Abstract-The Japan Hadron Facility (JHF) accelerator complex comprises a 50-GeV main synchrotron, a 3-GeV rapid-cycling synchrotron, and a 400-MeV linac. The accelerators provide high-intensity, high-energy proton beams for various scientific fields. These high-intensity, high-energy accelerators, especially the 50-GeV main synchrotron, impose tight demands on the injection/extraction septum magnets for a thin structure, large aperture and high operating field. But to manufacture high field septum magnets on the condition of a large aperture is very difficult because of its extraordinarily strong electromagnetic force due to the self-field. To cope with these tight demands, new design concepts of septa are required. An opposite-field septum magnet system is one of the solutions to realize a thin septa or very high-field septum magnets

I Sakai

M Muto

M Tomizawa

S Fukumoto

Y Arakaki

Y Ishi

Y Mori

Y Shirakabe

CiteSeerX

242 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 12, NO. 1, MARCH 2002
Opposite Field Septum Magnet System for the
Separation of Charged Particle Beams
I. Sakai, Y. Arakaki, M. Tomizawa, Y. Shirakabe, M. Muto, Y. Mori, Y. Ishi, and S. Fukumoto
Abstract—The Japan Hadron Facility (JHF) accelerator
complex comprises a 50-GeV main synchrotron, a 3-GeV
rapid-cycling synchrotron, and a 400-MeV linac. The accelerators
provide high-intensity, high-energy proton beams for various
scientific fields. These high-intensity, high-energy accelerators,
especially the 50-GeV main synchrotron, impose tight demands
on the injection/extraction septum magnets for a thin structure,
large aperture and high operating field. But to manufacture high
field septum magnets on the condition of a large aperture is very
difficult because of its extraordinarily strong electromagnetic
force due to the self-field. To cope with these tight demands, new
design concepts of septa are required. An opposite-field septum
magnet system is one of the solutions to realize a thin septa or very
high-field septum magnets.
Index Terms—Charged particle beams, magnet, septum.
I. INTRODUCTION
I N PARTICLE accelerators, various septum magnets havean important roll to separate the charged-particle beams.
The conventional septum magnet produces a magnetic field
only in the inside of the septum conductor, and there is ideally
no magnetic field outside of it. Usually the septum conductor
and its support are required to be as thin as possible. But the
septum conductor inevitably receives the electromagnetic force
produced by the self-field. Therefore, in the case of a high-field
septum magnet, the severe electromagnetic force on the septum
conductor and leakage flux to outside of the septum are serious
problems for designing its structure. To solve these problems,
an opposite-field septum-magnet system has been developed
for the beam injection/extraction of circular accelerators. A
cross-sectional view of the opposite septum magnet is shown
in Fig. 1. In this case, the same grade of opposite magnetic
field is produced outside of the septum, which is on the side of
the circulating beam, by another septum magnet; both septum
conductors are mechanically contacted to each other. The
electromagnetic force on the septum conductors and leakage
flux cancel out each other. Furthermore, the beam-separation
angle is twice as large as that of the conventional single septum
magnet.
To use this opposite-field septum magnet for beam in-
jection/extraction for a circulating beam, the magnetic field
of the circulating beam side must be compensated by other
Manuscript received September 24, 2001.
I. Sakai, Y. Arakaki, M. Tomizawa, Y. Shirakabe, M. Muto, and Y. Mori
are with KEK High Energy Accelerator Research Organization, 1-1 Oho,
Tsukuba-Shi, Ibaraki-Ken, 305-0801 Japan (e-mail: izumi.sakai@kek.jp).
Y. Ishi and S. Fukumoto are with Mitsubishi Electric Corporation, 1-1-2,
Wadasaki-Cho, Hyogoku, Kobe, 652-8555, Japan.
Publisher Item Identifier S 1051-8223(02)03467-X.
Fig. 1. Cross-sectional view of opposite-field septum magnet.
sub-bending magnets, as shown in Fig. 4. Fortunately, these
sub-bending magnets increase the angle of the injection/ex-
traction beam orbit with the circulating beam orbit. To obtain
same injection/extraction angle as that of the conventional
s ptum magnet, we need a half-length opposite-field septum
magnet and two quarter-length sub-bending magnets located
up-stream and down-stream of the main opposite-field septum
magnet.
The opposite-field septum magnet system is useful to realize
