Proceedings of the 11th Annual Meeting of Particle Accelerator Society of Japan
August 9-11, 2014, Aomori, Japan

PASJ2014-SUOM03

SUPERCONDUCTING ACCELERATOR DEVELOPMENT FOR ILC
Hitoshi Hayano, and STF group
High Energy Accelerator Research Organization (KEK), Tsukuba, Japan
Abstract

2 STF ACCELERATOR

The superconducting RF test facility (STF) in KEK is
the facility to promote R&D of superconducting
acceleration cavity and cryomodule for International
Linear Collider (ILC). The STF accelerator is a test
accelerator using the developed superconducting cavities
and cryomodules of ILC technologies to demonstrate
stable and high power electron beam acceleration. The
injector part of the STF accelerator was installed,commissioned and operated in 2011-2013 as the Quantum
Beam Project. It consists of the L-band photocathodeRF-gun (normal-conducting cavity), two superconducting
9-cell cavities, and the Compton chamber which wasutilized 4-mirror laser accumulator. The X-ray generation
experiment in the Quantum Beam Project wassuccessfully performed and reported in 2013. Now, theaccelerator is under installation of the 12m-cryomoduleand 6m-cryomodule for further beam acceleration. All ofthe STF accelerator developments together with future
prospect of ILC are summarized in this paper.

1 INTRODUCTION
The purpose of construction and operation of STF
accelerator under the planned program STF-phase-2 is todemonstrate the ILC Main Linac accelerator technology
and to experience operation of high current and high beam
power superconducting accelerator. Under this program,to encourage the superconducting accelerator expert isalso the purpose. During STF accelerator construction andinstallation starting from 2009, we conducted severalexperiments, such as S1-Global cryomodule experiment[1], and Quantum-beam experiment for a compacthigh-flux X-ray generation [2]. In the S1-Global
cryomodule experiment, we demonstrated ILC-likecryomodule built and operated by the international
collaboration. In the compact high-flux X-ray generation
demonstration referred as ‘Quantum Beam experiment’
which was founded by the MEXT (Ministry of Education,Culture, Sports, Science and Technology in Japan), theelectron beam source and beam capture cryomodule weredemonstrated its performance with high quality electron
beam generation and acceleration. The Quantum Beamaccelerator part is now used to the STF accelerator, as aninjector.
The powering scheme of cryomodule is to use TDR(Technical Design Report [3]) system which is using
multi-beam-klystron and Marx modulator, supplying theRF power to 39 cavities with flexible dividing ratio. The
demonstration of the TDR RF scheme is one of milestoneof the STF accelerator construction and operation.

The Inverse Compton Scattering (ICS) experiment line,
which was build under the program, "Quantum-beamproject" was disassembled on April-May 2013. The
injector part of the Quantum-beam accelerator, which
consists of photo-cathode RF gun, and the capture
cryomodule, are used as an injector of the STF accelerator.
Two 9-cell superconducting cavities in the capture
cryomodule were tested and successfully reached its
gradient up to 40MV/m and 32MV/m. The cavities wereinstalled in the capture cryomodule and connected to the800kW DRFS klystron power system in the tunnel. Their
klystron system is now moved to the 1-st floor of the STF
building, in order to make more open space in the tunnel.
The beam source consists of the photocathode RF gun
cavity, which is normal-conducting 1.3GHz cavity, andthe new ultra-violet (UV) laser system. Synchronized40.625MHz seed laser output of infrared is thinning out to2.708MHz, amplified by 2-stage photo-amplifiers of 1ms
length and then converted to UV by the crystals. The UV
pulse train is injected into the Cs2Te photocathode on themolybdenum cathode base, and the electron beam train isextracted. The commissioning and operation of the RF
gun was done in ICS experiment on 2012-2013 using theQuantum-beam UV laser system. By using the new UVlaser system, beam extraction test is under operation
successfully but with 1/10 beam intensity.
The downstream beam-line of the injector includes
ILC-type cryomodule, CM-1, the half-size cryomodule,CM-2a, and the beam dump. Overall view of the STF
accelerator plan is illustrated in figure 1.

