Permanent Magnets for the 500 MeV Accumulator Ringof the Intensity Doubler Project in KEK-PS.
E. Nakamura, K. Egawa, S. Igarashi and K. Takayama, KEK, Tsukuba, JapanK. Ogata, Tokin Corporation, Sendai, Japan
Abstract

2 PERMANENT MAGNET

A 500 MeV accumulator ring (AR) is proposed as apart of the Intensity Doubler (ID) Project in KEK-PS[1, 2]. The AR will be installed in the same tunnel of the12 GeV main ring. Its circumference will be 340 m. A
total amount of 112 permanent magnets are required for
the AR and beam transport lines from the booster ringand into the main ring; 88 gradient magnets, 12quadruples, 8 bending magnets and 4 Lambertson
magnets, with a wide mechanical aperture of a +/-50
mm transverse good field region and one or two meters
long. The field is driven by strontium ferrite permanent
magnets. The field is shaped by precision iron polepieces. The temperature dependence of the ferrite is
crucial to control the beam orbit in the actual tunnel
environment with a variation of +/-3 degrees at least.
The use of Ni(30%) Fe(70%) alloy operating near its
Curie point is expected to compensate a gap fielddegradation due to the temperature variation. A
prototype gradient permanent magnet 1 m long with a70 mm center gap height is assembled to investigate its
field performance and defects to be solved. In the paper,
field measurement results will be presented andinterested issues such as multipole components in field
and a temperature compensation will be discussed.

The basic design of the permanent magnet is shownin Fig. 1, with a 70 mm gap at center, a 230 mm
horizontal aperture. This is a 1m long straight magnet
and designed to give the center field of 0.13 T and thegradient of 0.31 T/m. Overall dimensions are 326 mm
high by 450 mm wide by 1,160 mm long. The weight isabout 660 kg. The field quality is required to be lessthan 1 × 10-3 across the designed good field region.
Figure 2 shows the 2-D calculation result withPOISSON/PANDIRA. Three bricks of a ferrite, 100.6mm × 222 mm × 25 mm, are stacked at both pole sides
and drive flux vertically. The entire assembly isenclosed in a flux return shell 25 mm thick. Solid “barstock” components are used throughout rather thanlaminations. The pole tip steel is SUYP1 low carbon
steel and the flux return is construction grade (SS400)
steel. The field of the magnet is terminated by flux
clamp/end plates, which are magnetically connected tothe flux return shell.

1 INTRODUCTION[%]

Figure 1: Cross-section of a prototype magnet.
101.0

Normalized dB/dx

Permanent magnets have been chosen for the AR tosave costs to make and operate power supplies forelectromagnets. The remanent field of strontium ferrite
is weak, Br ~ 0.39 T, but it is cheap and sufficient tocontrol 500 MeV proton beams. The main gradient(combined-function) dipole magnets 1.5 m long are
required to have a flux density 0.15865 T and field
gradient 0.3836 T/m for focusing or -0.3338 T/m for
defocusing. In Fermilab, various types of similar
gradient magnets have been developed for many years
and almost characteristics of the ferrite, such astemperature stabilization and multipole correction, arereported precisely [3, 4]. The required field for gradientmagnets of KEK-AR is close to type RGF of therecycler ring (RR) in Fermilab, but those aperture mustbe larger, a 70 mm gap at center, a 100 mm horizontal
good field region. The beam pipe is supposed to be thesame one used in the KEK 12GeV-PS with a diamondshape 68 mm vertically and 206 mm horizontally.

100.5

100.0

99.5

-50

0

Horizontal direction x[ mm ]

50Figure 2: Horizontal good field region in dB/dx
calculated with POISSON/PANDIRA.

3.1 Dipole and Quadruple Fields
Figures 3 and 4 show the field measurement results offlux density and its gradient, that is dipole andquadruple components, respectively. The open circles
fitted with solid lines indicate the flux density andgradient at the longitudinal center of the magnet. The
closed circles fitted with dotted lines indicate
longitudinal integrated values, BL eff and B ’ L eff . Notethat all the data about dipole and quadruple fields are
given as absolute values in figures and sentences.
At center, the dipole field is 0.127 T and thequadruple field is 0.307 T/m. The measured ratio of
dipole and quadruple field at the longitudinal center hasa slightly difference 0.2 % from the designed value, andthe ratio of integrated values at horizontal center has
0.15 % difference, those will be able to be correctedwith end-shims. The error in quadruple field is kept
within 1 % in required good field region +/-50 mm, but
its integrated value of quadruple field has a larger
gradient, sextuple component, 2.7 %. This is
intrinsically caused by the difference in longitudinal
leakage field at both ends due to the finite size of thepole tips. These errors are easily correctable by meansof end shim correction technique.

B [ T ] or BL [ Tm ]B(s=0) [Tesla]
BL [Tesla meters]

0.14

0.13

0.12-50050

Horizontal direction x [ mm ](b)
B(x,0,s) [T]

0.08

0.040.00-800 -600-400-200

0200

Longitudinal direction s400

600

[mm]

800

Figure 3: (a) Measured flux density at longitudinal
center and integrated field; (b) longitudinal profiles at
each horizontal position: solid lines for -50 ≤ x < 0and dotted for 0 ≤ x ≤ +50 / 10 mm step.

0.321

B'(s=0) [Tesla/m]B'L [Tesla](a)2.7 % width

All the data reported here were measured by a 2-D
field mapping with flip coil system. The signal coiloutput determines a dipole component. A quadruple
component is estimated from a difference in direct
outputs from two coils, which are located 10 mm apart.

