The 13th Symposium on Accelerator Science and Technology, Suita, Osaka, Japan, October 2001

Status of the RIKEN SRC

H. Okuno, J. Ohnishi, N. Fukunishi, T. Mitsumoto, S. Fujishima, T. Tominaka, N. Sakamoto, 0. Kamigaito, M.
Fujimaki, K. Ikegami, Y. Miyazawa, M. KaseA. Goto andY. Yano, RIKEN, Wako, Saitama 351-0198, Japan Japan

Abstract
The RIKEN SRC (superconducting ring cyclotron) is
under construction. Significant changes have been madein the last stage of the design works. In the new design thecyclotron is almost completely covered with soft-ironslabs of about 1 m in thickness, which give additional
magnetic and radiation shield. They suppress the strayfield from the sector magnets, decreasing magnetic
motive forces for the maximum bending power. This
makes all critical parts of the SRC more feasible andstable to work. The new design and status of theconstruction are described in this paper.

1 INTRODUCTION
A K2500 superconducting ring cyclotron with 6-sectors(SRC) is being constructed at RIKEN as the final energy
booster of the new cyclotron cascade in the RI beamfactory (RIBF) [1]. The maximum magnetic rigidity of
extracted beams is 8 Tm which exceeds that of 350
MeV/nucleon lf8+ beam. The mean. extraction radius is5.36 m and the harmonic number is 6.
Significant changes have been made in the last stage ofthe design works. In the new design, the cyclotron isalmost completely covered with soft-iron slabs of about 1
m in thickness, except in the central region as shown inFig. I. The "iron cover" yields the following good results:(1) The SRC can be self-radiation-shielding.
(2) The stray field in the valley is reduced from 0.5 T to
0.1 Tat the maximum. The sector field needed to bend,for example, U58+ 150 MeV/nucleon is accordingly
reduced from 4.3 T to 3. 7 T because of this reductionof negative stray field, with the K-value of the SRC
being kept at 2,500 MeV. According to this lower
flutter, we relocate the operating domain of thevertical betatron frequencies from the values between
1.0-1.5 to those between 0.5-1.0. The decrease of thevertical betatron frequencies causes an increase in thevertical beam size by a factor of about 1.4, but thispresents no problem because of sufficient acceptanceof the injection and extraction elements. This decrease
also makes the tolerances of the main and trim coils
and poles tighter, but they are still tolerable.
According to the reduction of the maximum sector
field, the maximum magneto-motive force required isalso reduced from 5.4 MA/sector to about 3.8
MA/sector. The maximum stored energy and theelectromagnetic forces exerted on the main coil are
greatly reduced from 390 MJ to 235 MJ and from 400
Ton/m to 260 Ton/m, respectively. This reduction offorces allows us to adopt a warm-pole scheme; there is
now no concern about brittleness of the iron pole at
low temperature. The small stray field in the valley
allows us to use cryopumps, motors, control devices,and so on in a safe situation.
(3) Superconducting magnetic channels for beam
injection and extraction are unnecessary. Now all theinjection and extraction magnetic channels inside gapof the sector magnets are normal with the moderate
power consumption, and their structures are similar to
those for the RRC and the IRC[2].
(4) The shift of the injection and extraction trajectory
depending on the negative stray field strength isgreatly reduced.
(5) The stray field outside the SRC is reduced to about
200 gauss at the maximum near the yoke and thevertical side shield-wall. We need neither the active
magnetic shielding which was adopted in the olddesign nor the thick iron plates enclosing the huge
SRC vault. We place the rf oscillators near the SRC
like they are for the RRC and the IRC. The SRC vault
is now very safe for those working at the site evenins.ide the cyclotron.

/Magnetic Shieldrf ResonatorFig. 1: A schematic illustration of the SRC.