a thin septum magnet or a very high field septum magnet for
beam injection/extraction of particle accelerators. In this paper,
e introduce physical fundamentals and technical aspects of the
opposite-field septum-magnet system.
II. PRINCIPLE OF THEOPPOSITEFIELD SEPTUM MAGNET
SYSTEM
A. Magnetic Field Configuration
As shown in Fig. 1, the opposite-field septum magnet has
three conductor blocks in a pole gap. The central conductor
forms a septum conductor on which double current flows and
makes an opposite magnetic field in both side gaps with both end
conductor blocks. These magnetic fields have the same value
of opposite signs and face each other across the central septum
conductor. For the injection/extraction septum magnet, one side
gap is used for circulating beams and the other side gap is used
for injection/extraction beams. In the case of equal pole width,
the yoke of the magnetic material is unnecessary. But in the case
of an unequal pole width, the yoke of the magnetic material must
absorb excessive flux.
1051-8223/02$17.00 © 2002 IEEE
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SAKAI et al.: OPPOSITE FIELD SEPTUM MAGNET SYSTEM FOR THE SEPARATION OF CHARGED PARTICLE BEAMS 243
Fig. 2. Comparison of the magnetic field distribution between the
conventional septum magnet and the opposite-field septum magnet by a 2-D
simulation.
In the any case of normal septum magnets, the magnetic flux
inevitably leaks out from the septum because of the finite per-
meability of the core, especially under high-field operation. The
leakage flux can be eliminated by a magnetic shield plate. But
the septum thickness is increased for that purpose. On the other
hand, in the case of an opposite field-type septum magnet, the
leakage flux is canceled out by each other. A comparison of the
magnetic field distribution between the normal septum magnet
and the opposite-field septum magnet by a simulation using the
computer program “Poisson” is shown in Fig. 2, which shows
an operation of 0.775 T in the structure of the model magnet.
In the opposite-field configuration, the leakage flux from the
septum conductor and the electromagnetic force on the septum
conductor cancel each other out. We can thus easily obtain a
steep septum magnetic field without any magnetic shield. The
force-free structure also releases us from serious problems con-
cerning the septum support of normal septum magnets.
B. Opposite Field Septum Magnet and Sub-Bending Magnets
System
A schematic layout of the conventional septum magnet and
that of the opposite-field septum magnet are shown in Figs. 3
and 4, respectively. The conventional septum magnet produces
a magnetic field only inside the septum magnet, and there is no
magnetic field outside of it, i.e., there is no magnetic field on
the circulating beam orbit. On the other hand, the opposite-field
septum magnet makes a magnetic field of opposite sign on the
circulating beam orbit.
To use this opposite-field septum magnet for beam injec-
tion/extraction, the magnetic field of the circulating-beam side
must be compensated by other sub-bending magnets. For perfect
Fig. 3. Conventional septum magnet.
Fig. 4. Opposite-field septum magnet system.
compensation, two sub-bending magnets with equal kick angles
and values half that of the main opposite-field septum magnets
must be settled symmetrically around the septum magnet.
As shown in Fig. 4, the horizontal aperture of these sub-
bending magnets covers the injection/extraction beam orbit, so
that the injection/extraction angle of the beam orbit with the
circulating beam orbit is enhanced to the same amount as the
opposite-field septum magnet. To obtain the same injection/ex-
traction angle as the conventional septum magnet, we need only
half the length of the opposite-field septum magnet and two
quarters of the length of the sub-bending magnets, which are
located up-stream and down-stream of the main opposite-field
septum magnet. As a result, to obtain the same injection/ex-
traction angle as that of conventional septum magnet with the
same magnetic field strength, the total length of the opposite-
field septum magnet system is equal to that of the conventional
septum magnet, apart from the problem of the longitudinal end-
effect and the projection of the coil ends.
III. T RIAL MANUFACTURE OF A THIN SEPTUM MAGNET
The distinctive features of the opposite-field septum magnet
are a “force free septum” and a “cancellation of leakage flux.”
To verify these characteristics, a model septum magnet having
a very thin septum (0.3 mm) was made, and experiments in-
