Proceedings of the 12th Annual Meeting of Particle Accelerator Society of Japan
August 5-7, 2015, Tsuruga, Japan

PASJ2015 FROL03

SUPERCONDUCTING ACCELERATOR DEVELOPMENT AT STF FOR ILC
Hitoshi Hayano, and STF group
High Energy Accelerator Research Organization (KEK), Tsukuba, Japan
Abstract
The superconducting RF test facility (STF) in KEK is
the facility for developing technologies of theInternational Linear Collider (ILC) main linacs. The STF
accelerator is a test linear accelerator using
superconducting cavities (SC cavities) and cryomodulesof ILC technologies. The injector of the STF accelerator
consists of the L-band photocathode RF-gun(normal-conducting cavity), two superconducting 9-cellcavities for pre-acceleration of electron beam. The
12m-cryomodule (8 SC cavities and 1 SC quadrupole
magnet with beam position monitor(BPM)) and6m-cryomodule (4 SC cavities) are followed after theinjector. They will be powered by 10MW multi-beam
klystron and the distribution wave-guide system, and theaccelerator field and phase are controlled precisely by thefast digital feedback at low-power level. For monitoring
SC cavity alignment change during cool-down, the wire
position monitors (WPM) are used inside of thecryomodules. Recent developments of the STF
accelerator such as cryomodules, high power components,
low-level controls, SC quadrupole magnet, BPM andWPM are discussed and summarized in this paper.

alignment of cavities inside cryomodule during cooled
state.

2 STF ACCELERATOR
The injector part of the STF accelerator consists ofCs-Te photo-cathode RF gun (1.3GHz DESY/FNAL
design normal conducting gun cavity), and the capture
cryomodule. Two 9-cell superconducting cavities in thecapture cryomodule were tested and successfully reached
its gradient up to 40MV/m and 32MV/m. They are
powered by 800kW pulsed klystron (DLDS klystron).
The downstream of the injector includes ILC-typecryomodule, CM-1, the half-size cryomodule, CM-2a,connecting beam lines and the beam dump. Overall viewof the STF accelerator plan is illustrated in figure 1.

1 INTRODUCTION
The purpose of construction and operation of STF
accelerator under the 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. During STF
accelerator construction and installation in theSTF-phase-2 program starting from 2009, we conducted
several experiments, such as S1-Global cryomoduleexperiment [1], and Quantum-beam experiment for acompact high-flux X-ray generation [2]. The Quantum
Beam accelerator part is now used to the STF accelerator
injector. The main accelerator by the long cryomodules
were constructed from 2013, assembled in to the STF
tunnel in 2014. The first cool-down test of these new
cryomodules was performed from October 2014 toNovember 2014. The laser for the photocathode RF-gun
was replaced from the one of the Quantum-beamexperiment beam pattern to the one of ILC beam pattern.
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
newly installed cryomodule includes cavity beam position
monitor and split-assembly superconducting quadrupole
magnet, together with WPMs and stretched wires to check

Figure 1: Planned view of the STF accelerator in 100m
length of the STF tunnel. Left side is photo-cathode RF
gun, followed by capture cryomodule, bunch compressor
1, CM-1, CM-2a, then bunch compressor 2 and the dump.
The accelerator will include two stage bunch compressor
for future 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
dump. For undulators and user area of FEL need to be
expanded more to the downstream. The connecting beamline is under design, not yet installed. The 2K cold-boxes
for 2K liquid helium supply to CM-1 and CM-2a isinstalled at the front of CM-1, the wall-side.

- 164 -Figure 2: Current view of the STF accelerator.

Proceedings of the 12th Annual Meeting of Particle Accelerator Society of Japan
August 5-7, 2015, Tsuruga, JapanPASJ2015 FROL03

The current installation picture of the STF accelerator isshown in figure 2. The accelerated beam energy will be
418MeV, assuming 31.5MV/m gradient for 12 cavities inCM-1 and CM-2a, and 40MeV energy at the exit of thecapture cryomodule. The accelerating beam train length is
0.9ms with 5.7mA peak intensity and 2.7MHz bunch
repetition in a train with 5Hz train repetition, which are
met with ILC beam specification.