Figure 1: Planned view of the STF accelerator.
The accelerator will include two stage bunch compressor
for future use as a FEL application. The first stage bunch
compressor is a chicane at the entrance of CM-1. The
second stage compressor is a chicane in front of the beam- 128 -Proceedings of the 11th Annual Meeting of Particle Accelerator Society of Japan
August 9-11, 2014, Aomori, JapanPASJ2014-SUOM03dump. The beam line is under design, not yet installed. Itwill be installed in 2015.
Another 2K cold-box for 2K liquid helium supply to
CM-1 and CM-2a was installed at the front of CM-1 but at
the wall-side.
The current installation of the STF accelerator is shownin figure 2. The accelerated beam energy will be 418MeV,
assuming 31.5MV/m gradient for 12 cavities in CM-1 andCM-2a, and 40MeV energy at the exit of the capture
cryomodule. The accelerating beam length will be 1mswith 5.7mA intensity, which are met with ILC beamspecification.

The picture of CM-1 cold-mass assembly is shown infigure 3. The installation of conduction cooled splittable
magnet is shown in figure 4.

Figure 3: Picture of the ILC-type cryomodule CM-1
assembly in the tunnel, by using rail-system.

Figure 2: Current view of the STF accelerator.

3 ILC-TYPE CRYOMODULE
The CM-1 is the ILC design cryomodule whichcontains eight 9-cell superconducting cavities and a
conduction-cooled splittable superconducting quadrupole
magnet together with a beam position monitor in thecenter position. The magnet uses conduction cooled,splittable structure, which are newly introduced concept
in TDR. The fabrication of the conduction-cooled
quadrupole magnet by the collaboration with FNAL wascompleted in September 2013. The beam position monitor
using re-entrant cavity structure was also fabricated.
Since we can accommodate only 4 cavities chain
connection in the existing cleanroom, and also canaccommodate only half-size of the ILC cryomodule coldmass at the tunnel carry hatch, then, we have done the ILC
cryomodule assembly in the STF tunnel by connecting thetwo 4 cavities chain, and inserting the connected 8cavities into the 12m-cryomodule vacuum vessel. The
infrastructure such as rail system for the ILC cryomodule
assembly, which requires more than 24m length space,
was constructed in the downstream part of the STF tunnel.
The plan was also changed to connect one more half-size
cryomodule, CM-2a, together with ILC-size cryomodule,CM-1. The assembly of the connected cryomodule with
8+4 cavities was finished at end of June 2014.

Figure 4: Conduction-cooled splittable quadrupolemagnet was installed in the center of CM-1,together with BPM chamber.
Figure 5 shows installed CM-1 and CM-2a in the STF
tunnel, at downstream of the capture cryomodule. The
2-nd cold box supply a liquid helium into these connected
cryomodules. The vacuum vessel of these connected
cryomodule is pumped down together by the one pumpingsystem. Many temperature sensors, two set of the wire
position sensors are installed to monitor temperature ofvarious points and position movement during cool-down
and warm up.

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August 9-11, 2014, Aomori, JapanPASJ2014-SUOM03

Figure 5: Picture of ILC-type cryomodule followed by the 6m cryomodule in the STF underground tunnel.
The RF power scheme with control of power dividing
ratio and phase is adopted as a cost effective baseline ofTDR. The Kamaboko-tunnel has thick concrete wall in
the centre makes the room divided to the accelerator
tunnel and klystron tunnel. The TDR RF power source inthe klystron tunnel which consist of a 10MW multi-beamklystron (MBK) and a Marx modulator, supply 1.3GHz
1.6ms pulse RF power into 39 cavities with circulator ineach input. In order to supply RF power effectively to20% spread of gradient performance cavities, the power
can be split with flexibility by a variable hybrid (Pkcontrol). Also, a phase of RF input can be controlled by aphase shifter in each of cavity input line. A coupling ofcavity (loaded-Q, QL) can be controlled by an input
coupler insertion length. In order to control each cavity
power input and loaded-Q of each cavity amongvector-sum controlled cavities, the above variableadjustment (Pk-QL control) are controlled remotely.
Figure 6 shows waveguide installation outside thecryomodule, CM-1 and CM-2a, to accommodate thesefunctions.

while. For demonstration of TDR klystron driver supply,
SLAC P2 Marx modulator is under fabrication by SLAC
and KEK collaboration. STF will procure one more MBK
for this purpose, also.

4 CAVITY DEVELOPMENT
Total number of the cavities for the STF accelerator was14, while 15 cavities were fabricated. They are 2 for thecapture cryomodule, 8 for CM-1, and 4 for CM-2a. The
rest one is spare. The inner surface treatment and the fieldtest are allowed twice in TDR. However, in STF,
maximum 4 treatment and field test were done for thepurpose of research and development. The field gradientperformance in the first treatment, and the final gradientperformance are shown in figure 7. Some of the cavity
stopped their treatment at the first treatment, some of thecavity in the second.

