JAERI-Conf 95-021

PA35
Design Study of Sector Magnet for
the RIKEN Superconducting Ring Cyclotron (I)T. Mitsumoto, A. Goto, T. Kawaguchi, Y. Tanaka, T. Kubo,
H. Okuno, T. Tominaka, S. Fujishima, andY. Yano
The Institute of Physical and Chemical Research (RIKEN)
2-1, Hirosawa, Wako-shi, Saitama 351-01, Japan
Abstract

Table 1 Required maximum energy of the SRC.

Design study of the sector magnets for a
superconducting ring cyclotron, which is adopted as a
post-accelerator of the existing ring cyclotron, is carriedout. Superconducting main coils as well as superconducting trim coils for rough fitting to isochronous
fields are adopted for the sector magnets. Isochronous
field distributions and betatron tunes are calculated.

1. Introduction
An "RI beam factory" has been proposed as anext facility-expanding project of the RIKEN
Accelerator Research Facility (RARF)[1]. The "RIbeam factory" aims at production and acceleration ofradioactive isotope beams covering the whole mass
region. It requires the energy of ion beam to be higher
than 100 MeV/nucleon. To meet this requirement, wehave adopted a superconducting ring cyclotron (SRC) as
a post-accelerator of the existing RIKEN Ring Cyclotron(RRC).
The SRC is expected to boost the energy of ion
beam from the RRC up to 400 MeV/nucleon for lightheavy ions like carbon ions and 150 MeV/nucleon forvery heavy ions like uranium ions. The sector magnet
of SRC have to be flexible enough to generateisochronous fields in a wide range of energies and forvarious q/A's. In this report we describe the feature ofthe sector magnet together with field calculation andorbit analysis.

300 MeV/nucleon
400 MeV/nucleon
150 MeV/nucleon
Figure 1 shows a schematic drawing of theSRC. The number of sectors for the SRC has beenchosen to be 6. Diameter of the SRC is 20 m. The pole
length is more than 3 m, the sector angle is around 23
deg., and the weight of one sector magnet is nearly 900
tons. From the matching condition with the RRC, theinjection radius of the SRC was selected to be 2/3 of theextraction radius of the RRC; the SRC operates with aharmonics of 6, while the RRC with a harmonics of 9.
Velocity gain factor is 2.26. RF frequency range is from18 to 38 MHz. Main parameters of the SRC are listed intable 2.

0 1 2 3m

2. General Description
The maximum acceleration energy of the SRC
was determined by experimental requirements. Themaximum energies for typical ions are summarized intable 1. Beam currents are expected to be more than 100
pf.lA for 400 MeV /nucleon light heavy ions such ascarbon and oxygen ions and about 0.2 pf.lA for 150
MeV/nucleon uranium ions. The minimum accelerationenergy of the SRC is about 60 MeV/nucleon for veryheavy ions.

- 124-Fig.1 Schematic drawing of the SRC.
Table 2 Main parameters of the SRC.

6
Number of sectors
6Harmonics
2.37m
injection
Average radii
5.36 mextraction
18 - 38 MHz
RF Frequency

JAERI-Conf 95-021

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Fig.2 Isochronous fields for typical ions for(1) 400 MeV/nucleon 12<:6+,(2) 300 MeV/nucleon 84 KJ36+,(3) 150 MeV/nucleon 238U80+ and(4) 58 MeV/nucleon, q/A =0.25.
Isochronous field distributions for typical ionsare shown in Fig.2. The maximum required field in thesector magnet becomes nearly 4 T. Field difference
between at the injection radius and at the extraction
radius on the sector axis is 0.7 T for 400 MeV/nucleon
ions and 0.1 T for below 100 MeV/nucleon ions.
Therefore both main coils as well as trim coils for coarse
fitting have to be superconducting.

