Proceedings of the 5th Annual Meeting of Particle Accelerator Society of Japanand the 33rd Linear Accelerator Meeting in Japan (August 6-8, 2008, Higashihiroshima, Japan)

R&D of a Septum Magnet Using MIC coil
Kuanjun Fan 1 ,A), Hiroshi Matsumoto A), Koji Ishii A), Noriyuki Matsumoto B)
A) High Energy Accelerator Research Organization (KEK)
1-1 OHO, Tsukuba, Ibaraki, 305-0801, JapanB) 2NEC/Token
Abstract
The J-PARC that is under construction at JAEA will provide a high-intensity, high-power proton beam forexperiments. A high-power beam will inevitably result in serious radiation, particularly at beam injection and extraction
area, where the uncontrolled beam loss is large. The fast-extraction abortion system of the 50-GeV main ring comprises
kickers and a group of septa, which includes low-field septa SM11/12/21/22 and high-field septa SM30/31/32/33. Since
the beam clearance is very narrow at the upstream, the low-field septa coils are ceramic insulated to deal with thesevere radiation damage. The high-field septa coils are polyimide insulated. In future when the beam power reaches
0.75 MW, very severe radiation damage is expected due to the high-intensity beam. Among these high-field septa, theSM30 suffers the severest radiation damage. To cope with this problem, a new septum coil using MIC (Mineral
Insulated Cable) is proposed. However, the problems arising from the MIC coil are: large leakage field due to the thickinsulation thickness, and eddy current effects due to the coil sheath. This study quantitatively investigates these effects
and gives the solutions.

The J-PARC that is under construction at JAEA is ajoint project between KEK and JAEA, which comprises aLinac, a 3-GeV RCS and a 50-GeV main ring. The 50GeV main ring will provide proton beam with power ashigh as 0.75 MW for different experiments, such as
neutrino oscillation experiment and nuclear physics
experiment. A high-power, high-intensity proton beam
will inevitably result in serious radiation, particularly at
beam injection and extraction area, where the beam
clearance between the beam and the septum coil is very
narrow and the uncontrolled beam loss is very large. The
radiation will destroy the insulation material between
individual coils and degrade the magnet performance.
Thus, very strict requirements are imposed on the designof these septa.
The layout of the 50-GeV ring is shown in Fig. 1. It isa three-fold symmetric lattice, which has three 116.1 mlong straight dispersion free sections. The straightsections are used of injection/dump, slow extraction andfast extraction/abortion, respectively. The fast-extractionabortion septa system is designed bipolar in order todeliver the extracted beam and the aborted beam inward
and outward of the ring. Fig. 2 shows the septa system.
At the upstream of the septa system, the low field septa
coils are ceramic insulated to deal with severe radiation
damage. The high field septa coils are polyimide
insulated. In future, the MR will operate with beam
power of 0.75 MW. The uncontrolled beam loss is
expected very large, which will creates severe radiation
damage to the polyimide coils. Among these high-field
septa, the SM30 suffers the severest radiation damage. To
1cope with this problem, a new septum coil using MIC coilis proposed.
Abortion

Fast extractionabortRF
C2

E3

neutrino
M3

D3
M2Neutrino

RCS

E1c

BT Collimators
Injection
Injection

D2
D1

E2

M1

Ring Collimators
Injectiondump
dump
Injection

Slow
extracttiio
onn

1. Introduction

C1Fig.1 Layout of 50-GeV MR
QFR

QDT
Vacuum tank
SM11/12 E

SM21/22 E

SM11/12 A

SM21/22 A

SM30E SM31E SM32E

SM33E

KM

SM30A SM31A SM32A SM33A

Low field septum magnets

High field septum magnets

Fig.2 Fast-extraction abortion septa system

2. MIC coil
The cross section of MIC is shown in Fig.3. Ceramicmaterial is filled in the between of the sheath and theconductor. Due to the inorganic insulation material, theE-mail: kj.fan@kek.jp-237-Proceedings of the 5th Annual Meeting of Particle Accelerator Society of Japanand the 33rd Linear Accelerator Meeting in Japan (August 6-8, 2008, Higashihiroshima, Japan)

MIC coil has higher radiation-resistant than theconventional organic insulation material. Table 1
compares the radiation-resistant ability among threedifferent materials.
SheathPipeincrease the separation between the magnet yoke and theseptum coil. Consequently, the leakage field increases.

CeramicCopper

Fig.4 Cross section of SM30To suppress the leakage field, the circulating beam
pipe has to be shielded. Fig. 5 compares the leakage fieldalong the circulating beam orbit with and without shield.
The beam pipe can be partly shielded by embedding two
3-mm iron plates in a SUS pipe, or completely shieldedby using an iron pipe. In both cases, the leakage field canbe decreased greatly.