3 CRYOMODULE DEVELOPMENT
3.1 Superconducting Cavities
Total number of the cavities used for the STF accelerator
was 14, while 15 cavities were fabricated. They are 2 forthe capture cryomodule, 8 for CM-1, and 4 for CM-2a.
The rest one is spare. In STF, the inner surface treatment
and the field test were allowed maximum 4 treatment andfield test. The summary of the field gradient performancein the first treatment, and the final gradient performanceare shown in reference [4]. The average gradient in thefinal field test was 34.2MV/m. The maximum gradientwas 41MV/m and the minimum gradient was 12MV/m.
They will be tested their gradient performance in thecryomodule in October 2015.
3.2 Superconducting Quadrupole Magnet
The CM-1 is the ILC design cryomodule which contains
eight 9-cell superconducting cavities and aconduction-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 (see figure 3 for the concept). The fabrication of
the conduction-cooled quadrupole magnet by thecollaboration with FNAL was completed in September
2013 [5].

Figure 4: Picture of the splittable quadrupole magnet
installation attached to the BPM chamber, under thecryomodule gas-return-pipe (GRP).
3.3 Cavity BPM
The beam position monitor using re-entrant cavity
structure has been designed and developed since 2007 [6].
The resonant frequency of this cavity is 2.04GHz which is
selected to avoid accelerating cavity HOM bands. The
field pattern of the dipole mode of the re-entrant cavity isshown in figure 5. It has 4 coupling waveguide slots
attached to the re-entrant cavity. 4 pick-up antennas areused in the coupling waveguide. This is to supress
common-mode leak to the pick-up antennas. In order tokeep wide beam pipe acceptance, 80mm diameter beampipe, which is the same as the accelerating cavity beam
pipe diameter, is selected. The resolution was alreadydemonstrated to 0.4 μm with 1 x 10 10 electrons/bunch
beam intensity during beam test at ATF beam line.
Electrical center definition accuracy was also studied byusing special BPM test stand. The results was around 25
to 50 μm accuracy relative to mechanical center.

Figure 5: Electric filed intensity pattern (dipole-mode) of
the re-entrant cavity BPM.
Figure 3: One half of the splittable quadrupole magnet. 4
coils are attached to inside of the steel body.
The installation of the splittable magnet into BPM
chamber was successfully done as shown in figure 4, in
normal cleanness environment. This installation was done
after the connection of BPM chamber with gate-valves onboth end of cavity train by using local clean-room in thetunnel. The conduction strips attached on the coils andsteel body, made by pure aluminium, were connected to2K helium supply pipe, 5K shield and 80K shield.

Figure 6: cryomodule-installation-type BPM chamber. It
has long beam pipe for superconducting quadrupole
magnet installation. Also pumping port is there.

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Proceedings of the 12th Annual Meeting of Particle Accelerator Society of Japan
August 5-7, 2015, Tsuruga, JapanPASJ2015 FROL03

As shown in figure 6, cryomodule-installation-type BPM
chamber was fabricated for CM-1 installation. The
chamber has been experienced liquid-nitrogentemperature thermal shock for leak-tight check,high-pressure-water rinsing, and baking in the cleanroom.
3.4 WPMs for alignment detection
In order to measure alignment of cavities during cooled
state, the 70microns gold-plated-tungsten wires are
stretched from the both end plates of the cryomodules,
which is room temperature anytime. The total length ofwires are 19.5m each. One end of the wire is connected tothe N-type feed-through connector, and the other end ishang on the ceramics roller with 500g weigh for tension
force. They are inside of the vacuum vessel of thecryomodule, so they are in vacuum. 5 WPMs are attachedto the gas-return-pipe (GRP) and 16 of them are attachedto the cavities in only CM-1, as shown in figure 7.

Figure 8: The WPM pickup. 4 strip-line electrodes are on
the SMA feed-through connectors. The one end isterminated, the other is connected to the cable and out tooutside of the cryomodule. The pickups are connected byaluminum pipes each other for the good signaltransmission on the wire.

