TESTS OF THE ACCELERATING CRYOMODULES FOR THEEUROPEAN X-RAY FREE ELECTRON LASER

M. Wiencek, B. Dzieza, W. Gaj, D. Karolczyk, K. Kasprzak, L. Kolwicz-Chodak, A. Kotarba,A. Krawczyk, K. Krzysik, W. Maciocha, A. Marendziak, K. Myalski, Sz. Myalski, T. Ostrowicz,

B. Prochal, M. Sienkiewicz, M. Skiba, J. Swierblewski, J. Zbroja, A. Zwozniak,

The H. Niewodniczanski Institute of Nuclear Physics Polish Academy of Sciences, Krakow, Poland

AbstractThe European X-ray Free Electron Laser (XFEL) is cur-

rently under construction in Germany in Hamburg area.A 2.1 km long superconducting linear accelerator, part ofthe XFEL, consists of 101 accelerating cryomodules. TheXFEL cryomodule is assembled with eight superconduct-ing RF cavities, one cold magnet and Beam Position Mon-itor (BPM). The cryomodules are tested in dedicated testfacility before installation in the XFEL tunnel. The testingprocedures for the cryomodules were prepared with use ofDESY expertise from TTF (Tesla Test Facility) Collabo-ration and FLASH (Freie-Elektronen-Laser in Hamburg).This paper describes the full set of testing procedure andincoming and outgoing inspections as well.

INTRODUCTIONThe cryomodules for accelerating part of the XFEL are

assembled in CEA Saclay (France) and delivered to DESY(Hamburg). After delivery the cryomodules are beingchecked during so called incoming inspection and followedby their preparation for the tests. Next step is cooling downof the cryomodule to the temperature of 2K. Then, RF andcryogenics measurements are performed. The cryomoduleis being warmed up while the measurements are nished.The nal step is performance of the outgoing inspection.According to the current estimation 14 working days orga-nized in two shifts are needed to complete the test of onecryomodule. All of the steps mentioned above are done byThe H. Niewodniczanski Institute of Nuclear Physics Pol-ish Academy of Sciences (IFJ PAN) team [1]. Currentlyover 30 enginners and technicians from IFJ PAN are in-volved in testing the cryomodules and RF cavities in newAMTF (Accelerator Modules Test Facility) Hall at DESYin Hamburg. Performance of all cryomodules tests is a partof Polish in-kind contribution for European XFEL.

INCOMING INSPECTIONDuring transportation the cryomodules are equipped

with vacuum monitoring system and shock loggers. Datafrom these devices are read as the rst step of the incominginspection. If results of the read are acceptable the restora-tion of high vacuum in the coupler line is performed. Next,mechanical and electrical checks are performed. The me-chanical inspection consists of removal of transport caps

and visual checks of all anges and process pipes. Theelectrical inspection consists of checks of all connectorand feedthroughs and functionality test of cavities and cou-plers tuners. The measurements of the fundamental modespectra of each cavity follow the mechanical and electri-cal checks. Finally, resistance test of the magnet coils isperformed.

PREPARATION FOR TESTSAfter the incoming inspection the cryomodule is in-

stalled on the test stand. At the beginning the cryomoduleis mounted on a special trolley, transported to test stand andaligned. Next step is to connect the beam line to the vac-uum system in a clean room conditions. The mass spec-trometry is performed to ensure that there is no contamina-tion in the beam line. Example of the mass spectrometryresults are shown in Figure 1.

Figure 1: Mass spectrum of the beam line.

Then the following mechanical operations are per-formed:

• connection of process pipes,• closing sliding muffs,• leak check of the process pipes and the cryomodule,• wrapping the process pipes with MLI (Multi LayerIsolation),

• installation of the thermal shields,• wrapping the thermal shields with MLI,• pump down of the isolation vacuum

When the preparation is done the electrical and waveg-uides connections are made. Finally, a warm coupler con-ditioning is performed to clean impurities from couplers.

MOP054 Proceedings of SRF2013, Paris, France

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09 Cavity preparation and production

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COOL DOWN

Test of the XFEL module is performed at 2K, thus allcomponents of the cryomodule are step by step cooleddown to operation temperature. One can highlight fewphases in the cool down process. During the rst phase thetemperature of the module is lowered to 80K with manytemperature and pressure constraints. This prevents themodule from thermal stresses caused by large temperaturegradients. Around 80K most of dislocations in the mate-rial are blocked, thus, the cool down process of the mod-ule is proceed to 4.5K. When all vessels are lled up byliquid helium the last phase of the cool down can be per-formed. The pressure above liquid helium bath is loveredto 31 mbar. This provides temperature of 2K and the cooldown process is considered as nished. The cool down pro-cess is shown in Figure 2. Conditioning of the couplers isalso performed during the cooling down.

Figure 2: Automatic cool down procedure plot.

RFMEASUREMENTS AT 2K

The RF measurements in cryogenic conditions are di-vided in two phases. The rst phase is measurements atlow RF power while the second one is - at high RF power.The former measurements are performed by means of Vec-tor Network Analyser (VNA) an RF ampli ers if needed.The latter measurements are performed with klystron.

