The 9th Symposium on Accelerator Science and Technowgy, Tsukuba, Japan 1993

Superconducting Accelerator modules for the JAERI FEL
N. Kikuzawa*, M. Sawamura, R Nagai, E. Minehara, M. Tali.ao, M. Sugimoto,
M. Ohkubo and Y. Suzuki
Free Electron Laser Laboratory, Japan Atomic Energy Research Institute2-4, Shirakatashirane, Tokai·mura, Nali.a·gun, Ibaraki·ken, Japan*Department Nuclear Engineering, Kyushu University
6·11-1, Hakozaki, Higasi·ku, Fukuoka·shi, Fukuoka-ken, Japan

Table 1 Main parameters of the accelerator.

Abstract

ItemBeam Energy
Energy Spread
Peak Current
Pulse WidthRepetition

We have been developing and constructing a ,freeelectron laser (FEL) driven by a superconducting accelerator
at Japan Atomic Energy Research Institute, Tokai. A multirefrigerators
system,originallydesignedforasuperconducting accelerator module, has worked well formany weeks. And cold rf tests are performed after these are
installed in accelerator vault and connected with beam line.
In this paper, present statns of cooling system andresults of cold rf tests of the accelerator modules are described.

Specification
13.5 -20MeV<0.2%>lOA
40ps
l0.4MHzUndufatOif & Optical Cavity

I. INTRODUCTION
Construction of a superconducting linac driven FEL
system for far infrared oscillation has been continued atJAERI. An outline of the JAERI FEL program has beenreported elsewhere [1). The superconducting accelerator
consists of two pre-accelerator modules \\'ith a single-cellcavity and two main accelerator modules with a 5-cell cavity.
Main parameters of the accelerator are summarized in Tab. 1
and a schematic layout is shown in Fig. 1. The statns ofinjection system has been reported elsewhere [2].
All of the four superconducting accelerator modules
were made by Siemens A. G. and already installed in theJAERI FEL accelerator vault in this January. A multirefrigerators system for the superconducting accelerator
module has worked well to keep liquid He level for manyweeks. Cold rf tests are performed and the accelerator
produced its first beam in this June.
In this article, the status of the superconducting
accelerator modules is described briefly.

,,------'IILJ~
Fig. 1 Schematic layout of the superconductilng accelerator.

II. SUPERCONDUC TING ACCELERATORMODULEThe 500 MHz superconducting cavity was fubricatedfrom 3 mm Nb sheet with RRR = 150. The geometry of thecavity follows DESY one including higher order mode(HOM) couplers and so on. Evaluated values for the cavity
are an accelerating gradient of 5 MV/m at 4.2 Kanda quality
factor of 2xl09 or more. An external Q (Qextl value of a main
coupler is adjustable over 3.5 decades in range from 5xl05 tolxl09 . The Oext is adjustable to change beam currents. Acenter conductor of the main coupler is designed to be cooled
down by cold gaseous N 2 evaporated from a liquid N 2container.

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Figure 2 shows the cryostat and the multirefrigerators system for the JAERI superconducting
The multi-refrigerators system isaccelerator module.
originally designed to avoid a regulation under a domestic
Japanese pressure vessel code, and to develop a highlyWe use closed-cycle He gas
efficient cooling system.
refrigerators as a reoondenser and a shield cooler [3].
He portwires of copper to insulate from a vibration of the shieldrefrigerator. Thermal anchoring positions of the heat inflow
components are determined to divide the static heat loads into
two stages of 80 K and 40 K. Thickness of the heat shields
shall be 2 mm or more for thermal conductivity and 3 mm forconstruction.
The safety of the liquid He container is a severe
problem because cavities can not withstand high pressure.
We designed a large safety line with a low pressure burst disc.
For moderate pressure increase, the cooling power of the 4 K
refrigerator increase. When heat loads exceed the coolingpower of the 4 K refrigerator, safety valves relieve over 1.7kg/cm2 abs. lf all liquid He lost in less than 3 minutes, thecryostat and the cavity itself will be undamaged. Coldgaseous He goes out of the buildings through safety duct.

