Proceedings of ERL09, Ithaca, New York, USA

WG305

KEK ERL CRYOMODULE DEVELOPMENT
H. Sakai*, T. Furuya, E. Kako, S. Noguchi, M. Sato, S. Sakanaka, T. Shishido, T. Takahashi,K. Umemori, K. Watanabe and Y. Yamamoto
KEK, 1-1, Oho, Tsukuba, Ibaraki, 305-0801, Japan
M. Sawamura
Japan Atomic Energy Agency (JAEA), Tokai, Naka, Ibaraki 319-1195, Japan
K. Shinoe
Institute for Solid State Physics (ISSP), University of Tokyo, Kashiwa, Chiba 277-8581, Japan
Abstract
Development of a SC Cavity Injector Cryomodule andMain linac Cryomodule for the compact ERL (cERL) [1]is being continued at KEK since 2006. Design of an
injector cryomodule containing three 2-cell 1.3-GHz
cavities for Injector Cryomodule and two 9-cell 1.3-GHz
cavities for Main linac Cryomodule are almost completed.
Status of R&D and design details are reported.

stainless steel. During the test, we observed some thermal
instability (blue dots in Fig. 2), where both Q and gradientdecrease slowly. It is well known due to the heating ofpick-up antennae of HOM couplers. Heating of one HOM
coupler was detected by thermometer at around 16 MV/m,but finally we could keep 16 MV/m for 6 hours.

INJECTOR FOR CERL
An injector for cERL is required to accelerate a CW
electron beam of 100mA to 10MeV. In this application,
critical hardware components are not cavities but RF
input couplers and HOM dampers. Several combinationsof number of cavity and cells per cavity were examined,
and a three 2-cell cavity system was chosen for cERL.
Each cavity is drove by two input couplers to reduce
required power handling capacity and also to compensate
coupler kick. HOM coupler scheme was chosen for HOM
damping, and 5 HOM couplers are put on beam pipes ofeach cavity. Because of simplicity cavities are cooled byjacket scheme. Basic parameters of the cavity aresummarized in Table 1.
Table 1: Basic Cavity Parameters of injector
Frequency
1.3
GHz
Number of cell
2
R/Q
205
Ω
Operating Gradient14.5
MV / mNumber of Input Coupler
2Coupler Power
167
kW
Coupler Coupling Q
3.3 x 105
Number of HOM coupler
5
Operating TemperaturekFigure 1: 2-cell Cavity

２cavity
A 2-cell cavity is shown in Figure 1. It has a TESLAlike cell shape and larger beam pipe aperture of 88mm.
Two fully equipped prototype cavities were fabricated,
and the first cold test in a vertical cryostat was done in thelast March. The cavity gradient reached 30MV/m withsmall electron loading (Figure 2). The reason of low Q
value is due to losses at beam pipe flanges made of___________________________________________* sakai.hiroshi@kek.jp

Figure 2: Vertical Test Results.

Input Coupler
RF input coupler is the most critical component in thehigh power application of the superconducting cavity. The
most powerful CW coupler under operation is the KEK-B
couplers, which has a coaxial disk type window

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developed for TRISTAN SC cavities [2]. We made scaled
models to 1.3 GHz, as shown in Fig. 3 and 4.
Impedance of coaxial part is 41Ω, and the outer diameteris 82 mm.

Figure 3: Input Coupler for Injector Cavities.
Couplers will be assembled to cavity in the clean-room
before installation to a cryostat, so it should be short as
possible. Then thermal intercept becomes difficult, andrequires the 5k and 80k anchors at outer conductors. Innerconductors and the windows are cooled by water. Highpower test is scheduled in September.

introducing second stub and a boss as can be seen in Fig.5[3].

Figure 5: Two Stub HOM Coupler
Fig. 6 shows the H field distribution of the modified
HOM coupler, the H field is reduced by a half, to 2000
A/m at 15 MV/m. The first cold test was performed withthese HOM couplers. After some processing, we could
rise the gradient to 30 MV/m. Heating appeared in one
HOM coupler, but we could keep the gradient of 16
MV/m for 6 hours.