(a)

0.12

B' [ T/m ] or B'L [ T ]3 FIELD MEASUREMENT RESULTS

0.15

0.11

d By(x,0,s)/dx [ T/m ]Type SR-2 Strontium Ferrite (Tokin Corp.) is chosen
as the permanent magnet material because of its low
cost and of availability. Ferrite bricks have beenmagnetized using three pulses of 1.7 T, produced with aspare bending magnet of KEK 12 GeV-PS. The products
are not so even with +/-5 % difference, so those wereshuffled and selected to be averaged in each unit, whichis composed of three ferrite bricks. The difference influx of each unit is within 0.02 %.
The intrinsic temperature coefficient of the ferrite
material, -0.2 % / degree is canceled by interspersing“compensator alloy strip sheets” between the ferrite
units above and below the pole tips. The compensator isan iron-nickel alloy with a low Curie temperature andtherefore a permeability which depends strongly ontemperature. The type MS-1A (SUMITOMO SPECIAL
METALS Co., LTD) has been chosen because of larger
temperature of dependence of –1.5 % / degree around 20oC over 100 Oe. This shunts away flux in a temperaturedependent manner which can be arranged to null out thetemperature dependence of the ferrite. 12 strip sheets ofMS-1A with 1.2 mm thick were inserted every 100.6 mm
long brick and a 3 mm dead space.

0.318
0.315
0.312
0.309
0.306

-50

050

Horizontal direction x0.3[ mm ]

(b)

0.2

0.1

0.0-800 -600-400-200

0200

Longitudinal direction s400600[mm]800Figure 4: (a) Measured field gradient at longitudinal
center and integrated value; (b) longitudinal profiles at
each horizontal position: solid lines for -50 ≤ x < 0and dotted for 0 ≤ x ≤ +50 / 10 mm step.

3.2 Temperature dependence

0.321
0.318

B'Leff [T]The flux source, a ferrite SR2(TOKIN Corp.), has aweak temperature dependence of about -0.2 % / degree.
In order to compensate it, a 14.4 mm temperaturecompensator material composed of 12 strips andseveral-mm non-magnetic spacers are inserted every
100.6 mm wide brick. Figure 5 shows the measured
results of the temperature dependence of a gap flux
density (a solid line), for comparison with an equivalentdependence of the ferrite (a dotted line). The variation
against temperature is +0.022 % / degree andequivalent to 1/9 magnitude of a ferrite tendency. It is alittle overcompensation, but is tolerable and can be
adjusted by removing one or two sheets of temperaturecompensator strip.

0.3150.312
0.309-500Horizontal direction x50[ mm ]

Figure 6: Measured quadruple integrated field. The
dotted line indicates the field correction result withend-shims.

101[%]

4B / 0.12718 T

1009998

18202224

Pole Temperature

26[ oC ]

28

Figure 5: Measured temperature dependence of center
field. The dotted line indicates -0.2 % / degree.

SUMMARY

A prototype of permanent gradient magnet isassembled to establish a manufacturing process, tomeasure its magnetic flux density, its field gradient andto investigate multipole components and thosecorrection. Dipole, quadruple fields and those integrated
values at horizontal center are desirable. On the otherhand, an end-effect appears evidently in the horizontal
dependence of the integrated quadruple field. But it has
been confirmed to be possible to correct such extent
with end-shims.
This work is partly supported by the Grant-in-Aid for
Scientific Research on Priority Areas "Neutrino
Oscillation and Its Origin", No.12047228.

3.3 Preliminary check of field correction
Unnecessary error fields must be corrected. A
longitudinally integrated field correction is attemptedby a traditional way using pole end shims. Figure 4(a)shows a strong sextupole dependence of integrated field(squared data), which is approximated by BL ~ BL 0 BL 1 ( 1 + 0.135 x ) x. To eliminate the sextupole
component, each end-shim’s shape should be roughly
represented by s(x) ~ 0.0675 L x2 / {BL 0 / BL 1 - x (1 +0.15 x)} ; ~ 0.42 (x/50) 2 in mm unit. Figure 6 shows
results with and without simple parabolic concave endshims, s(x) ~ 0.75 (x/50) 2 in mm unit, which isforeseen overcompensation, for the purpose to find out
enough effects of the applied end shims for correction,
This curve is approximated with multi-layers of 0.5 mm
thick iron sheets with 160 mm width, which is smaller
than pole width, 230 mm, to make easily. The dottedline indicates the result with the end-shims. The sign ofgradient, which means a sextupole component, changes
to be inverse against the case without end-shims (asolid line) around center, as expected. More precise
correction will be planned by using more proper andaccurate end-shims cut by a numerical cutting machine.

ACKNOWLEDGEMENTS
We are greatly indebted to James T. Volk and hisstaffs for their useful comments and showing theirpermanent magnets and marvelous techniques. We wish
to thank Makoto Sakuda for his suggestions on the ARand supporting our work.

REFERENCES[1] K. Takayama et al., “KEK-PS Upgrade Scenario :500
MeVAccumulator
andSuperbunchAcceleration”, Proc. of EPAC 2000.
[2] K. Takayama et al., “Intensity Doubler : A Proposal
for a Major Upgrade of the KEK 12 GeV-PS”,
P2oth17 in this conference (HEACC 2001).
[3] Fermilab Recycler Ring Technical Design Report
Revision 1.2, Nov. 5, 1996.
[4] B. C. Brown et al., “Time dependence andtemperature stability of the permanent magnets for
Fermilab antiproton recycler ring”, IEEE Trans.
Applied Supercon., Vol. 10, No.1, (2000) pp.879-882.