157■研究紹介

From the European XFEL Accelerator to the International Linear
ColliderDeutsches Elektronen Synchrotron (DESY)

Wolf-Dietrich Möller
wolf-dietrich.moeller@desy.de
KEK 加速器研究施設

山本 康史
yasuchika.yamamoto@kek.jp
2018 年 2 月 1 日の状況，などを中心に解説していただいている。表 1 は，ILC とEuropean XFEL の簡単なスペックの比較を示したものである。
昨 年 ， ド イ ツ 電 子 シ ン ク ロ ト ロ ン 研 究 所 (DESY) に て
DESY における 1.3 GHz の超伝導空洞およびクライオモジュ
European X-ray Free Electron Laser (European XFEL) 計画のたールの研究・開発について簡単に説明すると，1990
年代初め
めの超伝導線形加速器のビームコミッショニングが始まった。こ頃に超伝導加速技術による
TeV-scaleのリニアコライダー
れは放射光実験のための加速器であるが，現在，多くの高エ(TESLA 計画)の実現を目指して，超伝導加速技術の性能向
ネルギー物理学者がその実現を期待している国際リニアコライ
上および価格の低下を目的とした TESLA Test Facility (後のダー (International Linear Collider (ILC)) 計画とも大いに関係
Free Electron Laser in Hamburg (FLASH)) を国際協力の下にしているのである。その理由は，主線形加速器に用いられてい建設したのがその始まりである。最終的に TESLA Test Facility
る超伝導空洞の技術が ILC とほぼ同じものであり，また規模とではクライオモジュールを用いたビーム加速までが実現し，超しても ILC の 1/20 スケールに匹敵するものだからである。山本
伝導加速技術によるリニアコライダー実現への扉を開いた。
は 2014~2015 年にかけて DESY に長期滞在し，European
2004 年 の International Technology Recommendation Panel
XFEL 建設の初期段階に接したことで，この計画が ILC に与え(ITRP) による超伝導加速技術の選択は，まさに TESLA Test
るインパクトについてよく理解している。そこで今回，当時の超Facility で実証した結果を受けてのものである。その後，計画は伝導空洞グループのリーダーであった Wolf-Dietrich Möller 氏
European XFEL へとシフトしたが TESLA 計画で築き上げた国に，如何にして超伝導空洞およびクライオモジュールの大量生
際協力の枠組みや達成した技術などはそのまま生かされること産を実現し，European XFEL 計画を成功に導いたのか，そしてになり，他の研究所や会社への技術移転に大いに役に立つこ
そこから ILC 建設の実現に向けてどのような知見が得られたのととなった。
かを概説していただくためにこの記事の作成を依頼した。本原
ここで昨年 11 月に ICFA による ILC の 250 GeV 開始案が承稿では，European XFEL 計画のための国際協力の枠組み作り，
認
さ れ た が ， 本 原 稿 で 用 い て い る ILC に 関 す る 数 値 は超伝導空洞およびクライオモジュールの大量生産に伴い発生Technical
Design Report (TDR) に掲載されている 500 GeV のした数々のトラブル，また達成した性能，ビームコミッショニング
場合に基づくものであることを予めお断りしておく。

始めに

表 1: European XFEL と ILC の基本スペックの比較※ILC のスペックは TDR に掲載されている 500 GeV の場合に基づく。また，加速勾配については±20 %のばらつきを仮定している。

計画名称
空洞周波数とセル数
高周波条件(幅，繰り返し)
空洞台数
カップラー台数
クライオモジュール台数
加速勾配と Q0 値@縦測定
加速勾配と Q0 値@クライオモジュール試験
ビームモニターと電磁石の位置
ビームエネルギー
超伝導部分のトンネル長さ
運転温度

European XFEL
1.3 GHz / 9-cell1.40 msec / 10 Hz
808
808
101
26.0 MV/m, 1.0 x 1010
23.6 MV/m, >1.0 x 1010
End of cryo-module17.5 GeV~ 2 km2K

ILC
1.3 GHz / 9-cell1.65 msec / 5 Hz
~16,000~16,000~1,85035.0 MV/m, 0.8 x 1010
31.5 MV/m, 1.0 x 1010
Center of cryo-module
250 GeV x 2~ 11 km x 2
2K

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1 INTRODUCTION
The European XFEL has officially started its operation on the
1st of September 2017 at Hamburg in Germany. With the deliveryof 27,000 ultra-short X-ray flashes per second and peak brillianceone billion times higher than every conventional X-ray source, itwill open up a wide field of completely new research opportunities for scientists and industrial users.
The construction of the European XFEL started in early 2009and involved close collaboration between Deutsches Elektronen
Synchrotron (DESY), the European XFEL GmbH and many otherorganizations worldwide.
The superconducting (SC) linear accelerator (linac) is one of akind in the world. With its 808 1.3 GHz cavities contained in 101
cryogenic modules (cryo-module)1 and a length of 1.7km, it is thelongest SC linac in the world. It provides electrons at an energyof 17.5 GeV needed to generate the European XFEL’s X-ray
flashes in a highly efficient way. As an exemplary collaboration,
21 institutes from nine different countries contributed to the European XFEL.
The basic superconducting accelerating technology of the European XFEL linac has been developed since the early 1990’s by
an international collaboration coordinated by the DESY laboratory in Hamburg in order to create a TeV-Energy Superconducting
Linear Accelerator (TESLA) [1], an electron-positron linear collider for particle physics studies. The name ‘TESLA Technology’
stands for this development. As it was realized that this SC linac
technology has ideal characteristics for an X-ray free-electron laser, proposals for a free-electron laser as a side branch of the linear collider and later as a self-standing facility were proposed by
DESY and finally realized as the European XFEL. In 2004, theInternational Technology Recommendation Panel (ITRP) gave
momentum to the newly named ILC [2]. The realization of the SC
linac for the European XFEL is a big progress in the TESLA technology, and at the same time a perfect example in view of theinternational organization of a future ILC construction. Nevertheless its 808 cavities represent only 5 % of the total number required for the ILC.