Figure 6: H-Field Distribution

Frequency Tuner
Figure 4: Prototype Input Coupler

HOM Coupler
We decided to use HOM couplers instead of beam pipe
HOM absorbers to damp HOMs, because absorbers are
not well established in cold and they need extra drift
space. Major HOMs are summarized in Table 2.
Table 2: Major HOMs
ModeTE111
TM110
TM011
TM020

Frequency
1.57GHz
1.63GHz
1.80GHz
1.88GHz
2.28GHz
2.31GHz
2.67GHz
2.69GHz

R/Q0.59 Ω/cm2
1.8 Ω/cm2
4.0 Ω/cm2
1.9 Ω/cm264 Ω
12 Ω
0.4 Ω
31 Ω

Measured QL
400
350
1000
900
2000
1600

TESLA HOM couplers are considered as the best choice,but it is well known that thermal instability appears above
10 MV/m in the CW operation. It is also well known thatheating happens at pick-up antennae of HOM couplers,
but it is not yet understood why niobium antenna becomesnormal conducting. One may expect that if the current
density at antennae is reduced, the threshold gradientincreases. TESLA HOM couplers are modified by
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We will use Slide Jack tuners [4, 5] which are used in
STF cavities as is shown in Fig. 7. Two pairs of wedge
are set on both side of jacket cylinder flanges and driven
by one shaft from outside of a cryostat. One piezo system
is put in series with a slide jack tuner, and will be replaceable from a cryostat opening. Stroke of the tuner is listedin Table 3.

Figure 7: Slide Jack Tuner

Mechanical Tuner
Fine Tuner

Table 3:
TypeSlide JackPiezo

Stroke1mm4µm∆f1.3MHz
2.6kHzProceedings of ERL09, Ithaca, New York, USA

Cryostat

WG305

MAIN LINAC FOR CERL

Fig. 8 and 9 show a cryostat containing three 2-cellcavities. All the cross section may become square.
Cavities are dressed with He vessel made of Titanium,and magnetic shields are put inside of He vessel. The
estimated cryogenic load in 100mA and 10MV operation
is summarized in Table 4. As is seen from this table, it iscritical to take dynamic load of input couplers and HOM
extraction cables. They will be anchored to 4.5k reservoir
panels put on both side of cavities, which works as athermal shield as well. Because of this difficulty theoperating gradient may be lowered.
Table 4: Cryogenic Load per Cavity

CavityInput Coupler
HOM Cable
Beam Pipe
Others
Total

Static
02W
1W
1W
5W
9W

2k
Dynamic
6W
4W
7W
0017W

Static
08W
5W
2W
10W
25W

4.5k
Dynamic
016W
14W
0030W

Nine-cell SC cavities used for the main linac are underdevelopment to achieve a stable accelerating gradient of
15 - 20 MV/m under the beam of 100 mA. These cavities
were designed [11] so that harmful higher-order-modes(HOMs) can be extracted through large beam pipes to theabsorbers set on the 80K temperature. Thanks to theprinciple of the energy recovery, we can reduce the input
power down to 20kW for each cavity. The cavity iscooled down to 2K by using jacket. Basic parameters ofthe cavity are summarized in Table 5. Because the heatload of HOM absorber is high, the sophisticated coolingis needed for main linac cryomodule.
Table 5: Basic Cavity Parameters of main linac
Frequency
Number of cell
R/Q
Operating Gradient
Unloaded QCoupler PowerCoupler Coupling Q
HOM load per HOM absorber
Operating Temperature

1.3
9897
15-20
>1 x 1010
20 (max)
0.5-2 x 107>1002GHzΩMV / mkWWKcavity

Figure 8: Injector Cryomodule

Fig. 10 shows a conceptual view of the KEK-ERL
model-2 cavity [6-8], which has been designed for cERL.
The TESLA 9-cell cavity was modified to meet ourrequirements. Its features are the following.
• Cell shape is optimized and large iris diameter of 80
mm is chosen to suppress HOMs.
• Eccentric-fluted beampipe is adopted to suppressquadrupole HOMs.
• HOMs propagate through the large beampipes andare absorbed by HOM absorbers mounted on bothsides of the cavity; one is 120mm as Large Beam
Pipe (LBP) and the other 100mm as Small Beam
Pipe (SBP) .

