The 11th Symposium on Accelerator Science and Technology, Harima .Science Garden City, 1997

The Booster Ring Cyclotrons for the RIKEN RI Beam Factory
Akira GOTO, Toshiyuki KUBO, Hiroki OKUNO, Takeo KAWAGUCHI, Toshinori MITSUMOTO,Toshiharu TOMINAKA, Shiro FUllSHIMA, Jong-won KIM, Kumio IKEGAMI, Naruhiko SAKAMOTO,Shigeru YOKOUCHI, Tetsuya MORIKAWA, Kazuo SUGII, Yasushi TANAKA and Yasushige YANOThe Institute of Physical and Chemical Research (RIKEN),
Wako-shi, Saitama, 351-01 JapanAbstract
An energy booster of the existing K540-MeV ring
cyclotron for the RIKEN RI Beam Factory is being
designed in detail to provide primary heavy ions, up touranium ions, with energies exceeding 100 MeV/nucleon.
The energy booster consists of a four-sector normalconducting ring cyclotron (K=930 MeV) and a six-sector
superconducting ring cyclotron (K=2,500 MeV). Generalfeatures of the booster ring cyclotrons as well as staus of
the design of them are described.

1 Introduction
The RARF (RIKEN Accelerator Research Facility)
houses a heavy-ion accelerator complex consisting of aK540-MeV ring cyclotron (RRC) as a main accelerator andtwo different types of injectors: a variable-frequency heavyion linac (RILAC) and a K70-MeV AVF cyclotron (AVF).
One of the remarkable features of this facility is capabilityof supplying light-atomic mass RI (Radioactive Isotope)
beams with the world-highest level of intensity produced by
the projectile-fragment separator, RIPS [1]. In order tofurther promote research fields by utilizing RI beams, theRARF constructs "RIKEN RI Beam Factory" as a next
facility-expanding project [2]. The factory takes the aim atproviding RI beams covering over the whole atomic-massrange with very high intensity in a wide range of energies
up to several hundreds MeV/nucleon.
To meet this requirement, we plan to construct acascade of a four-sector normal-conducting ring cyclotron
and a six-sector superconducting ring cyclotron after theRRC. The ring cyclotrons are expected to boost the energyof ion beams from the RRC up to 400 MeV/nucleon forlight heavy ions like carbon and over 100 MeV/nucleon forvery heavy ions like uranium.

2 Description of the Booster Ring Cyclotrons
2.1 General Features
In a previous design [3], a very large single
superconducting ring cyclotron had been studied as anenergy booster of the RRC. After further study, we haverecently modified the design in such a way that it is split
into two stages: a four-sector normal-conducting ring
cyclotron (IRC: Intermediate Ring Cyclotron, K=930 MeV)
for the first stage and a six-sector superconducting ringcylotron (SRC; K=2,500 MeV) for the second. This twostage scheme has two big advantages. Firstly, thesimultaneous utilization of the IRC beam is possible in boththe existing experimental facility and the new facility, whena part of the beam is charge-stripped and is transferred backto the existing facility. As an example, we give a 127
MeV/nucleon 160 7+ beam from the IRC, while the main partof which is injected into the SRC, a part of which is chargestripped to 0 8+ and delivered to the existing facility (wherethe magnetic rigidity of the 0 8+ beam can be accepted).
Secondly, the difficulty of fabrication due to huge length ofthe cold mass and huge electromagnetic force on it isdrastically eased compared to the single-stage scheme.
The maximum beam energy of the ·SRC is set to be
400 MeV/nucleon for light heavy ions, which should beachieved at 38 MHz, the maximum rf frequency stably
operated in the RILAC. This means that the velocity of theRRC output beam has to be amplified by a factor of 2.26 byboth the IRC and the SRC. Harmonic numbers of th~ IRC
and the SRC are chosen to be 7 and 6, respectively, whilethat of the RRC is 9, considering the maximum magnetic
field strengths and the injection and extraction radii. The
mean injection radius of the IRC is taken to be 7/9 times themean extraction radius of the RRC, and the velocity gain
factor of the IRC to be 1.50. (For 400 MeV/nucleon lightheavy ions, their beam energy from the IRC becomes 127
MeV/nucleon, which is almost the same as the maximum
beam energy of the RRC now available.) From the above
conditions, the mean injection and extraction radii of theIRC are 2.77 m and 4.15 m, respectively, and those of theSRC are 3.56 m and 5.36 m, respectively. The sector anglesof the IRC and the SRC are 51 deg. and 25 deg.,respectively. The maximum magnetic fields required for theIRC and the SRC are to be 1.9 T and 4.3 T, respectively.
The maximum energies of the IRC beams are 127
MeV/nucleon for light heavy ions up to around Ar, 102
MeV/nucleon for Kr30+, 58 MeV/nucleon for U 58+. Theirminimum energy is 25 MeV/nucleon. The maximum
energies of the SRC beams are 400 MeV/nucleon for lig_ht
heavy ions up to around Ar, 300 MeV/nucleon for JCrO+,
150 MeV/nucleon for U58+ and 100 MeV/nucleon for lf9 +.
Their minimum energy is 60 MeV/nucleon.
The structure and size of the IRC are similar to thoseof the RRC. Therefore, our effort has been concentrated
mainly on the design of the SRC, in particular, on thedesign of its sector magnet and injection system,