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The 13th Symposium on Accelerator Science and Technology, Suita, Osaka, Japan, October 2001

2 DESIGN OF THE SRC
Number of sector magnets
Sector angle
Gap width
Gap width for the beam
Dimension
DiameterHeightMean injection radius
Mean extraction radius
Orbital freauency
Harmonic number
Betatron frequency

0.5~1.0 (vertical)

Total wei ht

1.1~1.5 (Horizontal)
7800ton625 degree
640mm
90mm

18.8m
7.9m
3.56m
5.36m
3-6.3MHz

6Sector Ma net
Maximum magnetic field
3.8T
Maximum stored energy
235 MJ/sector (max.)
Accuracy of isochornism
1 X 104
Current Stability
5 x 10·6
Main coil
Magneticmotive force
3.96 MNsector (max.)
Nominal current
5000A
Superconducting trim coils
Number of the sets
4 set x 2(up/down)/sector
Maximum current
3000A
Normal trim coils
Number of the sets
22 set x 2(up/down)/sector
Nominal current
600A

RFsystem
Number of resonator
Gap
Turn seperation
RF range for acceleration
TunerMaximum RF voltage
Maximum RF power

4 (Accel.), l(Flattop)
Single gap> 4mm (w/o precession)
18 - 40.5 MHz
Flapping panel
600 kV/gap (Accel.)
150 kW/res. (Accel.)A layout and main parameters of the SRC is shown inFig. 1 and Table 1, respectively. The SRC consists mainlyof six sector magnets, four main rf resonators, a flattop rf
resonator, injection and extraction elements, magneticshield. Evacuation system, beam diagonics and He
cooling system are also installed for each purpose.
The sector magnet is 7.2 m in length and 6 m in height.
The weight is about 800 Ton per each. The sector angle is25 deg. The maximum sector field is 3.8 T, which is
required to accelerate 350 MeV/nucleon U88• ions. Maincomponents of the sector magnet are: a pair ofsuperconducting main coils, four sets of superconducting
trim coils, their cryostat, thermal insulation support links,
twenty~two pairs of normal conducting trim coils, warm~
poles and a yoke. Our goal of accuracy of the isochronous
field is less than 1x 104 • The details are described in [3,4].
The elements listed in Table 1 are installed for beam
injection and extraction. The structures of electrostaticand magnetic channels (EIC, EDC, MICs, and MDCs) aresimilar to those of RRC and IRC. Analysis of theirelectric and magnetic properties, however, was carefully
made because their required fields are much higher thanthose of RRC and IRC. The superconducting bending
magnet (SBM) for beam injection is required to generate
about 3.8 T along the trajectories. The SBM are underfabrication after the some test coils [3]. The extraction
bending magnet generate 2.04 T in the long trajectory of
3.8 m. Its structure was designed so that horizontal andvertical steerer were imbedded and profile monitor can beinstalled at the middle of the trajectory in order to extract
the beams with high efficiency [5].
RF system is designed to generate energy gain of about
2.4 MV/turn, which give as large turn separations as more

Injection and extraction S) em
1 EIC, 2 MICs, 1 SBM
1 EDC, 3 MDCs, 1 EBM
Evacuation s stem
Main evacuation system
Cryopump
14(10000 1/s), 2(5000 1/s)
Turbo molecular pump4(5500 lis)
Final ressure< 1 x 10~Pa
Beam Dia onics
Elements
1 MDP, 2 ERPs, 20 PP andBuffle slits
Ma~netic Shield
Wei htI3000Ton
He coolin s stem
Type of refrigerator
Claude cycle
Cooling capacity
800W @4.5 K+
4000W@ 70K+
110 1/h (P.L. cooling)
Control dewar
2100 litter_ Magnetic shield (side)

Injection elements
Extraction elements

Table 1: Main parameters of the SRC. See the text andFig. 1 for the meaning of the symbols in it.
Fig. 2: Plan view of the SRC.

