for the cryogenic tests of single cavities
pays, as soon as the failure rate of the single cavities
exceeds 1.2 %. Also the overall accelerating gradient of
the linear accelerator can be improved in the range ofmore than 10% by sorting of the RF-cavities according totheir performance. For the fabrication of the TESLA
cryomodules more than 21 000 single cavities have to betested at a rate of about 24 cavities per day.
In addition, about 800 packages of superconducting
magnets (quadrupoles and steering dipoles) have to betested in vertical dewars at an average rate of one package
per day.
In general, with the exception of the tests of prototypes,
the tests will aim only at quality insurance. For thecavities as well as for the cryomodules the test resultsfrom the test facility will be used as a fast feedback into
the series production in order to avoid failures and toincrease the performance.

2 DEFINITION OF THE TESTPROGRAMMES
2.1 Test Programme for the Vertical Tests ofSingle Cavities and Magnet Packages
The maximum accelerating field and the corresponding
unloaded quality factor Qo of each single cavity will bemeasured at a temperature of 2 K in a bath cryostat. To
increase the throughput of the vertical tests stands, eightcavities are assembled in one cryostat insert. According toestimates of the test schedule [2], about 20 hours areneeded for one complete test run for eight cavities,
including the assembly and disassembly of the insert tothe cryostat and the cool-down and warm-up procedures.
The inserts will be assembled at the cavity preparation siteand delivered to the test facility. Cavities, which will havepassed the tests successfully, will be sent to thecryomodule manufacturer. Cavities with low performance
will be fed back into the preparation procedures.
The superconducting performance of the magnet packages
will also be tested in the bath cryostats. In particular thecurrent performance will be monitored. Two magnet
packages will be tested at a time in one cryostat.

2.2 Test Programme for the Accelerator Module
Test Benches
The cryogenic performance tests include the integral
leak check of all vacuum systems and cryogenic process
tubes, the test of the instrumentation, measurements of thestatic heat loads and of the maximum accelerating field of
the cavities. The corresponding unloaded quality factor Qo
will be monitored by cryogenic heat loss measurements.
During the performance tests of the cavities, also the xray radiation and the related dark currents will bemeasured by means of two ionisation chambers at eachtest bench and by dark current monitors at both ends ofthe beam tube of the cryomodules, respectively.
The pre-conditioned RF-couplers will be furtherconditioned. The RF-phase will be adjusted in the order of+/- 20 degrees. There will be the possibility to treat thecavities by means of high peak power processing [3].
The instrumentation of superconducting magnet packages
will also be tested.
About 170 cryomodules will be equipped with cryogenic
valves, temperature sensors, flow sensors and liquid levelindicators (string interconnection equipment). Also this
instrumentation will be tested on the test benches.
According to estimates of the test schedule [2], about
eight to nine days are needed for the complete test of one
accelerator cryomodule, including the mechanicalassembly and disassembly on the test bench.

helium liquefaction rate of about 50 g/s has to be supplied
on average from the helium plant. Peak liquefaction rates
will be buffered by means of two 10 m3 liquid helium
storage reservoirs.

3 LAYOUT OF THE ACCELERATORMODULE TEST FACILITY
3.1 General Layout
From the given test rates and the estimates of the test
schedules [2], it follows that 6 test benches forcryomodules and at least 3 vertical dewars have to beinstalled in the module test facility.
The number of test benches includes an overcapacity of
about 20 % for the test of 25 % of the modules. In order toincrease the efficiency of the vertical tests, 6 verticaldewars are foreseen(see Fig.1).
Each individual module test bench has to be surrounded
by a concrete shielding of 0.8 m thickness in order toestablish radiation safety in all parts of the test hall. A
roof shielding of 0.8 m thickness will cover the test
benches. Lead plates will be installed at both ends of thetest benches in addition. There will be an inner space of
23 m x 4.4 m x 2.5 m for one test bench. Three test
benches will be grouped together as a block.
The vertical dewars will be inserted into ground pits andcovered by movable concrete shielding blocks of 1.2 mthickness. The cryostat inserts will be transported fromthe storage area into the cryostats by use of the hall crane.

Figure 1: Ground Plan of the Accelerator Module Test
Facility
The test facility can by connected to any helium
refrigerator plant, which can supply the capacities
mentioned, in particular to the HERA refrigerator orTESLA refrigerator on the DESY site.