2 THE EUROPEAN XFEL
2.1 Layout and Performance
The European XFEL consists of the main components: injector,
SC linac, beam distribution system, undulators, photon beamlines and instruments in the experimental hall (the layout andview shown in Figure 1). The whole facility is 3.4 km long andstarts with the electron injector at the DESY site in the North Westpart of Hamburg, and ends in the neighbouring Federal State ofSchleswig Holstein, south of Schenefeld, where the experimental
hall is located. The entire facility is constructed in tunnels 6 to
38 m underground with an inner diameter of up to 5.3 m.
The injector consists of a normal conducting RF electron gun
where the electrons with high bunch charge and low emittance are
extracted from a solid cathode by a laser beam. Accelerated by
one 1.3 GHz cryo-module, it delivers electron bunches at an energy of 130 MeV. The longitudinal beam profile is improved by asuperconducting third harmonic 3.9 GHz cryo-module [3, 4],while the uncorrelated energy spread is increased by a laser heater.
At the end of the injector, the 600 µs-long electron bunch trains
have a typical bunch charge of 500 pC.
The further acceleration of the electrons is performed in thethree main sections of the main linac. The four cryo-modules in
the first section are operated at a moderate gradient below the European XFEL design gradient of 23.6 MV/m. From the second
linac section, consisting of 12 cryo-modules, the beam is ejectedwith a relative energy spread of 0.3 % at 2.4 GeV. In the third
linac section with a length of about 1 km, the 84 cryo-modules
accelerate the electron beam up to the design energy of 17.5 GeV.
Between the injector and the linac sections, three bunch compressor sections with their dipole-magnet chicanes and further focusing elements and diagnostics are installed. Once the electron
beam leaves the accelerator, it is directed by fast kicker magnets
into one of the two electron beam lines that feed the three different
undulator sections.
The availability of extreme short photon pulses (10-100 fs) andaround 1012 X-ray photons in one pulse with a very high coherence allows experiments with ultra-fast snapshots of the dynamics of the atomic motion and molecular rearrangements.

Figure 1: Layout and view in the SC linac of the European XFEL. The cryo-modules are hanging from the roof whereas the RF klystrons and theelectronic equipment is assembled beneath.

1

ILC では Type A（9 空洞）と Type B（8 空洞）の 2 つのクライオモジュールが存在するが，European XFEL では 8 空洞が収納された 1 タイプだけである。

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2.2 European XFEL History and Organization
DESY scientists initiated an international collaboration to develop and test the superconducting accelerating technology for
TESLA in 1992. The TESLA Test Facility (TTF) was built in order to demonstrate the high performance, reliability and efficiency of such a technology. As it was realized that this test facility has ideal characteristics to realize a FEL, undulators and allnecessary infrastructure were added in order to generate laser
light. In 2000, the scientists of the DESY TTF generated for the
first time in the world short wave laser light in the ultraviolet
range (80-180 nm), using the SASE FEL principle [5]. This was
the birth of the idea to use the TESLA technology for a future Xray laser laboratory based on the TESLA technology. A technicaldesign report (TDR) for an X-ray laser laboratory as a side branchof TESLA was published in 2002. Already in 2003, the German
government decided to cover around half of the investment cost,provided the rest would be delivered by European partners. Thisdecision led to intense negotiations on funding and participation.
In July 2006, the DESY XFEL project group and the European
XFEL project team, established in Hamburg through the MoU,
published a TDR for the proposed European XFEL facility [6].
After the official start of the project in 2007, the international
partners entrusted the construction and operation of the facility to
the non-profit European X-Ray Free-Electron Laser Facility
GmbH, which was founded in October. The representatives from
9 partner countries signed the European XFEL Convention andthe Final Act in the Hamburg city hall as chair holders (three morepartner countries signed later or will sign very soon). In the same
year the construction of the European XFEL started officially.

3 THE SUPERCONDUCTING LINEAR ACCELERATOR
3.1 The TESLA Technology
The main goal of the TESLA collaboration was to improve theperformance of the superconducting cavities and at the same time
reduce the cost. With the experiences from LEP, CEBAF, TRISTAN and HERA where superconducting cavities were operated
very successfully, and newer developments from laboratories allover the world, a new infrastructure for treatment, assembly andtesting was built up at DESY – the TESLA Test Facility. In thisfacility, every step of the cavity fabrication and treatment could
be controlled and investigated, from the raw niobium material,
the deep drawing and welding of cells to the final chemical treatment and cleaning of the active inner surface. Once the new
TESLA nine cell cavity at a frequency of 1.3 GHz and optimized
cell shape was designed, the fabrication was handed over to industry and optimized in very close collaboration.
The infrastructure allowed extensive testing of the cavities andall ancillaries, with and without electron beam. Already the firsttests showed a big step in the performance: the accelerating gradient reached 25 MV/m. By changing the RF surface cleaning
process to electro-polishing and adding a final baking at moderatetemperature (120 ºC), the achievable gradient could be improved
further and reached an average of > 30 MV/m with the highest
gradients over 40 MV/m.
The cryogenic experience with superconducting magnets at
Fermilab, CERN and DESY led to the development of new cryomodules containing 8 cavities including focusing elements andbeam diagnostic. They are assembled in a string without additional cold/warm transitions. This new technology improves the2

DESY では基本的に超伝導空洞の性能評価のみを行っており，組立作業については会社や他の研究所（後述）で行っている。

filling factor, saves cryogenic power and simplifies the assemblyof big systems. The TESLA technology cryo-module was bornand first brought to successful operation in the TTF, later renamed
FLASH facility. The success of the TESLA technology was notjust due to the cavity performance and cryo-module design, butalso the parallel development of reliable RF power couplers, frequency tuners and other ancillaries. The goal of the TESLA collaboration to reduce the cost per MV by a factor of 20 was successfully proven.
For the European XFEL, a total of 101 cryo-modules containing 808 cavities were produced, which represents by far the largest application of superconducting RF and of the TESLA technology to date.