2.2 IRC
Fig. 1 shows a schematic layout of the IRC. The IRC
consists mainly of four sector magnets, two main rf
resonators and a flattop resonator, and injection andextraction systems. Total weight of the sector magnets is
2,400 tons.
Initially the main coil was d.esigned to besuperconducting. Further investigation, however, hasrevealed that the power consumption can be as small as 320-53-The 11th Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

Sector oMagent

IFig.l Layout of the IRC.
kW in total even for a normal-conducting coil. Therefore,
we have designed the main coil to be normal-conducting.
The main rf resonators are of a double-gap type with astructure similar to that for the RRC [4]. Their resonant
frequency ranges from 18 MHz to 38 MHz, which is thesame as that for the RILAC and the RRC. The flattop
resonator with the third harmonic frequency is of a singlegap type.

2.3 SRC
Fig. 2 shows a schematic layout of the SRC. The SRC
consists mainly of six sector magnets, three main rf
resonators and a flattop resonator, and injection andextraction systems. Total weight of the sector magnets is4,000tons.
Magnetic fields and related characteristics werecalculated with a three-dimensional computer program
TOSCA [5]. Equilibrium orbits and betatron tunes werecalculated with the computer program that had beenoriginally developed for the RRC. Shape of the sector
magnet has been optimized with those programs in order toavoid any dangerous resonances [6].
Fig. 3 shows a cross-sectional view of the sector
magnet. Main components of the sector magnet are superconducting and normal-conducting coils, poles and a yoke.
Two kinds of super-conducting coils are used: a pair ofmain coils and a group of trim coils. The maximum currents
required for the main coil and the trim coils are 5,000 A and
500 A, respectively. The maximum ampere-turns of a pairof main coils is 6 MA, which is large enough to generate
the designed field strength of the sector magnet. A group ofnormal-conducting trim coils are also arranged on the upperand lower sides of the beam chamber. A remarkable featureof this sector magnet is the cold-pole arrangement [7]. This
arrangement gives an easier mechanical support against thehuge magnetic force on the main coils, and gives thereduction of ampere-turns and of magnetic forces,
compared with a warm-pole arrangement. The pole pieces
are separated from the iron yoke and are cooled down
together with the main coils in the cryostat. Two coilvessels that accommodate the main coils are attached to theside of the upper and lower pole pieces that are linked eachother by pole links. The pole gap is designed to be 380 mm,
considering sizes of the trim coils and of the magnets forinjection and extraction to be installed in the gap space. The
cold-mass weight is estimated to be about 50 tons. The coilvessels are made of stainless steel. One of the important
issues in the design is how to fix the coil vessel to the sideof pole piece. The detailed design study is in progress,
considering the stresses due to the thermal contraction andelectromagnetic force. The vertical force of 1,150 tons is
supported with the pole links between the upper and lower
cold-poles. The radially shifting force of 100 tons, which isgenerated by the arrangement of six sector coils and theasymmetric configuration of the coils and irons, issupported with a large-size thermal insulated supports
between the cold-mass and the side yoke.
We have started to construct a full-scale model sector
magnet of the SRC without a yoke to verify particularly themechanical design of magnet and the cryogenic design ofthe main and trim coils, which are designed to be
cryogenically stable [8]. The superconductors forthe main
and trim coils have already been complete. Construction of
the whole system is scheduled to be completed by themiddle of 1998.
For the field isochronization, a major correction ismade with superconducting trim coils, and the remnant
error field is corrected with normal-conducting trim coils.
By use of five sets of superconducting trim coils, the error
field reduced within ± 10 gauss, or ± 0.1 % of theisochronous field. Further fitting using twenty sets ofnormal-conducting trim coils was then carried out. The-54-Sector
MagnetSingle-gap
Resonator

Fig. 2 Layout of the SRC.