Figure 10: A conceptual view of the KEK-ERL model-2
cavity for the main linac. HOM absorbers are on the bothsides.

Figure 9: Injector Cryomodule

In order to validate the cavity shape of KEK-ERL
model-2, fabrication and surface treatment processes weretested on two single-cell Nb cavities, C-single and Esingle, which are shown in the left figures of Fig. 11. Csingle has the same cell shape as that of the central cell of
the 9-cell structure. E-single has the shape of the end cell
equipped with both beam pipes of the 9-cell cavity.

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After 4th and 5th vertical test, we investigate the innersurface by using the optical inspection camera [10]. We
found the large tip on 8-9 iris around 150° as shown inFig.14, which is just opposite side of 330° of the strong
radiation point of green dotted circle of Fig.13. Fromthese results, we thing tip of the iris point is one of theradiation sources and it is needed for recovering fromfield emission to grind this tip.
Figure 11: (left )two single-cell cavity one is C-single(top) and another is E-single cavity (bottom). (right)
results of the vertical tests of single-cell cavities.
We have first fabricated two Nb single-cell cavities andtested them successfully [9]. The result of vertical tests
shows the right figure in Fig.11. C-single and E-single
finally satisfied the specification of 20 MV/m with theunloaded-Q of 1×1010. Then, we fabricated a prototype 9cell cavity, and carried out its vertical tests. The result ofvertical tests is shown in Fig. 12. We successfully tested
the prototype cavity up to a field gradient of 15 MV/m at
2K, and achieved an unloaded-Q of higher than 1010 at 10MV/m in five vertical measurements. We also observedthat the Q-value decreased due to field emissions above
the field gradient of 10 MV/m.

Figure 14: picture of the inner surface on 8-9 iris around150° by using the optical inspection camera.

Input coupler
Minimum input power is restricted by the cavity
detuning due to the microphonics from the cryomodule.
We start to consider that the maximum detuning
frequency is 50Hz caused by the microphonics. Fromthese results, we determine that the maximum input
power is 20kW and loaded Q is 2×107 [8].
Figure 12: (Left) preparation for the vertical testing the 9cell cavity. (Right) the first result of the vertical tests.

R 1060details now under designing

dry N2 gas outp umpmovable
35

10R8φ16φ46Warm window

movab le96

R5

RF & Cryomodules

atomosphare

300Kcavity
25 ‹Figure 13: (Left) schematic view of the roating mapping
system (Right) the result of radiation mapping.

80K

80K

5K80
BAi rp ump

B

In order to investigate the cause of this problem indetail, we prepared cavity diagnostics by using rotating
mapping system after 4th measurements; one is carbon
resistor for measuring heat spot and another is Si PIN
diode for measuring X-ray radiation map. Fig.13 shows
one of the results of X-ray radiation mapping at 4thvertical measurement. We have strong radiation peak on
8-9 iris around 330° and also see the broad radiation
traces on 1-6 irises on 4th and 5th vertical measurements.

vacuumCold window
5K

20125

202

125267

dry N2 gas in460

cool airgas inpump
cool airgas out

Figure 15: schematic view of input coupler for main linac.
Fig. 15 shows the schematic view of input coupler formain linac. The STF-BL input coupler was modified tomeet our requirements [11]. Its features are the following.
• Change the impedance from 50Ω to 60Ω to reduce
the heat load of inner conductor. Furthermore forced
air cooling was applied to inner conductor.
• Purity of ceramic material was changed from 95%to
99.7% to reduce the heat load of ceramic.
• Cold ceramic size is same as warm one.
• Variable coupling was applied from QL = 5×106 to
2×107 for the short pulse conditioning of input power
coupler in commissioning.
In order to check the heat load and temperature of input
coupler by feeding the high power, first we made the test
stand for testing the components of the input power
couplers, especially ceramic windows and bellows, bydelivering the maximum 30 kW CW average power froman IOT as shown in Fig. 16.