ill. EXPERIMENTA L RESULTS

Fig. 2 Structure of the superconducting accelerator moduleand multi-refrigerators system.
A 4-K refrigerator cools down and recondenses
vapored cold gaseous He around a heat exchanger of the 4-K
refrigerator in a He container. This 4 K refrigerator's
efficiency is about one half of liquefier's one, but it has notransfer losses of liquid He since gaseous He is recondensed
in the liquid He container. So total efficiency of the coolingsystem is as good as that by use a liquefier. A prototype of
the 4 K refrigerator has cooling power of 8 W at 4.5 K. A
limited cooling power of the 4 K refrigerator restricts heatloads of the cryostat. An rf loss of the 5-cell cavity is about
50 W with Eacc = 5 MV/m and Q~ = 2xi09 and reduced toless than 1 W with 1 %duty factor. We assume that heatloads must be less than one half of cooling power of the 4 K
refrigerator for safety margin. So evaluated value of totalheat loads must be less than about 4 W and of static heatloads Jess than 3 W.
We adopt a shield refrigerator to reduce static heatloads to less than our requirements. The shield refrigerator
cools down 40 K and 80 K heat shields to minimize heatinvasion to 4 K region from 300 K region. The shieldrefrigerator generates two stage cooling points, 40 W at 40 Kand J 20 W at 80 K, respectively. Cold heads of the shieldrefrigerator are connected with each heat shield by flexible

Acceptance tests of the accelerator modules was
carried out during last year at facility in German. Results ofthese tests are summarized in Tab. 2. The cryostat showed
stand by losses of about 3.5 W at 4.5 K. The static heat loads
do not depend on the scale of cryostat. The major heat leaks
is from signal wires, for example thermometers.
Table 2 Test result of the accelerator modules.

----------,--------------.-------------Cryostat
Qo
Eaccmax
Module(at5MV/m) (at Q0=lxl09 )standby losses(at4.2 K)
3.6W
7.3 MV/m
2.5xl09
Pre #19
3.0W
6.8MV/m
2.lxl0Pre#2
3.5 w
6.4 MV/m
2.2xl09
Main#l9
3.5W
6.4MV/m
2.0xl0Main#2~~~~--~~~--~~~~~----~~~--JThe cold rf tests have been done at JAERI in April
this year. The cool down is continuously done by the shieldrefrigerator and 4 K refrigerator from room temperature.
After temperature of the liquid He container is below 20 K,
about 2 weeks after, liquid He is transferred from dewars tothe liquid He container. The pre-accelerator module has
about the 100 litter liquid He container, but liquid He of about
160 litter (80 litter x 2) is needed due to the transfer losses.
Cooling power of the 4 K refrigerator is about 11 W or more,which is enough for our requirements. After the transfer, the4 K refrigerator keeps liquid He level over one month.

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Temperatures of the heat shields are lower than ourevaluation.
l.Oe+lO--D•u•• . • ~0eo :o•evaluated values, Q0 > 2xl09 at Eacc = 5 MV/m. The Q
degradation due to slow cooldown is not observed.
00' l.Oc+09=•The superconducting linear accelerator has beenconstructed and tested. The multi-refrigerators system has
worked well for many weeks to keep liquid He level To
reduce the vibrational amplitude due to the shield refrigerator,
we modified supporting rods.
The performances of the cavities are better than•

---

rr-

Pre#l

ACKNOWLEDG EMENTo Pre#20

The authors would like to thank Drs. M. Ishii and Y.
Kawarasaki for their continuous encouragements andinterests on this project.

• Main #lD

1\iain #2

l.Oe+Oll024

68

REFERENCES

Eacc(MV/m)

Fig. 3 Performance of the accelerating cavities.

[1] M. Ohkubo et al., Nucl. Instr. and Meth. A296 (1990) 270.
[2] M. Sawamura et al. Proceedings of the 8th Symposium on

The performances of the superconducting
accelerating cavities are shown in Fig. 3. It seems that Q Accelerator Science and Technology, Saitama, Japan, 1991
The [3] N. Kikuzawa et al. Proceedings of the 8th Symposium on
degradation due to slow cooldown is not observed.
Accelerator Science and Technology, Saitama, Japan, 1991
frequency sensitivity of the cavities for He pressure is about
35 kHz/(kgf/cm2). The He pressure is expected to vary by 0.2
kgf/cm2 . The He pressure will be controlled by a electricheater to keep a constant pressure.
Due to the high Oext value with which
superconducting cavities are operated, vibration is an inherent
problem. The vibrational amplitude of the cavity due to theshield refrigerator is larger than that due to the 4 K
refrigerator. To reduce the \'ibration by the refrigerators, the4 K refrigerator is mounted on a lifting frame and connected\\'ith bellows and the shield refrigerator is supported on antiThe vibration affect not only
vibration support rods.
fluctuation of frequency but also fluctuation of ·the coupling
factor of the main coupler. The vibrational amplitude andspeed of frequency fluctuation can be compensated by piezo
electric tuners. Vibration of vacuum vessel vibrate a center
conductor of the main coupler. The fluctuation of thecoupling factor is a serious problem. To reduce the vibrationfrom shield refrigerator, we make changes for the better in the
support of shield refrigerator by mounted on supporting archover the accelerator module.

IV. CONCLUSION-442-