3.2 The International Collaboration
The European XFEL chair holders contribute to construction
and cost in cash or in kind (IKC). A total of 602 M€ was contributed by 79 IKC’s from 21 institutes off 9 countries with the largest
IKC and other contributions from DESY. The IKC’s have theform of component delivery, secondment of staff or both. The intensive coordination, monitoring and integration of the IKC’s
were essential to ensure a successful completion of the project.
Eight laboratories contributed to the superconducting cryomodule: DESY Hamburg in Germany, CEA Saclay and LAL, Orsay in France, INFN, Milano in Italy, IPJ Swierk and Soltan Institute in Poland, CIEMAT in Spain and BINP in Russia. CEA
provided a completely new infrastructure for the cavity string andcryo-module assembly. The prompt availability of all cryo-module sub-components was essential for the smooth delivery of theaccelerating cryo-modules to DESY for the test and final assembly in the tunnel. In this respect, the delivery of any sub-component in the supply chain was essential for the overall projectschedule. Weekly video meetings of all contributing collaborators
were conducted to follow up the schedule, and, if necessary, takeaction to secure the schedule.
Other important contributions were the RF high power system
consisting of 27 multi beam klystrons (10 MW each) including
the low level RF system (DESY), the cryogenic equipment for the
SC linac (BINP Novosibirsk) and the cryogenic plant itself(DESY). The warm part consists of 3 km room temperature beamline (BINP Novosibirsk) and 700 transport magnets (Efremov Institute in St. Petersburg, BINP Novosibirsk, Stockholm University). Major contributions to the electron beam diagnostic camefrom PSI in Switzerland, CEA in France, INR Moscow in Russia
and DESY. 460 beam monitors, 64 screens 12 wire scanners and
36 toroids as well as bunch compression and beam arrival monitors, electro optical devices and transverse deflecting systems are
essential for a state of the art beam diagnostic.
In addition to the electron accelerator equipment, many otherimportant assignments like civil construction, electric power andcool water supply had to be coordinated.

3.3 Technology Transfer to Industry for the Cavity
Production
The European XFEL is the first superconducting RF (SRF)
project where not only the mechanical fabrication of the superconducting cavities, but also the very delicate chemical surface
preparation was done by industry. The cavities were delivered to
DESY ready for the performance test in vertical cryostat.2 A major effort was necessary to transfer the technology to the industry,
and at the same time the vendors had to invest in the construction
and commissioning of significant new infrastructure in order to160meet the stringent fabrication requirements and production schedule. A large team from the responsible laboratories INFN MilanoLASA and DESY consisting of about 25 experts of different
fields worked in very close collaboration with both contracted
companies (Ettore Zanon S.p.a. Italy (EZ) and RI Research Instruments GmbH, Germany (RI)).
The main cavity fabrication principle was the so called “build
to print” strategy. Therefore, DESY provided a detailed specification for the mechanical fabrication and the surface preparation.
Both vendors where required to extensively document each production step, and the expert team closely supervised the well-defined procedures and tolerances. Before starting the cavity fabrication, the whole infrastructure and procedures at the vendors,
like clean room assembly, High Pressure Rinse (HPR), vacuum
pumping and purging, and chemical polishing was tested step bystep. 16 extra cavities prepared by DESY had to undergo eachtreatment step performed at the new infrastructure at the vendors,
and were measured by DESY before and after this test cycle, before the new infrastructure was proven to be ready for mass production. As a result of the “build to print” strategy, DESY accepted all cavities built according to specification without performance guaranty, and took the responsibility for the recovery ofpoor performing cavities.

4 COMPONENT PRODUCTION AND TESTRESULTSAs described before, many components had to be fabricated athigh quality, delivered in time and tested to complete the whole
European XFEL. Here we will concentrate only on a few key
components of the electron accelerator: superconducting RF cavities, RF power coupler and the superconducting accelerating
cryo-module.

4.1 Superconducting RF cavities fabrication
The TESLA superconducting cavity is made out of solid niobium and consists of 9 elliptical shaped cells at a frequency of
1.3 GHz. Two Higher Order Mode (HOM) couplers, one power
coupler port and one pick-up port are located on the beam tubes.
A helium tank and the 2-phase helium pipe, made out of titanium
are welded to the cavity (Figure 2). The niobium material for thecavities was ordered by DESY and delivered by three companies.
After an intensive quality control including scanning of the active
RF surfaces at DESY3, the material was distributed equally to thetwo cavity vendors EZ and RI. For the RF measurements during
fabrication and field flatness tuning, DESY developed and delivered two dedicated semi-automatic machines to both vendors4 [7].
The surface preparation is one of the most critical processes of
the cavity fabrication. It determines predominantly the later cavity performance. For both vendors, the surface preparation started
with a bulk electro polishing (EP) and a subsequent 800 ºC annealing. For the final polishing, two alternative recipes were usedaccording to the company’s abilities: EZ applied a final bufferedchemical polishing (BCP) of 10 µm, whereas RI applied a final
EP of 40 µm5. Both vendors applied an extensive High Pressure
Rinsing (HPR) with ultra-pure water at a pressure of 100 bar. After the assembly of the HOM coupler antennas, the high Q input3

空洞形状に成型する前のニオブ板の段階で，不純物の割合を渦電流探査法によって調べ，多いものは除外している。
4電子ビーム溶接を行う前のハーフセルの段階で周波数を揃えるためにトリミングの量を自動算出する装置，および完成した 9 セル空洞の各セルの周波数を自動調整する装置のこと。

antenna and the vacuum valve in an ISO4 clean room, well-defined vacuum pumping and 120º C baking as final step of the
preparation were applied. Close monitoring of the infrastructure
and the procedures of the surface preparation was essential toachieve a high cavity performance.

Figure 2: 3-D model of the series European XFEL cavity equipped withantennas ready for performance test at DESY.