Measurements at Low RF PowerThere is a necessity to measure attenuation of the cables

inside the module at 2K. This measurement is performed asa S11 measurement and has to be done inside the test stand.All the other mentioned measurements can be done outsideof the test stand using cables for high RF power measure-ments. After that a fundamental mode spectra of each cav-ity is measured. Then the cavities are roughly tuned to thedesign resonance frequency (1.3 GHz). In parallel, an ex-ternal quality factor of the couplers is set by coupler mo-tors. Finally, the High Order Mode (HOM) Spectra mea-surement is performed. The results panel is shown in Fig-ure 3.After these operations test stand is closed and secured by

the Personal Interlock.

Figure 3: HOM Spectra measurements application.

Measurements with igh RF owerThere is no possibility to tune the cavities to the exact

resonance frequency with the VNA. Therefore the ne tun-ing with use of the LowLevel RF (LLRF) system is needed.This is the rst operation performed with the klystron. Af-ter ne tuning, the cavities are calibrated at low gradient (5MV/m) to determine the following parameters:

• Kt - calibration coef cient used to calculate the gra-dient,

• Qext - External quality factor adjusted during tuningwith VNA; measurement of Qext is much more accu-rate at high RF power

• Qprobe - Quality factor of the Probe antenna• QHOM1 and QHOM2 - Quality factor of the HOMsantennas

A new software for cavities calibration has been writtenby IFJ PAN Team (Figure 4)

Figure 4: Calibration application.

After calibration the cavities are tested one by one. Be-cause of the power distribution system there is a necessityto detune all cavities except one. Most important results ofthis measurement are listed below:

• Quench limit [MV/m],• Operating gradient - Value of Gradient when radiationdo not exceed 10−2 mGy/min [MV/m],

• Xrays start - Value of Gradient when radiation starts[MV/m],

• Xrays quench - Value of radiation just before cavityquench [mGy/min],

H P

Proceedings of SRF2013, Paris, France MOP054

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• Pfor - power used to drive a cavity to the quench limit

The measurements results of a pre-series XFEL cry-omodules (XM-3) are shown in Figure 5. After completingthe performance test, all cavities are again tuned to the res-onance frequency in order to perform the cryogenic mea-surements.

Figure 5: Results of the performance test.

CRYOGENICMEASUREMENTSThe cryogenic measurements are divided into two

phases. The rst phase is a static measurements, whenthere is no powering inside either cavities or the magnet.The second one, so called Heat LoadsMeasurements is per-formed with the powered cavities.

Static MeasurementsThe thermal performances of the accelerator modules is

a vital issue to estimate the load budget of the refrigeratorsystem. Variety of measurement considers the losses oncryogenic lines such as 2K, 4.K and 40/80K are performed.

Heat Loads MeasurementsThis measurements are performed by two operators: one

from cryogenic site and another from RF site (RF operatorapplication is shown in Figure 6). First, the zero measure-ment is performed in order to obtain the cryogenic loadwithout power. Then all cavities in cryomodule are pow-ered to certain level of the gradient. Next the series ofmeasurements at different gradients are proceeded. Duringthose measurements a few points are obtained: at gradientjust below the Xrays start, when radiation exceeds 10−2mGy/min (if available). Next at 23.6 MV/m (XFEL goalgradient) and just below the lowest cavity quench limit. Ifthere is only one cavity with low limit in cryomodule thecavity is detuned. The cryomodule is operated without theweakest. The cryogenic calculation of heat loads are shownon Figure 7. From cryogenic load total Quality factor of allcavities inside the cryomodule is calculated. Thus, Qualityfactor from this measurements is averaged for each cav-ity. Software for this measurements has been written byIFJ PAN Team.

Figure 6: Heat Loads measurement - RF operator applica-tion.

Figure 7: Heat Loads measurement - Cryogenic calcula-tions.

WARM UP, DISASSEMBLY, OUTGOINGINSPECTION

After measurements at 2K the cryomodule is warmed upand disassembled from the test stand. Next, the outgoinginspection is performed in order to con rm that there wasno damages during the tests procedure. Results of outgoinginspection is put into Outgoing Inspection Report. Finally,the cryomodule is handed over to the next work packagesfor further installation.

ACKNOWLEDGMENTThis work is being funded by the Ministry of Science

and Higher Education in Poland (MNiSzW) as a part of thePolish contribution to construction of the European XFEL.We would like to thank the DESY management and expertsfor their support during our work.

REFERENCES[1] A. Kotarba, P. Borowiec, W. Daniluk, M. Duda, B. Dzieza,

W. Gaj, E. Gornicki, D. Karolczyk, K. Kasprzak, L. Kolwicz-Chodak, J. Kotula, A. Krawczyk, K. Krzysik, W. Maciocha,A. Marendziak, K. Myalski, Sz. Myalski, T. Ostrowicz,B. Prochal, M. Skiba, M. Stodulski, J. Swierblewski, M.Wiencek, J. Zbroja, A. Zwozniak, ”Proc. SPIE”, vol 9803,8903-97 (2013)

MOP054 Proceedings of SRF2013, Paris, France

ISBN 978-3-95450-143-4

246Cop

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09 Cavity preparation and production

K. Technical R&D - Large scale fabrication