.JAERI-Conf 95-021

PA36
Design Study of Sector Magnet for
the RIKEN Superconducting Ring Cyclotron ( II )T.Kawaguchi, T.Kubo, T.Mitsumoto, T.Tominaka, S.Fujishima,
H.Okuno,Y.Tanaka, K.lkegami, A.Goto, and Y.Yano
The Institute of Physical & Chemical Research (RUffiN)
2-1, Hirosawa, Wako-shi, Saitama 351-01, Japan

Abstract

A conceptual design of the superconducting sector magnet for the RIKEN
superconducting ring cyclotron is described.
A comparison of the warm pole system withcold pole one is discussed. Magnetic forces,
coil support system, coil cooling method andcryogenic system are also described.
1. Introduction
The superconducting ring cyclotron
proposed for the RIKEN RI beam factoryneeds six units of superconducting sector
magnets, each of which must generate themaximum magnetic field strength of 4 T in
the beam
orbital area.
We usesuperconducting coils for the sector magnet toobtain a compactness in size and to saveelectric power and cooling water. The yoke
and pole made of magnetic iron are arrangedin the sector magnet to reduce the ampere
turns of the superconducting coils and tominimize the leakage magnetic flux.
2. Structure of Superconducting
Sector Magnet
Figure 1 shows an overview of thesector magnet. A cryostat is not drawn in thisfigure. The main components of the magneticelements are superconducting coils, poles anda yoke. We use two kinds of superconducting
coils: a pair of main coils and a group oftrim coils. Both coils are located upper andlower sides with respect to the mid plane. We
have studied two ways of arrangements forthe pole. One is a warm pole system and theother a cold pole one. Figure 2 shows theboth systems and a cryostat. As for thewarm pole system, the poles are directly
connected to the yoke in room temperature
region, and thus the cold mass at 4.5 K
consists of superconducting coils and coilvessels. As for the cold pole system, themain superconducting coil is wound aroundthe pole directly, so the cold mass consists ofsuperconducting coils, poles and coil vessels.
From points of view the mechanical rigidity
and magnetic force, we have decided to use
the cold pole system. However, a serious
problem on the cold pole system is thedifference of thermal contraction between
the pole and the coil vessel during thecooldown of the cold mass from 300 K to
4.5 K. We are currently investigating thisproblem.
3. Magnetic Field and Forces
A three dimensional display of themagnetic field on the median plane
calculated using the OPERA (TOSCA) codeis shown in Fig.3. Figure 4 shows thecomparison of expanding forces on thestraight section of the main coil between for
the warm pole system and the cold pole one.
It is clear the force in the cold pole system
can be reduced by one-third of that in thewarm pole system. This effect is due to theshort distance between the main coil andthe pole in the cold pole system. Figure 5
shows the changing of the maximum
magnetic field (at a radius of 6 m on themedian plane) and the magnetic forces Fx,Fzin the cold mass (consists of main coils and- 127-

JAERI-Conf 95-021

cold poles), as a function of the ampere turns oftwo (a pair ot) main coils . The magnetic force
Fz in the vertical direction is supported with twocoil links which are arranged between the upper
cold mass and lower one. The force of 300 tons
maximum causes a mechanical deflection of
about 4 mm maximum of the cold mass. The
magnetic force Fx in the radial direction isgenerated with a configuration of the six sector
magnets . Each cold mass is pushed toward
outer radius by the forces from sector magnets at
both-side. The maximum force of Fx is estimatedto reach about 500 tons per each magnet. It isvery difficult to support this force by thermal
insulating supports .which locate in between thecold mass and the vacuum vessel. To support
such a large Fx, we are investigating a cold ringof 2.6 m in diameter and 200 mm in thickness
which connects the six cold masses in thecentral region of the ring cyclotron.
4. Superconducting

Coils

The main superconducting coil has atriangle shape with two long straight sections ofabout 4 m length. This force is supported by
the coil vessel and the cold pole. It is very hard,particularly for the coil in non-circular shape, toprevent the coil's wire movement that causes acoil quench. However, we should avoid the coilquench,because the total magnetic energy storedin six magnets reaches as· much as 300 MJ.
Quench-free is indispensable for maintain areliable long-time operation for the cyclotron.
Taking the above matter into account, we apply
fully cryogenic-stable cooling for both the main
coil and the trim coil adopting a method ofconservative liquid-helium bath cooling. The
operation currents of the main coil and trim coilare roughly set to be 5000 A and 500 A,respectively. In order to maintain the cryogenic
stability, the average current densities of themshould be less than 40 Nmm2 and 50 Nmm2,respectively.

5. Cryogenic

A helium refrigerator having a capacity of
1 kW at 4.5 K stage will be used for six
sector magnets and the beam injection &extraction channels. Weight of the total
cold mass of six magnets is about 360 tons.
It will take almost one month for thecooldown of the cold mass from room
temperature to 4.5 K by the helium
refrigerator.
6. Conclusion
Conceptual design of the superconducting sector magnet for the RIKEN
superconducting ring cyclotron has been
carried out. The arrangement of the coil andpole has been studied, and we have decided
to adopt the cold pole system in order tosupport and reduce the magnetic forces inthe coil. We are planing to construct amodel of the superconducting sector magnet
to confirm our design.
References

1. Y.Yano, "RIKEN RI-Beam Factory
Project", This symposium.
2. T.Mitsumoto et al., "Design Study ofSuperconducting Sector Magnet for theRIKEN Superconducting Ring
Cyclotron ( I )", This symposium.
SUPERCONDUCTING COIL
& COLDPOLE

em~600.0

X1200.0..J-System

The total heat leak of six magnets isFig.l Overview of Superconducting Sector Magnet
roughly estimated to be 500 W at 4.5 K.
(for three dimensional magnetic field calculation)-128-JAERI-Conf 95-021

lOcm~---t··

MIDPLANE·----Wann Pole Svstem
1000"'I..,_800

wa:(.)!? 600~z~ 400::10Cl~- 0zz

0·--~···-····--·········

200...............................!.---···-it)(

w0Cold Pole System

012345COIL AMPERE lURNS PER 1WO COILS (MA)Fig.2 Comparison

Fig.4 ·comparison of Expanding Magneic Forcesin Coil Straight Sectionof Pole Arrangements

Component -BZ
Minimum= -21983.3, Maximum= 40251.9
1em

Z600.0"I6006mE~zwC)····-- ---l---·······----~--------+--·----500"-4······-···-··--+···· ....................-+.------··· ..........1 ......... _. __400~0300w

12'32200-'

110000X-200.0-.J -!Y"-··--+-4----~-------t-1Ir-"-~

cZ-400.0

N-100-2Y-600.0' 1--200012345COIL AMPERE lURNS PER TWO COILS (MA)Fig.3 Three Dimensional Distribution of Magnetic Fieldin MidPlane-129-.

5Fig. 5 Field Strength & Magnetic Forcese~~~z