agreement of the magnetic field on the median plane canbe obtained for both the six-symmetry system and thesystem with only one set. This method shortened thecalculation time to be less than 25% of that of the sixsymmetry system.
One important point in the design of the sector
magnet is to optimize its geometry so that the magnetic
forces acted on the main coil can be as small as possible.
The expanding force acted on the radial part of the main
coil reaches the oder of several hundreds tons. One way
to support such a main coil is to put the pole in thecryostat and to fix the main coil directly to the pole
(cold-pole method). Detailed study has been carried out
in ref. 3. In the case of the cold-pole magnet with asector angle of 23 deg., the excitation current is 2.9
MA/sector.
Conceptual sketch of the superconducting trimcoils is shown in Fig.4. If the trim coils can becontrolled by many independent currents, fields can begenerated flexibly and precisely. But cryogenic system
requires the minimum number of current leads to reduceheat load. Three sets of superconducting trim coils thus
bring a realistic solution. Maximum excitation currentof one set of trim coils is estimated to be 240 kA.

3. Sector Magnet and Field Calculation
Magnetic fields were calculated by the three- ·dimensional code TOSCA[2]. For the sector magnet
only a quarter of it was modeled because of its
symmetry. For the coils a complete set of them in only
one sector was taken into account in the calculation. An
example of modeled magnet for TOSCA is shown in
Fig.3. Strictly speaking, TOSCA requires all conductorsof the system to be taken into account. But owing toappropriate selection of reduced potential region, good-dac.o~~~ Middle ~~~Fig.4 Conceptual sketch of superconducting trim coils.
Three sets of trim coils are wound in three radial
regions.
4. Isochronous Field and Tunev~.oI

Fig.3 Example of modeled magnet for TOSCA.
Iron part and main coils are shown.

Equilibrium orbits and betatron tunes werecalculated by the computer program that had beenoriginally developed for the RRC. Results of the field
distributions by TOSCA were used in the orbit
calculations.
Because of saturation of the iron pole, the field
distribution is largely affected by coils' configuration.
Examples of field distributions are shown in Fig.5 andFig.6. In the region of the valley, negative field is- 125-JAERI-Conf95-021

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Fig. 5 Field distributions along the sector axis producedwith (1) Main coil and (2) Main coil+ Trim coilfor 400 MeV/nucleon 12C6+ ions.

Fig.7 Tune values at a sectorangle of23.0 deg. for(1) 400 MeV/nucleon 12C6+,(2) 150 MeV/nucleon 238u8<>+ and(3) 60 MeV/nucleon ions with q/A =0.25.
preferable. Figure 7 shows typical tune values
calculated for the sector angle of 23.0 deg. All the tunevalues are within the range from Vz=l.O to Vz=l.5.

5. Summary

2030Angle (deg)Fig. 6. Field distribution along the azimuthal direction at(1) r = 5.5 m , (2) r = 4.7 m and (3) r = 3.9 m.
created. This field brings large flutter and sharp fringing
field. Therefore, vertical focusing force is larger thanthat in a normal conducting ring cyclotron.
Using three sets of superconducting trim coils,
it was possible to adjust various distributions ofisochronous fields within ±0.02 T. By the optimizationof the configuration, it is expected to be less than ±0.01
T. Further fine adjustment will be done with trim coilsof room temperature.
In the case of high energy acceleration, verticaltune Vz decreases as the energy increases. The sector
angle was selected so that vertical tune value neveracross Vz = 1.0. Smaller sector angle causes larger
vertical focusing force. But from the viewpoint ofminimizing the maximum field, large sector angle is

Design study of the sector magnet for the superconducting ring cyclotron for the proposed RIKEN RI
beam factory has been carried out. Until now, it has
turned out that isochronous fields for various ions can begenerated by using superconducting main coils andsuperconducting trim coils within ±0.01 T. Detailed
design studies and further optimization are under way.

References[1] Y.Yano et al.,"RIKEN RI Beam Factory Project,"
this symposium.
[2] Vector Fields Limited, Oxford, England.
[3] Kawaguchi et al., "Design Study of Superconducting
Sector Magnet for the RIKEN Superconducting Ring
Cyclotron (II)," this symposium.

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