volving pulse excitation have been carried out.
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244 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 12, NO. 1, MARCH 2002
Fig. 5. Schematic layout of the composition of a excitation current using
single-power supply.
Fig. 6. The transverse cross section of the model magnet.
Fig. 7. The external appearance (fromA–A line in Fig. 6) of the septum
conductor of the model magnet.
A. Structure of a Model Magnet
A schematic layout of the composition of excitation-coil cir-
cuit using single-power supply is shown in Fig. 5. To make it
possible to excite the opposite field septum magnet with a se-
ries connection by a single power supply, the central septum
conductor must be divided into two parts. The transverse cross
section of the model magnet is shown in Fig. 6. In this case,
the central septum is divided into three parts. The upper and
bottom parts form one conductor and the middle part of the
septum forms the other. The thickness of the septum conductors
Fig. 8. The magnetic-field distribution of the calculated value by the computer
program “Poisson” and the measured values under several conditions.
is 0.3 mm. The total height of the upper and the bottom parts
is equal to the height of the middle part of the septum so as to
realize a uniform current density. These septum conductors are
fixed on a horseshoe-shaped insulating support, which enables
the injection/extraction orbit to pass in the septum magnet.
The external appearance (from– line in Fig. 6) of the
septum conductor of the model magnet is shown in Fig. 7. At
the fixing points of the conductor, the edges of the middle con-
ductor and the upper/bottom conductors are alternately shaped,
as shown in Fig. 7, to obtain a vertical aperture as wide as pos-
sible, and to maintain the uniformity of the magnetic-field dis-
tribution. The force free mechanism of the septum conductor
and its end structure has been verified trough a pulse-excitation
test up to 0.18 T (6 kA).
At the septum conductor, the pole face is notched to make the
insulation gap with the septum conductor. As shown in Fig. 6,
the notching shape is an equilateral triangle whose width is
10 mm and depth is 2 mm. The longitudinal pole ends are cut
along the contour of hyperbolic curve to lamination at the pole
end. These cutting effects on the escape any magnetic saturation
and eddy current of the field quality must be carefully examined
by magnetic-field measurement and computer simulations.
B. Field Quality Near the Septum
An opposite-field type septum magnet has already been de-
veloped for both negative-ion charge-exchange injection and
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SAKAI et al.: OPPOSITE FIELD SEPTUM MAGNET SYSTEM FOR THE SEPARATION OF CHARGED PARTICLE BEAMS 245
Fig. 9. The layout of an opposite field septum magnet.
Fig. 10. The effective length of an opposite field septum magnet.
positive-ion multi-turn injection in our laboratory [1]. At that
time, quite the same cross-sectional structure of the core and
the single-turn septum was applied, and the optimum shaped of
the pole and the size of the septum for a uniform and sharp edge
distribution of the magnetic field were carefully examined.
The magnetic-field distribution of the calculated value by the
computer program “Poisson” and the measured values under
several conditions, which were acquired through optimization
process, are shown in Fig. 8.
The notched shape of the pole face, which had the same size
as this trial manufacture, was fixed in advance, and the size of
the septum conductor was changed by trial and error. The calcu-
lated values by “Poisson” were agreed well with the measured
value. The field distribution near the septum is very sensitive to
the cut-off quantity of the septum. These deformations disturb
the field distribution near to the septum. Fortunately, however,
regarding the disturbance of the field distribution, the notched
TABLE I
pole face and the cut-off septum are complementary to each
other. In this way, the optimum shapes of the pole face and the
septum conductor were decided, and a field uniformity of less
than 1% was obtained [1].
IV. THE CALCULATION OF THE END FIELD IN AN OPPOSITE
FIELD SEPTUM
To inspect the field quality and leakage flux including the
end effects, three-dimensional calculation was performed by a
finite-element-based program, called “MAFIA.” Silicon steel
(50RM600) is used as a H–B table. Fig. 9 shows the layout of
an opposite-field septum magnet for calculation. The length for