4.2 Superconducting RF Cavity Testing
A dedicated Accelerator Module Test Facility (AMTF) wasused at DESY for the quality control of the cavity and cryo-module production.
The cavity’s acceptance test was divided in three parts: incoming inspection, cold vertical test and outgoing inspection. Since
the vendors did not take any responsibility for the cavity performance, the incoming inspection took a major role in order to guaranty a high quality of fabrication. After the verification of all fabrication documents and the mechanical and electrical inspection(RF measurements of mode spectrum) the cavity was accepted forthe vertical performance test.
In order to keep the overall project schedule, it was necessary
to test up to ten cavities per week. Two vertical test cryostats withindependent RF systems and six cryogenic inserts accepting four
cavities per each cool down were available and used in AMTF.
All cavities were tested with assembled HOM feedthroughs
and a RF input antenna with fixed coupling. The test followed astandard automated procedure, and included not only the quality
values (Q0) versus the accelerating electric field (Eacc), but also
the frequencies of the fundamental and the TM010 passband6. Xrays were measured inside the concrete shielding above and below the cryostat in order to estimate the field emission.
Starting operation in February 2013, the vertical test infrastructure reached full operation in October 2013 with an average testing rate of approximately 40 cavities per month (peak test rate
was 15 per week). The last test of the series production was finished in March 2016.

4.3 Superconducting RF Cavity Test Results
The acceptance criteria was not only based on the maximum
gradient Eacc ≥ 26 MV/m, but also included the quality factor Q0> 1010 and the X-ray threshold measured below and above thecryostat. After 270 cavity test results were available, the numberof necessary retreatments was analysed and the acceptancethreshold for the usable gradient 7 was reduced to Eusable ≥ 20
5

一般的に，縦測定前の最終表面処理は電解研磨の方がより信頼度が高く，かつより高い加速勾配まで到達できるが，European XFEL のスペ
ックは 23.6 MV/m であるため化学研磨でも十分である。
6
9 セルのため基本モードである TM010 も 9 個存在する。
7
Q0 値および X 線量も許容値を満たしている加速勾配の最大値のこと。「実際に使用可能な加速勾配」ほどの意味である。

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MV/m in order to optimize the number of vertical tests while still
maintaining an average gradient in the cryo-module ≥ 23.6 MV/m.
There have been mainly two reasons for additional tests: theusable gradient of the cavity was considered unacceptable and thecavity had to be retreated, and there were technical issues with thecavity or with the vertical test. In Figure 3, the fractions of cavities with one or more test are shown.

Figure 3: Fraction of cavities which have 1, 2, 3 or 4 vertical tests.

In Figure 4, the maximum gradient, the usable gradient and theyield curves for the cavities ‘as received’8 are shown. Since cavities with additional treatment are excluded, these results represent best the nominal production process of the two vendors.

Table 1: Statistics of the “as received” vertical cavity tests

Average gradient
RMS
Yield ≥ 20 MV/mYield ≥ 26 MV/m

MV/m
MV/m

Maximum
31.4
6.8
92%
85%

Usable
27.7
7.2
86%
66%

During the three years of cavity production, a clear improvement of the average gradient was determined. In the last 18 monthof the production, the number of cavities with gradients below
20 MV/m (see Figure 5) was significantly reduced. This led inconsequence to an overall reduction of retreatments and verticaltests.
Out of the 831 fabricated cavities, 313 retreatments were performed for a total of 237 cavities. Approximately 49 % of the retreatments were due to unacceptable RF performance, whereas
51 % were caused by other nonconformities like e.g. vacuum
leak10. It proved to be very efficient to retreat the cavities with anunacceptable RF performance first with HPR. This was especiallytrue for cavities whose performance was field emission limited.

Figure 5: The relative fraction of cavities with a “usable gradient” in theindicated range is shown during the quarterly production of the cavities.
The cavities are divided in three categories of gradient limitations “asreceived”: 26 -28 MV/m, 20-26 MV/m and <20 MV/m. In the second halfof the production, a clear reduction of the cavities with gradiens below
20 MV/m can be seen.
Figure 4: The distributions and yield curves for the maximum achieved
gradient (blue) and the usable gradient (orange) for all cavities “as received”. Yield is defined as those cavities which have a gradient greaterthan or equal to the specified value. (The darker colour represents theoverlap of the two histograms.)

As shown in Table 1, the average maximum and usable “as received” gradient is 31.4 MV/m and 27.7 MV/m, respectively. The
usable gradient is about 4 MV/m lower due to low Q0 or X-raysby field emission above threshold.
One should note that about 50 % of the cavities were limitedby thermal breakdown, whereas all other cavities were either limited by the administrated power limit of Pfor ≤ 200 W9, i.e. the true
physical breakdown limit is higher. The power limited tests were
dominantly caused by a low Q0 below the acceptable threshold,which leads to a higher power consumption.

8

空洞を会社から受け入れた状態のこと（再表面処理を含まない）。1 回目の性能試験の結果だけを抽出することで製造品質の推移が分かる。
9
空洞の性能試験を行う際，ケーブル損傷や高周波機器の故障を避けるため，通常 200 W 辺りで投入パワーを制限する。

If after an additional HPR the usable gradient was still below
the performance limits, the cavity was retreated with BCP. An EP
retreatment was not used, because there was no experience withthe cavity already welded in the helium tank. Figure 6 shows thegradient before and after the retreatment with HPR and BCP. It isclearly visible how efficient the retreatment was.
Figure 7 and Table 2 show the finally accepted maximum andusable gradient distribution and statistic of the cavities ready forthe string assembly (including retreated cavities). A few cavities
with a gradient below 20 MV/m were accepted for string assembly because of schedule reasons.

10

いわゆる「低温リーク」のこと。超流動ヘリウムは摩擦の無い液体のためリークする確率が飛躍的に高まる。常温でヘリウムガスを用いて行うリーク試験では，この種のリークを発見するのは困難である。

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Figure 8: The distribution and yield of Q0 measured at 23.6MV/m at thefirst and final accepted test. Very few cavities were accepted with a Q0
value below the specified 1010.
Figure 6: The usable gradient before and after a retreatment with High
Pressure Rinse (HPR) and Buffered Chemical Polishing (BCP).

4.4 RF power coupler
The RF high power coupler is the connecting part between the
RF transmission line and the cavity; it provides the electromagnetic power to the cavity and the particle beam. In addition to thisRF function, it also has to provide the vacuum barrier for the
beam vacuum, and is also the connecting part between room- andcryogenic temperature. High Power Couplers (HPC) are one of
the most critical parts of the RF cavity system in an accelerator.