Fig. 3 Cross section of MIC
Table 1: Comparison of radiation resistant
AOrganic
B C

Insulation Epoxy
Resistant>107 Gy

Inorganic

Polyimide MgO>109 Gy>1011 Gy

Due to the excellent property of radiation resistance,
MIC is widely and successfully used in the design ofhigh radiation resistant magnet in J-PARC[1].
However due to the fabrication technology
restriction, MIC has several demerits: the size of MIC
cannot be determined precisely, the cross section shapeis restricted to square, the insulation thickness cannot
be very thin, the metallic sheath may induce eddy
current in pulse operation.
The existing MIC magnets are normally DC dipole
magnets and Q magnets, whose interesting gap field isdetermined mainly by the profile of the magnet yoke. The
coil size, coil shape and the sheath have little effects onfield quality.
For a pulsed septum magnet, using MIC coil may
create several problems, such as: the leakage field
increases due to the large coil-yoke separation, septum
thickness increases due to the thick MIC insulation layer,
eddy current effects due to the metallic coil sheath, andcooling difficulties due to the high temperature caused byhigh current density.

Leakage field along beam line
SUS /Iron pipe
1. Without shield
2. Part shield
3. Full shield
Shield3mmMagnet coreFig.5 Comparison of leakage field with/without shield
However, the end fringe field is still as high as 150 Gs
even with a fully shielded pipe. To suppress the end
fringe field more, a thin shield plate can be placed in
frond of the magnet yoke as shown in Fig. 6. The end
fringe field can be shielded greatly.

Shield plate, 1cm

3. SM30 design study

Magnet core

The cross section of the high-field septum SM30 isshown in Fig. 4. It is a bipolar design so that the extracted
beam and the aborted beam can go different direction.
This geometry has a merit that the leakage field can be
partly bypassed by the magnet yoke.

Fig.6 Shield plate to suppress end fringe field

3.1. MIC Insulation thickness effects

3.2. Eddy current on beam pipe

The multi-turn septum conductor can be made bybrazing individual MIC coils. However, the thickinsulation layer, the fabrication and installation errors will

The septum magnet works in pulse model to escape thethermal problem. The rise time is 1.9 s for 50-GeV
operation. During the rise time, the eddy currents will be-238-Proceedings of the 5th Annual Meeting of Particle Accelerator Society of Japanand the 33rd Linear Accelerator Meeting in Japan (August 6-8, 2008, Higashihiroshima, Japan)

induced in the beam pipe, which may affect thecirculating beam. Since all pipes are electric connected
through the flanges, the eddy current can flow between
circulating/extraction and circulating/abortion beam pipes,
which enhance the leakage field a lot. Fig. 7 shows theeddy current flowing.

3.3. Eddy current in sheath
Eddy current can be induced in the metallic sheath also.
To study its effects on the leakage field, suppose only
septum coil is made of MIC. During the rise time, theeddy current generated in sheath is shown in Fig. 10.

Eddy current on sheath

Eddy current flowing

Fig.10 Eddy current in MIC sheathFig.7 Eddy current flow between pipes
Without shield, the leakage field longitudinal
distribution with and without eddy current is compared asshown in Fig. 8. It clearly shows that within the range ofbeam pipe, the leakage field increases; external of thebeam pipe, where no eddy current exist, the end fringe
leakage field in two different cases are the same.

The Sheath material is assumed to be copper or SUS,
Fig. 11 compares the leakage field integral in the threedifferent cases. It clearly indicates that the eddy current insheath can be neglected no matter it is made of copper orSUS.
0.001

Without shield
No sheath
Sheath-cu;

Leakage field
1. DC 2.Eddy current

Sheath-SUS

Field integral (T*m)

0.000-0.001-0.002-0.003-3-2-101

2

3

Transverse coordinate (cm)Fig. 8 Comparison of leakage with/without eddy current

Fig. 11 Leakage field integral in different cases

When the circulating beam pipe is shielded as shown in
Fig. 5, the leakage field integral transverse distribution isalmost same in two different cases as shown in Fig.9. It
means that the eddy-leakage field can be completelyshielded.
DC shd1
edy-shd1

0.004

Field integral (T*m)

0.002

0.000

Summary
The high-field septum SM30 of the fast extraction
system will suffer very severe radiation damage in thefuture high-power beam operation. High radiation
resistant septum is needed for the stable operation.
Preliminary study shows that it is possible to design sucha radiation resistant septum by using MIC coils. The
problems arising from the MIC coil can be resolved andthe field quality can satisfy the requirement. Detailed
studies are needed and experiment will start soon.

-0.002

Reference-0.004

-3

-2

-1

0

1

2

3

Transverse coordinate (cm)Fig.9 Comparison of leakage field integral after shield[1] K. H. Tanaka, et al. “Development of radiation resistantmagnets for JHF/J-PARC”, IEEE Trans on applied
superconductivity, Vol. 14, No. 2, June 2004

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