The Jlth Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997
Yoke

Cold PoleFz Support

Trim Coil(normal)

Trim Coil(super)masses of the six sector magnets
weigh 360 tons, and therefore it
will take 011e and a half months
for cooling them from room
temperature to 4.5 K. By taking
the two-refrigerator operation,
the magnets can be . kept ·at
between 5 K and 6 K even when
one of the two refrigerators
breaks down. No liquid nitrogenin this cryogenic system is used
for simplicity of the c.oolingsystem.

4 Summary
Two ring cyclotrons withnormal-conducting four-sector
and superconducting six-sector
magnets, respectively' aredesigned as an energy booster ofFig. 3 Cross-sectional view of the sector magnet of the SRC.
the existing ring cyclotron for"RII<EN RI Beam Factory". A
cascade of the two ringfield error is less than ± 0.05 % for the case of the fivecyclotrons is aimed at accelerating heavy ions up t.o : e.g.,superconducting trim coils, which meets our criterion of
400 MeV/nucleon for light heavy ions like carbon, 300
field isochronism.
MeV/nucleon for krypton ions, and over 100 MeV/nucleon
The injection system consists of four bending magnets
for uranium ions. The six-sector ring cyclotron SRC adopts(BMl, BM2, BM3 and BM4), three magneti~ channels
a cold-pole arrangement and a cryogenically-stable method(MICl, MIC2 and MIC3) and an ~lectro~tatic ch~nnel
for the main and superconducting trim coils. A full-scale(EIC), as shown in Fig. 2. The MI~ .s ~e mserted ~n the . model of the sector magnet of the SRC is being made topole gaps. The problem in the SRC t.nJection sys~em ts ~atverify the design. Construction of the two ring cyclotrons
the beam trajectory changes depending on the kind of ton
has been approved by the Government and is scheduled todue to the difference of fringe field in the valley region. For
be completed in the year 2002.
example, the EIC is requrred to be radially movable byabout 10 em and its curvature should be changeable toReferences
some extent. The apertures of the other elements should
also be wide enough to accept the difference of trajectories.
[1] T. Kubo et al., N.I.M., 870 (1992) 309.
. , .
The four bending magnets and the MIC3 are[2] Y.Yano et al., "RIKEN RI Beam Factory Project , tn
superconducting. The extraction system co~sists of an
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Proc. lOth Symp. on Acct. Sci. and Techn.ol.,(EBMl and EBM2), as also shown in Fig. ~· The t~oHitachinaka, (1995) 457.
bending magnets and MDC3 are superconducting. Detatls[4] T. Fujisawa et al., "The Radiofrequency System of theof orbit analysis in the injection and extraction systems andRIKEN Ring Cyclotron", N.I.M., Al92 (1990) 1.
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[10], respectively.
[6] T. Mitsumoto et al., "Orbit Analysis for the RIKEN
The main rf resonators are designed to be of a singleSuperconducting Ring Cyclotron", in this proceedings.
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double-gap resonator particularly m the IDJection regton.
for the RIKEN Superconducting Ring Cyclotron (ll)",
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Proc. lOth Symp. on Acct. Sci. and Technol.,instead of from 18 MHz to 38 MHz. This is because it isHitachinaka, (1995) 127.
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proceedings.
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[9] S. Fujishima et al., :"Analysis of the Injection ~is twice the frequency of the RILAC, the RRC and the IRC:
Extraction Trajectories in the RIKEN Superconductingfrom 36 MHz to 46 MHz. The flattop resonator with theRing Cyclotron", in this Pr-oceedings.
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[10] H. Okuno et al., "Design Study of the Injection ~dTwo refrigerators having a capacity of 500 W each. atExtraction Systems for the RIKEN Superconducting
4.5 K will be used for cooling of the six sector magnets plus
Ring Cyclotron", in this proceedings.
the beam injection and extraction magnets. A total of cold--55-