4.5 RF power coupler fabrication

Figure 7: The final maximum and usable gradient distribution and yieldof the accepted cavities for string assembly (including retreatments).
Table 2: Statistic of the finally accepted (after retreatment) cavity performance

Average gradient
RMS
Yield ≥ 20 MV/mYield ≥ 26 MV/m

MV/m
MV/m

Maximum
33.0
4.8
98 %
94 %

Usable
29.8
5.1
97 %
79 %

As shown in Figure 8, the number of cavities with a Q0 below
1010 could be reduced by HPR, but not completely eliminated.
The superconducting cavity production and the final result for
the vertical tests are summarized in the published papers [8, 9].
11高周波窓に用いられるセラミックに窒化チタンコーティングを施すこと
で二次電子放出係数が下がる。この結果，マルチパクタ放電が抑制される。

As mentioned before, the HPC was developed together withother ancillaries in the frame of the TESLA collaboration. Likefor the cavity fabrication, also for the HPC the so called “build toprint” strategy was applied. The HPC had to be fabricated,cleaned to the same standard as the cavity, assembled on a test
stand and shipped to the coupler test facility at LAL, Orsay,
France. Two suppliers were chosen: the consortium THALES,
France / Research Instruments RI, Germany for 670 HPC’s, andCommunication & Power Industries CPI, Beverly, USA for 150
HPC’s.
As shown in Figure 9, the HPC consists of four main parts: The
cold coaxial part with the inner conductor coupling the RF power
to the cavity and the warm coaxial part providing the connection
to the cryo-module outer shell. Both include a cylindrical ceramic
as vacuum barrier. The warm transition provides the connection
to the RF source and for tuning the coupling to the cavity, a remotely controlled motor is attached to the inner conductor.
The HPC fabrication involves many fabrication processes asmachining and joining techniques with different materials, and in
addition coating techniques for copper and titanium-nitride(TiN)11. In order to achieve the highest possible quality, the fabrication was adapted to the abilities of the supplier. Therefore, anindustrial study was already initiated two years in advance of the
project. As a result, many improvements and simplifications fromthis study as well as a review of all manufacturing tolerances
could be implemented for the HPC mass production.

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Figure 9: Section of the European XFEL High Power Coupler model,
main parts are the cold part, warm part, warm transition and tuning motor.

As the main joining technique for the HPC, metallic and ceramic subassemblies brazing was chosen. This technology has agood reproducibility and allowed a production in big batches. After the subassemblies were brazed, the metallic parts had to becopper plated. Due to the demanding specification in thickness
and quality of the copper plating at one hand, and the complicatedgeometry of the components which includes different diameters,
cones, bellows and big flanges, on the other hand, the copper plating turned out to be the most demanding step in the whole HPC
production. Many mechanical and electrical parameters were
measured and documented in an extensive quality insurance program before and attendant to the mass production. An objective
definition of the copper surface quality criteria for the visual inspection was developed and agreed on between the RF- and the
production experts. Using this definition, a weekly visual inspection of all copper plated parts in presence of all involved parties
was held.
After the final fabrication step of joining the TiN coated ceramic together with the metallic coaxial coupler parts by electronbeam welding, the vacuum parts were cleaned to the same standard (ISO4) as the superconducting cavities and assembled in pairsof two on dedicated test stands. The coupler test stands wereshipped under clean conditions packed in shock absorbing transportation creates from both vendors in Germany and US to LAL
Orsay, France. The production rate was 8 to 10 HPCs per week.

4.6 RF power coupler testing and conditioning
LAL as the in-kind contributor for the HPCs, developed andinstalled a complex infrastructure for the handling and testing at
a weekly rate of 8 – 12 HPCs. It includes clean rooms for assembly, baking ovens and a high power test stand with a 5 MW RF
power source. A RF power distribution system allowed a parallel
RF test and conditioning of 4 test stands (8 HPCs) at a time, as
shown in Fig 10.
After an incoming inspection at LAL, consisting of visual inspection, vacuum leak check and residual gas analysis, the two
HPCs on its test stand were vacuum baked at 150 ºC in order toremove residual gasses. The following assembly on the RF power
test facility was carried out again under clean conditions in a cleanroom. A normal RF test and conditioning cycle needed less than
one week including the assembly and disassembly. The rejection
rate was 13 % mainly due to unacceptable electrical discharge inthe HPC, but also caused by copper plating defects.

Figure 10: European XFEL High Power Coupler test stands at LAL, Orsay, consisting of four RF power lines (shown here), each populated with
two HPCs located in an ISO4 clean room.

In spite of the many difficulties during the HPC fabrication,
especially in the beginning of the production, finally the qualityof the accepted couplers was very high. This big success was
based on and only possible due to the strong collaboration between the experts at the laboratories LAL/DESY and at the companies involved.

4.7 Cryo-module assembly
CEA, Saclay, France took the responsibility to assemble 100
Cryogenic Modules (CM) over two years at a throughput of 1 CM
per week as an in-kind contribution for the European XFEL. The
challenges were demanding:x
Handle more than 10,000 individual parts per CM during
integration and recognize and avoid possible non-conformities.
x
Avoid RF performance (accelerating gradient) degradations
and keep the quality of all components high.
xManage many transfer interfaces with several groups at
DESY, European laboratories and companies. This means
to respect many different incoming quality inspections withdifferent quality standards.
Assembling of the eight cavities to the so called ‘CM string’ isthe first step in the cryo-module assembly and the most important
for the preservation of the cavity performance. In this assembly
step the cavities, HPCs, beam monitors, steering magnets andgate valves are connected together and the clean beam vacuum isclosed. No pollution of the very sensitive SC surface of the cavities is allowed; therefore it was done in an ISO4 clean room. Dedicated procedures, special tooling and well trained personnel aswell as intensive quality controls are needed to ensure the highperformance of all components.
The next steps were done outside the clean room and include
the assembly of all parts outside the beam vacuum, like frequency
tuner, helium supply connections, thermal insulation and cryogenic shielding. Finally the outer cryo-module vessel and thewarm HPC parts are attached.
The CEA in-kind contribution was subcontracted to the workforce of the industrial contractor ALSYOM under the supervisionof CEA staff.
The preparation, assembly and control work is described in 145
written procedures and first set up by CEA during the prototyping phase at DESY and CEA. In a pre-series of three CMs, ALSYOM as the executing company updated the procedures andbrought them in a usable form for the French operators.

