The superconducting TESLA linear accelerator will consist of about 1800 cryomodules. Before their installation in the TESLA tunnel, the acceleratorcryomodules have to be qualified after the assembly in random tests. The qualification includes the check of the general mechanical dimensions and the measurement of the cryogenic performance of all systems, in particular,the performance of the cavities. In addition, all about 21000 single cavities have to be tested before the assembly in the cryomodules at a rate of about 24 per day. The paper presents the layout of the test facility

Möller, W D

Petersen, B

Sparr, B

Wolff, S

Petersen, B.

Moller, W. D.

Sparr, B.

Wolff, S.

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The TESLA Accelerator Module Test Facility

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THE TESLA ACCELERATOR MODULE TEST FACILITYB.Petersen,W.D.Moeller, B.Sparr, S.Wolff,DESY, Notkestrasse 85, D-22607 Hamburg, GermanyAbstractThe superconducting TESLA linear accelerator willconsist of about 1800 cryomodules. Before theirinstallation in the TESLA tunnel, the acceleratorcryomodules have to be qualified after the assembly inrandom tests. The qualification includes the check of thegeneral mechanical dimensions and the measurement ofthe 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 INTRODUCTIONThe Tera-eV Energy Superconducting LinearAccelerator (TESLA) will consist of about 21000superconducting 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 40-80 K and 5-8 K temperature levels respectively.Before their installation in the TESLA tunnel, the about1800 accelerator cryomodules have to be qualified. Thequalification includes the check of the general mechanicaldimensions and the measurement of the cryogenicperformance of all systems, in particular, the performanceof the cavities. At the start of the cryomodule seriesproduction the tests will cover 100 % of the cryomodules,in order to check and adjust the fabrication. As soon asthe 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 singlecomponents, 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 likeHERA and LHC, the low performance of singlecryomodules can be tolerated for the operation of the linacto some extent.For the time being, the performance of singlesuperconducting RF-cavities can only be monitored bycryogenic tests. Industrial studies [1] have shown that theeffort, needed for the cryogenic tests of single cavitiespays, as soon as the failure rate of the single cavitiesexceeds 1.2 %. Also the overall accelerating gradient ofthe 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 TESLAcryomodules more than 21 000 single cavities have to betested at a rate of about 24 cavities per day.In addition, about 800 packages of superconductingmagnets (quadrupoles and steering dipoles) have to betested in vertical dewars at an average rate of one packageper 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 intothe series production in order to avoid failures and toincrease the performance.2 DEFINITION OF THE TESTPROGRAMMES2.1 Test Programme for the Vertical Tests ofSingle Cavities and Magnet PackagesThe maximum accelerating field and the correspondingunloaded quality factor Qo of each single cavity will bemeasured at a temperature of 2 K in a bath cryostat. Toincrease 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 performancewill be fed back into the preparation procedures.The superconducting performance of the magnet packageswill also be tested in the bath cryostats. In particular thecurrent performance will be monitored. Two magnetpackages will be tested at a time in one cryostat.2.2 Test Programme for the Accelerator ModuleTest BenchesThe cryogenic performance tests include the integralleak check of all vacuum systems and cryogenic processtubes, the test of the instrumentation, measurements of thestatic heat loads and of the maximum accelerating field ofthe cavities. The corresponding unloaded quality factor Qowill be monitored by cryogenic heat loss measurements.During the performance tests of the cavities, also the x-ray radiation and the related dark currents will bemeasured by means of two ionisation chambers at eachtest 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 packageswill also be tested.About 170 cryomodules will be equipped with cryogenicvalves, temperature sensors, flow sensors and liquid levelindicators (string interconnection equipment). Also thisinstrumentation will be tested on the test benches.According to estimates of the test schedule [2], abouteight to nine days are needed for the complete test of oneaccelerator cryomodule, including the mechanicalassembly and disassembly on the test bench.3 LAYOUT OF THE ACCELERATORMODULE TEST FACILITY3.1 General LayoutFrom the given test rates and the estimates of the testschedules [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 ofabout 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 surroundedby a concrete shielding of 0.8 m thickness in order toestablish radiation safety in all parts of the test hall. Aroof shielding of 0.8 m thickness will cover the testbenches. Lead