3.2 Cryogenic Supply

3.3 Cryomodule Test Benches

For the continuous operation of the test hall, cooling
capacities of about 10 KW at 40-80 K, 1.0 KW at 4.5 K
and 0.6 KW at 2.0 K are needed. In order to reduce theeffort of the helium distribution system, only the 40-80 K
and 4.5 K circuits will be branched to the test benches andthe test dewars. At the cryomodule test benches as well at
the vertical cryostats the 2 K liquid helium will besupplied by the isenthalpic expansion of the 4.5 K helium,
which will be sub cooled to 2.2 K before the expansion bymeans of counter flow heat exchangers. Distributed warm
compressor systems will be used to lower the vapour
pressure of the helium baths to 31 mbar [4]. As a result, aThe cryogenic supply of the test benches is divided into
two 'layers', in order to operate the test benches
independently from each other, to avoid air condensation
on cryogenic valves during the exchange and installationof modules, and to reduce the consequences of leaky
valves. One layer are the feed-boxes of each module test
bench, the other layer are the sub-cooler & valves-boxes,
to which a group of three module test benches will beconnected (see Fig.2). In order to supply the cryogenic
40-80 K and 4.5 K shield circuits as well as the 2 K
circuits of the cryomodules, each cryomodule test bench
has to be equipped with a feed-box, a feed-cap and anend-cap. The feed-box will be fixed on the ground inside
the shielding of the test bench and will contain cryogenic
valves for each supply and return tube, a small 4.5 K subcooler and an 2 K counter flow heat-exchanger. The 4.5 K
sub-cooler of the feed-box is used to stabilize the supply
temperatures of the 4.5 K and 2 K circuits. All safety ventlines and relief valves are connected to the feed-box. The
feed-cap, which is attached to the feed-box will containall connection flanges to the cryomodule. The end cap
will be fixed to the module support structure and willcontain only the short circuits of all cryogenic tubes

cryogenic valves for all process tubes of the different
transfer line branches, and all corresponding warm-up andcool-down tubes and valves connected to the transferlines. The sub-cooler has to compensate the heat losses ofthe transfer lines from the refrigerator to the test benches.
As a result of the combination of feed-boxes and subcooler & valves-boxes all cryogenic supply and return
tubes are separated from the test bench by two cryogenic
valves in series. The sub-cooler & valves-boxes will be
supplied from the test hall distribution-box (see Fig.2).
The 2 K vapour return tubes of three-test bench feedboxes are connected to one 300K-helium compressor unit
respectively. In the exhausts of the compressor units thepumped mass flows can be measured by warm gas flow
meters. At a constant liquid helium level in the 2 K liquid
reservoirs of the end caps, the heat load of the 2 K circuit
can be monitored with a resolution in the order of +/- 0.1
W by means of these flow meters.

3.3 Vertical Test Cryostats
Also for the supply of the vertical dewars two 'layers' ofvalves in the dewars and in distribution-boxes will beinstalled (see Fig.2). The design of the vertical dewars canbe scaled from the vertical test dewars of the TESLA test
facility [4]. The vertical dewars will contain a 2.2 m3
liquid helium volume, cryogenic valves and a 2 K counter
flow heat-exchanger, installed in the vacuum jacket.
Three dewars will be connected to one distribution-box
respectively. The distribution-boxes will contain all
warm-up and cool-down connections and valves. One 10
m3 liquid storage dewar will be connected to each
distribution box respectively, in order to buffer the loads
for the refrigerator. The storage dewars will be supplied
from the test hall distribution-box and will also be used as
sub-coolers.
Figure 2.: Block Diagram of the Cryogenic System
CP= cryogenic plant (HERA or TESLA), HDB= hall
distribution box, SCVB= sub-cooler & valves box, MTB=
module test bench (consisting of a feed-box, a feed-cap
and an end-cap), HC= helium compressor, SD= storage
dewar, DB=distribution box, VD= vertical dewar
and a 2 K liquid reservoir. The position of each end-cap
can be adjusted easily to the different lengths ofcryomodules.
Each cryogenic supply tube in the feed-box is equippedwith a Venturi flow meter. In the feed-cap as well as in
the end cap all supply and return tubes are equipped withredundant thermometers. Redundant liquid levelindicators and pressure sensors are installed in the 2 K
reservoir of the end cap. The pressures of all cryogenic
circuits will be monitored.
Each group of three feed-boxes will be supplied via
transfer lines from one sub-cooler & valves-box. The
connecting transfer lines consist of 4.5 K supply, 4.5 Kreturn, 40K supply and 80 K return tubes. The sub-cooler& valves-box contains one 4.5 K sub-cooler and3.4 RF Systems
In order to avoid time consuming installations of the RF
equipment between the module tests benches, one 10 MW
klystron and modulator set will be installed at eachaccelerator module test bench, including a permanent RFdistribution system.
The vertical tests will be supplied by additional RFsystems, which will operate in cw-mode.

4 REFERENCES[1] ACCEL industrial study[2] W.D.Moeller, B.Petersen, B.Sparr: A Proposal for a
TESLA Accelerator Module Test Facility TESLA
Report No. 2001-08[3] J. Graber, PhD thesis, Cornell University, 1993
[4] G.Grygiel, U.Knopf, R.Lange, B.Petersen, T.Peterson,D.Sellmann, and J.Weisend II, Status of the TTF
Cryogenic System, Adv. in Cryogenic Engineering,Plenum Press, Vol. 41 a, pp. 847-854, N.Y. (1996)