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Assembly infrastructure of 2300 m2 was installed at CEA, supporting seven different workstations, as shown in Figure 11.

Figure 12: Cryo-modules in the test hall AMTF. In the foreground a fully
equipped Cryo-module with the tailored wave guide distribution system
attached ready for tunnel installation. The test stand shielding is visible inthe background.
Figure 11: The XFEL Village of 2300 m2 with its different workstations
is shown. The clean room, roll out and alignment area consists of twoparallel production lines in order to assure a through-put of one CM per
week.

After the start of the cryo-module production in March 2014, a
5-day throughput per CM was reached in mid-October 2014 after
assembly of 15 CMs. In January 2015, the 4-day throughput wasestablished with the 25th CM.
The assembly rate benefited mainly from improvements andnew developments of the clean room tooling and procedures. In
addition, more human resources in the incoming inspection andquality control reduced the impact of non-conformities of component supplies on the assembly schedule.

4.8 Cryo-modules test results
The CM performance (gradient and cryogenic heat load) as awhole as well as the individual cavity performance (gradient) was
measured on the three CM test stands in AMTF shown in figure
12.
In addition, the qualification of all subsystems, like frequency
tuner and piezo12, quadrupole magnet, beam position monitor and
HPC, was performed during these tests.
The CM performance is - with an average of 27.5 MV/m - well
above the European XFEL specification of 23.6 MV/m. At the
CM test stands, we were limited by available RF power to a maximum cavity gradient of 31 MV/m. When clipping the vertical
test results for better comparison also to 31 MV/m, the average
vertical test result of all cavities is 28.3 MV/m13. From these numbers, one can conclude a very small degradation of the cavity performance caused by the CM assembly. Besides, an improvementof the performance over time is visible in Figure 13.
In order to profit from the maximum gradient of any individual
cavity, the RF power distribution system in the SC-linac tunnel
was tailored for all cryo-modules.

12

冷却後，空洞周波数は機械的チューナーにより 1.3 GHz に調整される。一方，高周波パワーの投入中に電磁界応力による空洞変形（ローレンツ離調）を補整するために電気的ピエゾチューナーにより微調整が行われる。
13
縦測定では 200 W のパワー制限を設けているが，クライオモジュー
ル試験では 31 MV/m 相当のパワーまでしか投入できず，そのパワーレベルで試験が制限されるため，両者の加速勾配を比較する際は注意が必要である。

Figure 13: The graph shows the average cavity gradient of the vertical test
(VT, blue) and CM test (CM, orange) for all CMs. For better comparison,
the vertical results are clipped at 31 MV/m, the same gradient limit givenby the maximum available RF power on the CM test stands.

4.9 Performance degradation and cryo-module repair
The cryo-module performance degradation as described in thelast chapter is caused by only a fraction of the cavities. A repairof these cavities would imply to disassemble the module and repeat the whole cavity preparation. Since this effort could not be
afforded and the average performance of 27.5 MV/m is well
above the design performance we accepted the cryo-module performance as is14.
However, it was necessary to repair some cryo-modules forother reasons. One major problem at the beginning of the cryomodule assembly was the welding quality of the two-phase pipe
connecting the cavity helium tanks (see Figure 2). This titaniumlines as well as the other helium supply parts are subject to theEuropean Pressure Equipment Directive (PED) and underlay astrict quality control by X-ray15. The welding seams of the first
10 % of the cryo-modules showed an unacceptable high number
14

クライオモジュール収納後，性能劣化が起こってしまった空洞を元通りの性能に回復させる簡単な手段は今のところ存在しない。大量生産が行われている傍らで，クライオモジュールを解体し，しかるべき処置を施した後，再組立てを行う余裕は無いのが現状である。
15
溶接部に対する X 線探傷のこと。

165

of pores at a size > 0.5 mm and had to be repaired. This repair
was already done at the XFEL village at CEA. After improving
the cleanliness and an intense training of the welder, the problem
was solved.
As mentioned before, the HPC is one of the most sensitive partsof the cryo-module. Due to the high complexity, the handling andassembly needs special attention. In the beginning of the cryomodule assembly, the difficulties of coupler assembly had been
under-estimated. Few couplers had been damaged during handling in the clean room (1 %). Others suffered from bad assemblyof the RF contacts16 (1 %) and were damaged during operation on
the test stands at AMTF. These coupler parts were replaced during
or after test in AMTF.
A major problem at the HPC appeared at a small bellow closing
the warm coupler vacuum and connecting the inner coaxial partto the tuning motor. Due to a small design change at this bellow,
not tested in the pre-series, the bellow formed a small RF resonator and was able to pick up low RF power from the nearby capacitor. This lead to sparking and finally caused a leak in the bellow.
The analysis of this problem during the ongoing production wasvery difficult. After the 81st cryo-module we understood the problem and in consequence the RF leaking capacitors (Figure 9) of
all 808 HPCs were removed and replaced by shorts17.
Few leaks appeared in the beam vacuum and the helium supply
system. In four cases the beam vacuum leak and in two cases thetwo phase line could be repaired by only partially disassemble thecryo-module. Four cryo-modules are still under repair and havenot been accepted for the tunnel installation. Since the linac wasplanned with one additional RF power station (including fourcryo-modules), the final performance can be achieved also with
only 96 cryo-modules. The repaired modules will be installed at
the next major accelerator shut down.