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The 13th Symposium on Accelerator Science and Technology, Suita, Osaka, Japan, October 2001

than 12 mm for beam extraction without beam losses,
using the off-centering acceleration technique which isroutinely used for the RRC operation. A single-gap-type
resonator whose structure basically the same as that of theRCNP ring cylotron and the IRC was adopted for theacceleration resonator[6]. Canning of the resonators isundergoing now.
There are three kinds of vacuum: vacuum for beam,
thermal insulating vacuum for the superconducting coils,
and sub-vacuum in the resonator for decrease theoutgasing into the vacuum for beam. The chamber which
separate the beam vacuum from the thermal insulating
vacuum in the sector magnet was designed to be enoughrigid to open the beam chamber keeping the thermal
insulating vacuum. Expansion seals are adopted for theconnections with twelve chambers where the beams pass.
A high vacuum of less than 1x 10-6 Pa is required for as
high transmission as more than 99 % during acceleration.
One differential probe (MDP), twenty phase probes(PP) and radial probes in extraction regions (ERP) areinstalled for beam-monitoring. Their detailed designs arein progress based on the experiences in RRC operation.
Buffle slits for beam protection and monitoring are
installed at the entrances of most elements for beam
injection and extraction. The entrances of the MIC2 and
MDC3 is difficult to install the slits due to the narrowavailable spaces. The designs of their structures are inprogress, taking how to replace them in cases of any
troubles into account.
Some of the iron slabs of the magnetic shield are
bridged on the top and bottom of the valley regions
between the sector magnets. The others are placed
vertically between these top and bottom slabs so as tocover the space between the back yokes of theneighboring sector magnets. Cross-sectional and planviews of the magnetic shield are shown in Fig. 9. The
total weight of these six falling-U-shaped structures isabout 3,000. The vertical outside slabs are assembled toform a double-leafed hinged door to be opened when themaintenance is carried out for the rf resonators, vacuum
pumps, and so on in the valley region. Asymmetries ofsix sector magnets is about 2,100 L and that in the control
dewar is 2,000 L. By taking the injection bending magnet
into account, the refrigerator is designed to have acapacity of 600 W at 4.5 K for the coils, 3500 W at 70 K
for the thermal shields, and 4 g/s for the current leads.
The total equivalent refrigeration capacity at 4.5 K is thus
about 1.3 kW. The time required to cool-down the coldmass of 180 Ton in from room temperature to 4.5 K is
estimated to be about 20 days.

3 CONSTRUCTION SCHEDULE
Figure 4 shows the status of the construction of theSRC vault where six bases for· the sector magnets andmagnetic shield has been settled. Their construction will
finish until March of 2003 including installation of theinfrastructures. It takes the longest time to fabricate thesector magnets. Winding of the coils will start in thisautumn and all fabrications of will finish in March of2003.

6 REFERENCES[1] Y. Yano et al., "RI Beam Factory Project at RIKEN",
in this proceedings.
[2] S. Fujishima et al., "Design of the Injection andextraction Systems for the RIKEN SRC", Proc. 16th
Int. Conf. on Cyclotron and Their Applications,Michigan, (2001).
[3] H. Okuno et al., "Superconducting Magnets for The
RIKEN SRC", in this proceedings.
[4] A Goto et al., "Progress on the Sector Magnets for theRIKEN SRC", Proc. 16th Int. Conf. on Cyclotron andTheir Applications, Michigan, (2001).
[5] S. Fujishima et al., " Design of an Extraction Bending
Magnet for the RIKEN SRC".
[6] N. Sakamoto et al.,"Construction of the rf resonator
for the RIKEN Intermediate-stage Ring Cyclotron(IRC)", Proc. 16th Int. Conf. on Cyclotron and TheirApplications, Michigan, (2001).

the sector field due to inaccuracies of their size werestudied. The study let us to make a decision to machineeach slab "beautifully" because disturbances toaccelerated beams are estimated to be not negligible,
assuming accuracy of non-machined slabs.
The cooling system is a closed-circuit system withoutliquid nitrogen. It consists mainly of the six cryostats ofthe sector magnets, the cryostat of the injection-bending
magnet, a control dewar, a refrigerator, three compressors
and five buffer tanks. The control dewar of 2.5 m indiameter and 3 m in height is located on the top of the six
sector magnets. The superconducting conductors of thecoils are electrically connected to the power leads in theliquid helium vessel of the control dewar. The total
volume of liquid helium in the main coil vessels for theFig. 3: Status of the construction of the SRC vaults(April 2001). The temporary structure for the building
construction is also seen.

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