Figure 6: RF power distribution scheme of STF phase-2
cryomodule, according to TDR design.
As a RF power source, the 10MW MBK is used in theSTF accelerator. The conventional modulator using
bouncer-type compensator now drives this MBK, for aFigure 7: Cavity gradient performance of the cavity usedin the STF accelerator.

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Proceedings of the 11th Annual Meeting of Particle Accelerator Society of Japan
August 9-11, 2014, Aomori, Japan

PASJ2014-SUOM03

The condition to stop treatment was to perform greaterthan 35MV/m, however, some of the cavity proceeded
once more again to see what happen in the next. The
transition of the gradient performance is plotted in figure
8. One cavity stayed at low gradient, never improved with
4 treatments cycle. Other cavities improved drastically.

Figure 8: Transition of cavity gradient performance ofeach cavity.
Total number of the cavities for the STF accelerator was14, while 15 cavities were fabricated. They are 2 for thecapture cryomodule, 8 for CM-1, and 4 for CM-2a. The
rest one is spare. The inner surface treatment and the fieldtest are allowed twice in TDR. However, in STF,
maximum 4 treatment and field test were done for thepurpose of research and development. The field gradientperformance in the first treatment, and the final gradientperformance are shown in figure 7. Some of the cavity
stopped their treatment at the first treatment, some of thecavity in the second. The condition to stop treatment wasto perform greater than 35MV/m, however, some of thecavity proceeded once more again to see what happen in
the next. The transition of the gradient performance isplotted in figure 8. One cavity stayed at low gradient,
never improved with 4 treatments cycle. Other cavities
improved drastically. This seems to relate with cavity
fabrication, especially with electron-beam-welding
quality. The detail of the reason is under investigation.
The improvement of the performance has been achievedby the full use of temperature mapping, X-ray mapping
during the field test, and use of high resolution innersurface camera and local grinder repair method. The
electro-polish technology advance is also contributed tothe performance improvement [4]. The technology
advance is supported by the use of the surface analysis
instruments and their technologies. SEM, EDX,
micro-XPS, TOF-SIMS, Raman-spectroscopy, ICP,
laser-micro-scope, are used for the understanding of theNb surface after several treatment method and machinedmethod [5]. We can judge the treatment method is good ornot by using coupon attached single cell cavity at thesurface treatment facilities.

5 ILC PROSPECT AND STF
After TDR publish, the international linear collider
study team (GDE: Global Design Effort) was changed tothe LCC, Linear Collider Collaboration. LCC requests tothe Japanese Government (MEXT) to host the ILC in
Japan, and recommend the reasonable ILC construction
site. MEXT start to explore the project procedure, such asasking to Science Council of Japan, forming
knowledgeable council (Yushikisha-kaigi), etc. It willtake one or two years. In parallel, LCC promote theengineering study of ILC with more detailed and with sitespecific.
STF accelerator construction and operation isconcurrently proceeded with movement. The result of the
STF accelerator will strengthen the TDR and theengineering design of ILC. Since the use of FEL
application after ILC study in STF is unavoidable, thepreparation of FEL should be included in the design fromthe beginning.

6 ACKNOWLEDGEMENT
The author would like to thank all the members of STF
group for their effort to develop ILC superconducting
accelerator technology. The support of the LC supervisor,A. Yamamoto, S. Yamaguchi, and the support ofAccelerator Laboratory director, K. Oide, trustee of theexecutive board, Y. Okada, N. Toge, and director generalof KEK, A. Suzuki, are gratefully appreciated.

7 REFERENCE[1] Hitoshi Hayano, et. al. "Results Achieved by the S1-Global
Collaboration for ILC," Proc. of the 26th International Linear
Accelerator Conference, Tel-Aviv, Sep. 2012.
[2] Hirotaka Shimizu, et.al. “X-ray Generation by Inverse
Compton Scattering at the Superconducting RF Test Facility,”
submitted to N.I.M. A.
[3] ILC Technical Design Report;
http://www.linearcollider.org/ILC/Publications/Technical-Desi
gn-Report[4] Yasuchika Yamamoto, et.al. “Achieving high gradientperformance of 9-cell cavities at KEK for the international linearcollider,” N.I.M. A729 (2013) 589-595.
[5] Xin Zhao, et.al. “Surface characterization of Nb samples
electropolished with real superconducting rf accelerator
cavities,” P.R.S.T.A.B. 13, 124702 (2010).

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