5 LINAC ASSEMBLY
In total, 96 cryo-modules in 103 working weeks were installedin the European XFEL linac tunnel, whereas the initially projected installation rate was one CM per working week.
One of the critical operations during linac assembly was theclean handling of the beam vacuum. In order to not pollute thesuperconducting cavity surfaces and compromise the accelerating
performance, all beam vacuum operations followed the same
rules as on test stands and during CM string assembly: assembly
under ISO4 clean room conditions and slow pump and purge ofbeam vacuum. In addition, all components of the room temperature beam vacuum in a range of 50 m apart from CMs werecleaned and handled under the same clean conditions as the SC
components. Precautions were taken to ease the clean handling ofthese components: pre-assembly on girders in clean room outsideof linac tunnel; pre-alignment outside the linac tunnel; beam vacuum connections in the linac tunnel with local clean rooms.
In addition to the CMs, as main linac elements many othercomponents had to be installed. It was a big challenge to coordinate the different installation groups. A dedicated planning group
optimised the global process at a daily basis. Many improvements
were needed for a friction free succession of installation work andin situ tests while the component transports were going on.
The complex cryogenic system which works with superfluid
Helium at 2 K was completely welded in the tunnel to avoid cold
leaks.
16

金属同士をボルト締結する際にボルトが緩んでいると隙間が出来てしまい，そこで放電が発生する原因となる。
17
ここで述べられていることは非常に重要で，マイナーチェンジであると思って導入したアイテムが予想外のトラブルを引き起こし，結局全てのハイパワーカップラーの修理作業を余儀なくされた，ということであ

6 LINAC COMMISSIONING AND PERFORMANCE
The injector installation was finished already end of 2015 andthe first electron beam arrived at the dedicated beam dump on
December 18th 2015. During 2016, a regular 24/7 operation wasestablished. In this time, many of the commissioning procedures,
later used for the main linac, were developed, automated andtested.
The first cool down of the European XFEL linac was finishedon Dec. 24th 2016. The linac commissioning started in January
2017 after the last step of the cool down procedure to 2 K was
finished. Due to the consequent quality assurance procedures during installation, no cold leak in the cryogenic system was detected.
Since most of the high power RF systems had been alreadycommissioned directly after installation, the commissioning of
the low level RF together with the accelerating cryo-modules
could start directly after cool down. The commissioning team included 20 experts from DESY, SLAC and HZDR working at two
8 hour shifts per day. One maintenance day per week was used togather issues and investigate and solve them in the linac tunnel.
Many of the CMs showed multipacting and required RF conditioning. Due to a flexible timing system allowing shifting the RF
pulse of individual RF stations, parallel off beam operation andconditioning is possible.
After the first beam was accelerated to the undulators section,
the photon beam diagnostics and transport beam based alignment
was done.
Here are the main mile stones reached so far in 2017:x
April, 27th: first beam at main dump;xMay, 3rd: first lasing (0.9 nm);xMay, 27th: beam energy close to saturation at 1mJ;
x
June, 23rd: lasing at 1.5 Å, see Figure 14.
Currently, the European XFEL linac is being operated at 14
GeV (goal: 17.5 GeV), the flat top phase and amplitude stability
is well below the European XFEL specification and the measured
energy stability is ΔE/E = (0.0023 ± 0.0023) % (goal 0.01 %) [10].

Figure 14: First SASE light spot on FEL imager at 1.5Å on 2017, June
23rd.
る。大量生産を行う前に，個々のパーツ試験をどこまで行っておくかを注意深く見極める必要がある。

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In order to reach the design energy of 17.5 GeV, a task force
of experts was established. The fact that four CM (32 cavities) are
driven by one 10 MW multi-beam klystron with two power lines,
and tolerances in the customized power distribution, is causingthat a few low performing cavities can limit all other cavities in
the maximum possible operable gradient18. Main challenge is toinvestigate all RF stations on a single cavity basis and work out
solutions for operation at the maximum possible gradient.
The commissioning of the European XFEL was very successful, not only in respect to the schedule, but also for the achieved
performance. Regular FEL user operation started in September
2017.

7 SUMMARY AND OUTLOOK FOR THE
ILC
The European XFEL is the first accelerator where 816 superconducting cavities were fabricated, treated and assembled in industry in close collaboration with the responsible national laboratories. The TESLA technology has been successfully industrialized, and there are no reasons seen why this cannot be extrapolated to the needs of the ILC. The achieved average gradient after
retreatment (28 % of the cavities were retreated) of 30 MV/m isclose to the ILC specification, but a spread of 7 MV/m RMS is
too large. The necessary retreatment of 28 % of the cavities is still
very high. Performance degradation caused by handling, string
assembly and necessary pump and purge operations is still too
frequent. But it could also be shown that a string and CM assembly without degradation is possible. Crucial for the reach andpreservation of high cavity performance are a very close cooperation between laboratories and industry as well as constant quality assurance, quality control and feedback. In order to profit fromthe high performance of individual cavities for beam acceleration,
the power distribution has to be customized, which is difficult
when too many cavities are powered by a single RF power source.
A tunnel installation parallel to early equipment commissioning of many systems is necessary and implies a great accuracy
and a high level of organization.
The experience with the large and complex cryogenic system
at European XFEL is very good and encouraging for implementation at ILC.
To commission the large SC linacs of the ILC in a finite time,
much more automation is necessary [11].
For the now proposed 250 GeV centre of mass ILC as a Higgs
Factory, the needed number of SC cavities would reduce to about
8,000, which is a factor of 10 more than for the European XFEL.
Divided by the three regions Asia, Europe and America who plan
to participate in the ILC, the number of cavity mass production
reduces again and becomes assessable.
Still it will be a big challenge to organize the collaborations not
only between the laboratories and companies, but also between
the different regions. A success requires great diligence and detailed planning.