Proceedings of ERL09, Ithaca, New York, USAWG305

shows the design of the prototype of HOM absorber. Itsfeatures are the following.
• HIP bonding between ferrite and copper are applied
to keep the strong connection• Comb-type RF bridge is set to suppress the HOM
come from the bellows• Two kinds of thermal anchor at 80K and 5K wereapplied to absorb the heat load and reduce the static
loss from HOM absorber to the cavity.
H IP ferr i teComb - type RF Br idge

In test stand, warm window with bellows sandwiches thecold window and power go through the 2 warm windows
and one cold window. Temperatures of bellows andceramic windows were monitored. In high power test, thesudden temperature rise was observed when the powerincreases up to 8kW and finally the cold ceramic window
was broken as shown in Fig. 17. In low level test of coldwindow, we see the sharp resonance peak of 1.305GHz on
the S21 measurement. And we found this peak is shifted tolower frequency side when temperature is increased. We
also found that this frequency is same as the calculationof the unexpected dipole mode which stands on the chokeof the ceramic window. From these results, this peak will
induce the sudden temperature rise. In order to escape this
dipole mode, we plan to modify the ceramic window bychanging the thickness of ceramic.

2Figure 16: (Left) ceramic windows of input coupler withbellows. (Right) schematic diagram of coupler test stand.

80

Be l lows

80K The mr a l Anchor
4K Therma l Anchor

Figure 18: schematic view of HOM absorber
We plan the heat removal test by using this prototype,Cryostat
Fig. 19 shows the design of the main linac cryomodule
containing. Two cavities are set on one cryostat. Cavities
are dressed with He vessel made of Titanium, andmagnetic shields are put inside of He vessel. We also
apply the enlarged jacket size (dia. 300mm) of cavity forsmooth pumping of 2K by considering the heat load of40-50W per cavity. One coupler feeds the RF power toone cavity. Three HOM absorbers set on the cryostat. The
dynamic loss come from input coupler and HOM
absorber is mainly absorbed 80K thermal anchor and byadding the 5K thermal anchor the static loss is reduced tobelow 1W to cavities at 2K. We also use the slide tuner toour cavity.

Figure 17: (Left ) the measurement of temperature riseof warm ceramic windows (brown, orange line) and cold
window (red line) corresponding to the input power(black line). (Right) broken ceramic cold window

HOM absorber
The HOM damper is also important for the ERL. HOM
heat load is expected to be more than 100W. Therefore,
HOM absorber material is set in the large beam pipes at
the temperature of 80K. In order to investigate the enoughabsorption for high frequency at 80K, we measure theproperties of absorption of the 8 kinds of ferrite and one
ceramic. The detailed results will be shown in Ref.[12].
After the measurement, we decide to use new-type ofIB004, which was used to the HOM absorber of KEKB,
for the first prototype of HOM absorber for ERL. Fig.18

Figure 19: The design of main linac cryomodule.

SUMMARY
Development of Injector and main linac cryomodule
are in progress. Assembly of cryomodule is scheduledfrom 2011 to 2012.

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REFERENCES[1] S. Sakanaka, et al., proc of PAC09, TU5PFP081.
[2] S. Noguchi, et al., “Couplers-Experience at KEK”,
Proceedings of 4-th SRF Workshop, KEK, 1989,
KEK Report 89-21,1990, p.397-p.412.
[3] K. Watanabe, et al., “New HOM coupler Design for
ERL Injector at KEK”, Proceedings of 13-th SRF
Workshop, Peking University, Beijing, China, (2007).
[4] E. Kako, et al., proc. of PAC09, TU3RAI04.
[5] Y. Yamamoto, et al., PAC09, TU5PFP075.
[6] K. Umemori et al., APAC2007, p.570.
[7] M. Sawamura et al., PAC’07, p.1022.
[8] H. Sakai et al., Proc. ERL2007 ,p56
[9] K.Umemori et al., EPAC2008, p.925.
[10] Y. Iwashita et al., Phys. Rev. ST Accel. Beams 11,093501 (2008).

[11] E.Kako et al.,Proc. of the 3rd Annual Meeting
Particle Accelerator Society of Japan, Sendai, Japan,
2006, p136.
[12] M.Sawamura et al.. in these proceedings of ERL’09

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