the longitudinal direction is 0.4 m. The gap height and width
are 40 mm and 75 mm respectively. Two sets of coils are used
to generate the opposite field. The magneto motive force is 6 kA
turns. The magnetic flux density (B0) is 0.188 T. Fig. 10 shows
the transverse distribution of effective length on the median
plane. Since the support-metal fittings can not be equipped with
he end coil near to the center so as to maintain the thin septum,
the force from self field should be taken into consideration. We
conform that the end coils in the center push together, and then
offset.
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246 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 12, NO. 1, MARCH 2002
Fig. 11. The overall layout of the magnet.
V. DESIGN OF AHIGH-FIELD SEPTUM MAGNET FOR THE
SLOW-EXTRACTION OF 50-GeV PROTONSYNCHROTRON IN THE
JHF PROJECT
A. Requirement for the Slow Extraction Septum Magnet
The requirements for a high-field septum magnet used to
extract 50-GeV protons are as follows: Longitudinal space,
2630 mm; kick angle, 0.024 rad.; septum thickness,
30 mm; aperture of septum magnet for extraction,60 mm;
vertical aperture of main septum magnet and sub-bending
magnet for circulating beams, 120 mm. The acceleration
cycle of the 50-GeV proton synchrotron is 3.42 sec and its
extraction period is 0.7 sec. The field of the septum magnet
must remains on 0.7 sec every 3.42 sec. The tight space, thin
septum, large aperture and large kick angle for the beam energy
make it hard to realize reliable septum magnets for long-time
operation. Moreover, high-duty pulse operation makes it hard
to design the cooling system of the conductor.
B. Structure of a High-Field Opposite-Field Septum Magnet
Designed for the Slow Extraction of the 50-GeV Proton
Synchrotron
When the above opposite-field septum magnet is adopted for
the slow extraction of a 50-GeV proton synchrotron, although
the problem of electromagnetic force is greatly relaxed, coil
cooling is still a difficult problem. The overall layout of the
magnet and its parameters which we propose are shown in
Fig. 11 and in Table I. As shown in Table I, each coil is
composed of 64 turns, which are parallel for every turn.
In Table I, although the cooling performance to the average
heat loss is shown, since if a practical-condition pulse run is
carried out, the time change of the conductor temperature and
cooling-water temperature is also taken into consideration. In
this case, it is necessary to understand the exact heat-transfer
coefficient between a conductor and the cooling water. More-
over, it is also considered to be a subject on manufacturing to
maintain the water-flow balance of 64 circuits well; also verifi-
cation by trial production is required.
VI. SUMMARY
The opposite-field type septum magnet combined with sub-
bending magnets has unique features compared with those of
normal septum magnets: a force-free structure, cancellation of
the leakage flux and a large separation angle. A force-free struc-
ture is useful to realize thin septum magnets or high field septum
magnets. The current density of the septum is twice as large as
that of normal septum magnet, and the combination with sub-
bending magnets requires a larger size magnet system. However
the separation angle is twice as large as that of a normal septum
magnet, and the length of the septum magnet is needed to be
half that of the normal septum magnet. Further the force-free
structure easily permits pulse excitation for energy saving. As
a results, the total power loss of the opposite-field-type septum
magnet system is smaller than that of normal septum magnet.
The system is the effective solution to realize thin septum mag-
nets and high-field septum magnets.
ACKNOWLEDGMENT
The authors would like to express sincere appreciation to
Prof. H. Sato, T. Kawakubo, Y. Irie, N. Tokuda, S. Mitunobu,
and A. Yamamoto for their valuable discussions. They are also
indebted to K. Kitagawa and H. Yamaguchi for their technical
support.
REFERENCES
[1] I. Sakai, “Magnet system for both negative-ion charge-exchange injec-
tion and positive-ion multi-turn injection,” in14th Int. Conf. Magnet
Technology, Tampere, Finland, 1995.
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Opposite field septum magnet system for the separation of charged particle beams

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Opposite field septum magnet system for the separation of charged particle beams

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