4 FIRST COOL-DOWN EXPERIMENTThe helium 2K cold-box for the cryomodules in the STF
accelerator was installed in the STF tunnel and connectedto the main helium refrigerator. They were cooled down
to 2K and tested its low power performance in October
2014. The test has been performed to confirm coolingcapability of the cryomodules and the cooling system,
cavity low-power performance such as resonant
frequency and Q-value, test of superconducting
quadrupole magnet inside of the cryomodule, and GRP
and cavities alignment check. The cavities weresuccessfully cooled down to 2K. The low-power RF
performance of them was expected one for every cavity.

Figure 7: Positions of the WPMs (red circles) in thecross-section of the cryomodule. The upper one is
attached to the GRP, the lower one is attached to thecavity jacket side-tubs.
They are 4 strip-line electrodes pickup, which detects
burst 160MHz RF signal with 5μs width on the stretchedwire (figure 8). The induced signals on the strip aredigitized by the fast ADC of 1Gs/s 12 bits only for 512
points and FFT converted for 92ch multiplex. From the 4
peak amplitude of FFT 160MHz waveforms, the wire
positions are calculated and recorded with 10 secondsinterval.
The position measurement of GRP and cavities relative tothe wires shows relative movement. Since the cavities are
deep inside of cryomodule and thermally shielded bysuper-insulator, their position in a cooled state is hardlyrecognized. On the other hand, ILC requires their position
to be kept within 300μm offset and 300μrad tilt.
Confirmation of keeping alignment is important for thecryomodule assembly technology.

Figure 9: Temperature profile during cool-down. Red line
shows GRP temperature. Three other lines aretemperature difference between GRP top to bottom.
Deviation of temperature difference happens on steep
cool-down duration.
On the other hand, the conduction cooled splittable
quadrupole magnet was cooled only at 9K, while thetarget temperature was 5K. The reason was analysed afterwarm-up, to be insufficient thermal anchor or thermal
shield for incoming heat from current leads. It is now
modified to improve thermal shield from current lead.
During the cool-down, alignment of the cavities hang on
one long GRP of CM-1 is checked by the WPM. Relative
movement of the wire by WPM on the GRP are plotted in- 166 -Proceedings of the 12th Annual Meeting of Particle Accelerator Society of Japan
August 5-7, 2015, Tsuruga, JapanPASJ2015 FROL03

figure 10 for X direction (horizontal direction) and figure
11 for Y direction (vertical direction), during cool-downin one month. Big movements happened on steep coolingfrom 200K to 4K for Y direction, about 3.5mm on theboth end of GRP. It seems to happen by temperaturedifference between GRP top and bottom. For X direction,
the movement is not significant, however, very slow
damping of vibration of wire was observed.

Figure 12: WPM trend plot for Cavity #1, the most
upstream one, X-direction for red and orange, Y-direction
for blue and purple, together with GRP temperature
variation. Two set of plots are corresponding to cavity
support tab upstream and downstream. Variations arefollows from GRP movement.

Figure 10: WPM trend plot for GRP X-direction, together
with GRP temperature variation. Plots from top to bottom
are for GRP upstream to downstream positions.
Relatively big variation happens on the most upstream
WPM(red).

5 BEAMLINE OPTICS PLAN
Considering future extension of the STF accelerator for
possible FEL application, a design of beam energy 1GeV
with two-stage bunch compressor have been done asshown in figure 13. After capture cryomodule, the firstbunch compressor (BC1) has been designed with 45MeV
beam energy. Then beam is accelerated by 3 cryomodules
to 525MeV. The second bunch compressor (BC2) isplaced at this energy. There is a plan for extension of three
more cryomodules for 1GeV beam for undulators withnew tunnel extension.