plates will be installed at both ends of thetest benches in addition. There will be an inner space of23 m x 4.4 m x 2.5 m for one test bench. Three testbenches 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.3.2 Cryogenic SupplyFor the continuous operation of the test hall, coolingcapacities of about 10 KW at 40-80 K, 1.0 KW at 4.5 Kand 0.6 KW at 2.0 K are needed. In order to reduce theeffort of the helium distribution system, only the 40-80 Kand 4.5 K circuits will be branched to the test benches andthe test dewars. At the cryomodule test benches as well atthe 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 warmcompressor systems will be used to lower the vapourpressure of the helium baths to 31 mbar [4]. As a result, ahelium liquefaction rate of about 50 g/s has to be suppliedon average from the helium plant. Peak liquefaction rateswill be buffered by means of two 10 m3 liquid heliumstorage reservoirs.Figure 1: Ground Plan of the Accelerator Module TestFacilityThe test facility can by connected to any heliumrefrigerator plant, which can supply the capacitiesmentioned, in particular to the HERA refrigerator orTESLA refrigerator on the DESY site.3.3 Cryomodule Test BenchesThe cryogenic supply of the test benches is divided intotwo 'layers', in order to operate the test benchesindependently from each other, to avoid air condensationon cryogenic valves during the exchange and installationof modules, and to reduce the consequences of leakyvalves. One layer are the feed-boxes of each module testbench, 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 cryogenic40-80 K and 4.5 K shield circuits as well as the 2 Kcircuits of the cryomodules, each cryomodule test benchhas to be equipped with a feed-box, a feed-cap and anend-cap. The feed-box will be fixed on the ground insidethe shielding of the test bench and will contain cryogenicvalves for each supply and return tube, a small 4.5 K sub-cooler and an 2 K counter flow heat-exchanger. The 4.5 Ksub-cooler of the feed-box is used to stabilize the supplytemperatures of the 4.5 K and 2 K circuits. All safety ventlines and relief valves are connected to the feed-box. Thefeed-cap, which is attached to the feed-box will containall connection flanges to the cryomodule. The end capwill be fixed to the module support structure and willcontain only the short circuits of all cryogenic tubesFigure 2.: Block Diagram of the Cryogenic SystemCP= cryogenic plant (HERA or TESLA), HDB= halldistribution box, SCVB= sub-cooler & valves box, MTB=module test bench (consisting of a feed-box, a feed-capand an end-cap), HC= helium compressor, SD= storagedewar, DB=distribution box, VD= vertical dewarand a 2 K liquid reservoir. The position of each end-capcan 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 inthe end cap all supply and return tubes are equipped withredundant thermometers. Redundant liquid levelindicators and pressure sensors are installed in the 2 Kreservoir of the end cap. The pressures of all cryogeniccircuits will be monitored.Each group of three feed-boxes will be supplied viatransfer lines from one sub-cooler & valves-box. Theconnecting 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 andcryogenic valves for all process tubes of the differenttransfer 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 sub-cooler & valves-boxes all cryogenic supply and returntubes are separated from the test bench by two cryogenicvalves in series. The sub-cooler & valves-boxes will besupplied from the test hall distribution-box (see Fig.2).The 2 K vapour return tubes of three-test bench feed-boxes are connected to one 300K-helium compressor unitrespectively. In the exhausts of the compressor units thepumped mass flows can be measured by warm gas flowmeters. At a constant liquid helium level in the 2 K liquidreservoirs of the end caps, the heat load of the 2 K circuitcan be monitored with a resolution in the order of +/- 0.1W by means of these flow meters.3.3 Vertical Test CryostatsAlso 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 testfacility [4]. The vertical dewars will contain a 2.2 m3liquid helium volume, cryogenic valves and a 2 K counterflow heat-exchanger, installed in the vacuum jacket.Three dewars will be connected to one distribution-boxrespectively. The distribution-boxes will contain allwarm-up and cool-down connections and valves. One 10m3 liquid storage dewar will be connected to eachdistribution box respectively, in order to buffer the loadsfor the refrigerator. The storage dewars will be suppliedfrom the test hall distribution-box and will also be used assub-coolers.3.4 RF SystemsIn order to avoid time consuming installations of the RFequipment between the module tests benches, one 10 MWklystron 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 aTESLA Accelerator Module Test Facility TESLAReport 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 TTFCryogenic System, Adv. in Cryogenic Engineering,Plenum Press, Vol. 41 a, pp. 847-854, N.Y. (1996)

The TESLA accelerator module test facility

The TESLA accelerator module test facility

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