まとめと ILC 建設に向けて多くの困難やトラブルがあったものの，European XFEL 加速
器のコミッショニングがほぼ予定通り始まったことは，ILC にとっても朗報である。これにより ILC 建設のために必要な超伝導空18極端に性能の低い空洞が存在した場合，必要な高周波パワーも下がってしまう。その際，個々の空洞に投入するパワーの調整裕度を十分に持っていないと性能の良い他の空洞も低いスペックで運転せざるを得なくなってしまう。このことは ILC のような高加速勾配を目指す加洞およびクライオモジュールの大量生産が実現可能であることが裏付けられ，また多くの研究所や会社が関わる国際共同計
画をどのように推進していくのかという case study にもなった。
ILC では少なくとも 3 領域で同様の大量生産を行っていくことが仮定されているため，建設段階では領域間同士の緊密なコミュ
ニケーションも求められることになり，まさに global project と呼ぶべきものになるであろう。
European XFEL 建設で最終的に達成された主な成果は以下の通りである。
① 空洞性能が ILC スペックをほぼ満足するものであったこと② 研究所と民間会社との共同作業による空洞ストリングおよびクライオモジュールの組立て後に，空洞性能が大きく下がることはなかったこと(ILC では加速勾配で 10 %の低下を仮定しているが，European XFEL では 3 %程度であった)③ 加速器建設後のヘリウムリークがなかったこと④ ビームコミッショニングでは制御系がうまく機能したことによりスペックの 80 %まではすぐに到達したこと（さらに高い
エネルギーまではもう少し時間がかかる）
European XFEL 建設時に問題となったいくつかの事項についても述べておくことにする。これは ILC 建設時にも同様の問題が発生するものと予想され，その際に適切に対処できるよう教
訓としておくためである。
① クライオモジュールの不具合
ILC では空洞の生産台数には 10 %のマージンを取っているが，クライオモジュールについてはマージンが無い。
European XFEL ではトラブルにより結局，4 台のクライオモ
ジュールがトンネルにインストールされておらず，現在も修理・待機中である。
また，ILC では性能評価を行うクライオモジュールは全数ではなく 38 %が仮定されている。しかし，実際に European
XFEL ではクライオモジュールの性能試験時に多くの問題が見つかり，それに対処する必要があった。幸いなことに
DESY には 3 つのクライオモジュール試験用ピットがあったため，1 ヶ所でトラブルが発生しても他の 2 つを使って乗り切ることが出来たが，ピットが 1 つしかない研究所では一旦トラブルが発生するとすぐに滞ってしまうリスクがある。試験用ピットをいくつ設けるのかは，クライオモジュールの大量生産を考える上での一つポイントになる。
一方，クライオモジュールのトラブルで最も危険なのはビームラインのリークである。通常，空洞の連結作業時にリークチェックは行っているが，European XFEL では 6 台のクライオモジュールにビームラインのリークが見つかった（内 1 台はゲート弁の不具合による）。ビームラインのリークは空洞内に大量のゴミが混入してしまい，（程度にもよるが）field emission の原因にもなるため絶対に避けなくてはならない。実際，European XFEL ではリークを起こした 2 台のク
ライオモジュールに重度の field emission が観測され，結局，使用されていない。
38 %のクライオモジュールだけ性能試験を行うことについても検討の余地があると思われる。European XFEL ではク
ライオモジュールの大量生産の最終段階においてもビー
ムラインのリークが発生しており，この種のトラブルはある速器において特に問題となり，クライオモジュールにおける空洞性能の劣化を如何にして防ぐかが一つの鍵となる。

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②

③

④

⑤

割合で発生するものと仮定するのが自然である。性能試
験を行わずにトンネルに持ち込んだクライオモジュールにトラブルが発生した時に，トンネル内で対処できるならよいが，再び地上に持ち出して修理する必要がある場合
は手間が発生する。こういったリスクもよく検討する必要があるであろう。
クライオモジュールの熱負荷
European XFEL のクライオモジュールの冷却試験時に測定された静的熱負荷の平均値が 5.6 W であり[12]，これは TDR に記述されている静的熱負荷のスペックである
1.3 W と大きく異なっている。使用しているケーブルの本
数や冷却配管に多少の違いはあるものの，基本的な構造は同じであり，この差が生じた原因を早急に解明する必要がある。
ハイパワーカップラー (HPC) の不具合
多くの人は空洞の性能ばかりに目を向けがちであるが，カップラーの性能も同様に重要なのである。実際，
European XFEL では空洞よりもカップラーに多くのトラブルが発生しており，カップラーにも依然として多くの課題が残されているということが判明した。その内の一つはMöller 氏も指摘しているように銅鍍金のクオリティである。
トンネルインストール後のトラブル
2008 年に起こった LHC での半田付け不良によるトラブル
は多くの人々の記憶に新しいと思われるが，European
XFEL でもヘリウム配管における圧力試験中に機械的クラッシュが発生しており，ILC でもこの種のトラブルには十分に注意する必要があるであろう。
空洞へのパワー分配の最適化
European XFEL でもまだ調整中であるが，クライストロンから各空洞へ供給される高周波パワーの最適化がなされな
いと，空洞性能を最大限引き出すことが出来ない。ILC は最高エネルギーを目指す加速器であるため，パワー調整
は柔軟に行えるよう備えておく必要がある。

ACKNOWLEDGMENT
We like to thank all partners from the different institutes andindustries for their outstanding cooperation during the construction of the European XFEL. The Information and data in this article are from many different sources, which are too many to reference all of them here. We thank all authors and apologize to theunnamed authors.
Thanks also the JAHEP journal for the possibility to publish
this article.

謝辞
最後に私のドイツ滞在中にお世話になった方々に対し改めて御礼を申し上げたい。まず，私の滞在手続きや滞在費の工面などを受け持っていただき，また本記事の執筆も快く引き受
けてくださった Wolf-Dietrich Möller 氏に格別の感謝を申し上げます。また，同じグループで一緒に仕事をし，大いに助けていただいた Detlef Reschke 氏，Nick Walker 氏，Jacek Sekutowicz 氏，Jens Iversen 氏，Axel Matheisen 氏，Denis Kostin 氏，Alexey Sulimov 氏，Jan-Hendrik Thie 氏，Mateusz Wiencek 氏，にも感謝申し上げます。滞在中の雑務を担当してくださった
Katrin Lando 氏，Steffi Killough 氏，Michal Abramova 氏にも感謝申し上げます。滞在中のアパートの大家さんであった
Jürgen Aust 氏には細かな配慮をしていただき特に感謝申し上げます。ここには書ききれませんでしたが，他にも多くの方々に助けられました。ありがとうございました。

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