Figure 11: WPM trend plot for GRP Y-direction, together
with GRP temperature variation. Plots from top to bottom
are for GRP upstream to downstream positions. Fairly bigvariation happens on steep cool-down on both end of GRP(red and magenda).
2 set of WPMs for the cavities are attached to the bothsupport tabs in CM-1. So we can measure the cavity offset
movement and tilt movement, both. As an example ofWPM data for the cavity, #1 cavity two set WPM trend
data are plotted in figure 12, together with GRP
temperature variation. They should be followed amovement of GRP upstream position, because thecavity#1 was hang on GRP upstream end part. Plot shows
good traceability to GRP, however, reached positions byboth upstream and downstream WPM in cooled stateshow tilts on X and Y, both direction.

Figure 13: Planned beam line optics design for future STF
accelerator. Total 6 cryomodules and 15m undulator
section are assumed.
The BC1 compress the bunch length from 1.25mm to0.3mm with 0.068% energy spread. And the BC2compress the bunch length from 0.3mm to 0.05mm withthe same energy spread.

- 167 -Proceedings of the 12th Annual Meeting of Particle Accelerator Society of Japan
August 5-7, 2015, Tsuruga, Japan

PASJ2015 FROL03

6 RF POWER DISTRIBUTION AND
CONTROL PLAN
The RF power scheme with control of power dividing
ratio and phase is adopted as a cost effective baseline ofTDR. The TDR RF power source consist of a 10MW
multi-beam klystron (MBK) and a Marx modulator, andwaveguide system which distributes 1.3GHz 1.6ms pulse
RF power into 39 cavities with circulator in each input. Inorder to supply RF power effectively to 20% spread ofgradient performance cavities, the power can be split withflexibility by a variable hybrid (Pk control). Also, a phaseof RF input can be controlled by a phase shifter in each ofcavity input line. A coupling of cavity (loaded-Q, QL) canbe controlled by an input coupler insertion length. In order
to control each cavity power input and loaded-Q of eachcavity among vector-sum controlled cavities, the above
variable adjustment (Pk-QL control) are controlled
remotely. Figure 14 shows waveguide installation outside
the cryomodule, CM-1 and CM-2a, to accommodate thesefunctions. Figure 15 shows detail side view of thiswaveguide system for CM-1.

during RF pulse interval of 200ms makes it possible tocompensate acceleration voltage shortage by thisinterlock RF shutdown. So we can keep beam energy
constant by this control logics and communications in any
predictable events, in ILC.

Figure 16: Example of a pulse-by-pulse RF waveform
acquisition and analysis. There are sequence of RFwaveforms from 143 to 146 pulse number. The loaded Q
are calculated using tail slope for every pulse. We canknow variation of QL at 145 pulse, then control logic
shutdown the RF power supply. So, in 146 pulse, there is
no RF power, just noise.

7 ACKNOWLEDGEMENT
Figure 14: RF power distribution scheme of STF phase-2
cryomodule, according to TDR design.

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, and the support of Accelerator Laboratory
director, S. Yamaguchi, Accelerator 6th-division head, S.
Michizono, trustee of the executive board, Y. Okada, anddirector general of KEK, M. Yamauchi, are gratefully
appreciated.

8 REFERENCE
Figure 15: 2 set of RF power distribution for 4 cavities
set. In STF, 3 set of this waveguide packages are used for
12 cavities.
To control RF pulse flat-top with very flat in amplitude
and phase for beam acceleration, the fast digital feedbackis used by utilizing FPGA controller element together
with fast A/D and D/A. The software in the FPGA capture
the every RF pulse waveform digitally, and calculate
feedback value for amplitude and phase control, and also
calculate loaded Q value (QL) for every pulse. Once QL
has abnormal value, it can be predicted that cavity willquench in the next pulse or so. Then we can shut-down RF
power to cavities beforehand, in order to avoid any
over-loaded RF power in the cavities. So it is used for theQL interlock. Communications between other RF unit

[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] H. Hayano, “Superconducting Accelerator Developmentfor ILC”, Proc. Of The 11th annual Meeting of Particle
Accelerator Society of Japan, Aomori August 2014.
[5] Vladimir Kashikhin, “Splittable Quadrupole for LinearAccelerators”, AWLC2014, FNAL May 2014.
[6] H. Hayano, et. al., “Status of re-entrant BPM R&D for ILC
Main Linac”, AWLC2014, FNAL May 2014.

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