JAERI-Conf

95-021

PA27
DEVELOPMENT OF A COMPACI' STEERING MAGNETWITII EIGHT-POLE STRUCI'URE
Masanori IKEGAMI and Yoshihisa IWASIDTA
Nuclear Science Research Facility, Institute for Chemical Research, Kyoto University
Gokanosho, Uji, Kyoto 611, Japan
Abstract
A compact steering magnet has been devised. Thesteering magnet has "eight-pole structure". The strength and its
direction of the dipole field can be electrically changed. The
outer radius, the bore radius and the length of the magnet are120 mm, 14 mm and 60 mm, respeetively. The effective lengthis measured to be 97 mm and magnetic field of 0.4 kG isachieved. The steering magnet is very useful under severe spacelimitation.
1. Introduction

The 7 MeV proton linac at Institute for Chemical
Research, Kyoto University consists of 50 keV proton ion
source, 2 MeV R,FQ linac and 7 MeV Alvarez DTL. The linac
system has been operated since 1992[1]. Unfortunately, the ·output beam from the DTL has a slight angle against thedesigned axis, and then the beams are deflected away byquadrupole lenses installed downstream.
A conventional steering magnet system consists oftwo dipole magnets whose fields are perpendicular to eachother, while our facility does not have enough space for two
dipole magnets between the exit of the D'l'L and the quadrupole
magnets. Because the direction of the output beam is found to
be changed with operating conditions, a short steering magnet
system, which has the variable strength and the steering
direction, was required. Considering these requirements, wehave developed a new type of compact steering magnet which
has "eight-pole structure". The main features of this steering
magnet and the results of the field measurements are describedin the present paper.

(3)

The above considerations provide us a guide line for adesign of a dipole magnet with "multi-pole structure", The
extension to a magnet for higher order field is straight forward.
Applying the above considerations to an actual
magnet, the sinusoidal magnetic potential should beapproximated by segments where the magnetic potential isconstant and chosen to be proportional to the sine of theazimuthal angle. Further more, coil space should be reserved.
Taking these requirements in1t0 consideration, we adopt "eightpole structure" in which the sinusoidal magnetic potential isapproximated by eight poles (See Fig. 1 and 2). The magnetic
potential on the pole tips is chosen to be proportional to the sineof the azimuthal angle. In this structure, enough space isavailable for coils and we can g~ relatively large field strength.

2. Concept of the multi-pole structure
It is well-known that a circular distribution of electric
current, in which the current density is proportional to thecosine of the azimuthal angle, can produce a perfeetly uniform
dipole field [2][3]. In analogous way, it also can be shown thata magnet with "multi-pole structure" can produce a dipole fieldaround the axis.
Inside the aperture ro. where their is no electric
current (ignoring the beam), the magnetic field can be
expressed as the gradient of the magnetic potential cp, i. e.,

Fig. 1

Schematic view of the eight"pole structure

cppole 3(1)e (rad)The magnetic potential of the perfect 2N -pole field can be
written aspole 1poleS(2)ipole apole 7
where k2N is an appropriate coefficient representing the fieldstrength and e is the azimuthal angle relative to the horizontal
direction. In order to generate the dipole field (N = 1) inside theradius ro. the magnetic potential at r = r0 must have the form

Fig. 2Magnetic potential distribution at r = r0
The magnetic potential is linearly interpolated between poles.
broken line: sine curve, solid line: generated by eight poles- 100-

JAERI-Conf 95-021

We adopt thin poles to reduce leakage flux and have agood approximation of the sinusoidal magnetic potential.
It can be easily derived from eq. (l) and (3) that ineight-pole structure the field strength on the pole tips can bewritten as. n-1

B,.,. = B0 cos( 4n- 9)(4)(n = l, 2; 3, ... , 8)

where Br,. is the field strength on the n-th pole tip and 9 is theazimuthal angle of the dipole field relative· to the horizontal
direction. The azimuthal angle of the n-th pole tip measured
relative to the horizontal direction is (n-1)7t/4 radian (See Fig.
1). The excitation current in each coil can be written as}
nn-1s(4n-9)
I,.= I0 { cos(4n-9)-co(n(5)= 1, 2, 3, ... , 8)

where I,. is the excitation current in the n-th coil and Io is acoefficient representing the strength of the excitation current.
N-th coil is located between the n-th and (n+l)-th poles (SeeFig.1 ).
With this structure, we can change the field strengthand its direction only by changing the excitation currents andthen we can steer the beam in both horizontal and verticaldirections by a single element. It is very advantageous under
severe space limitation.

"3. Field calculation and design
The geometry of the steering magnet is shown in Fig.

3. The outer radius, the bore radius and the length of the magnet
are 120 mm, 14 mm and 60 mm, respectively. In order to havelong effective length, the length of the pole is longer than thatof iron yoke. Its iron yoke is octangular rather than circular for
sitnple fabrication.

(c)Fig. 4Flux Plot of a eight-pole magnet

Io =2.0 A. Mesh size is 1 mm.
Fig. 3

Geometry of the steering magnet(a) 9-101-=90" (b) 9 =78.75" (c) 9 =67.5"

JAERI-Conf 95-021

400300•0'--'-.._._._._.._._._._,u..-:_._._._._._._.__.._._.__._._......__._._~

02

Hannonic number N

34

5

6

Excitation current strength 10 (A)(a)

Fig. 6
Magnetic field strength as a function of the excitation
current strength I oFig. 5
Results of harmonic analysis(a) harmonic field components(b) azimuthal angle of the generated dipole filed 9iron pole

400

wza

Table 1 Main features of the steering magnet
iron poleiron yokecoil

number of poles
bore radius
axial length
axial length
number of coils
number of turns per coilcross section
axial length
excitation current

total axial length
effective length
maximum field strength

814mm
56mm
22mm
8 '49818 x40mm260mm<SA60mm
97mm>0.4kG

This geometry is designed by PANDIRA[4). Some
results of the calculations are shown in Fig. 4. In thecalculations, we found that a steering magnet with eight-polestructure can produce sufficiently good dipole field aro~d. theaxis, in which higher components of the field are neghg1blysmall. Some results of harmonic analysis are shown in Fig. 5.
The harmonic field components are normalized at 10 mm fromthe axis.
Four DC power supplies feed the excitation current
with an accuracy of± 1 %. Main features of the steering magnet
are shown in Table 1.
4. Magnetic field measurement
The steering magnet has been fabricated and somecharacteristics of its magnetic field are measured. The
dependence of the field strength at the magnet center on theexcitation current strength / 0 is measured by a Hall probe(FWBELL 4048) (See Fig. 6). The field strength of 0.4 kG isachieved and it is strong enough for the present purpose. Above
10 = 3 A, the field strength becomes to saturate. Judging fromthe results of the two-dimensional calculations with P ANDIRA,
the nonlinearity is mainly due to the iron saturation in theconnection region of the iron pole to the yoke.
. .
The magnetic field distribution on the beam ax1s ts
also measured with the Hall probe (See. Fig. 7). The effective
length of the steering magnet is calculated to be 97 mm fromthe measured results, which is considerably larger than the pole
length of 56 mm. It provides larger deflection angle for thebeams.

10050;;;; ........ .! ••0 ~~~~~.......~........_.__..__._.~~~~~
020406080

100120140

Distance from the pole center (mm)Fig. 7

Magnetic field distribution on the axis

5. Summary
A compact steering magnet with eight-pole structure,
which has variable strength and field direction has beendeveloped. It has been installed in the beam line. The outerradius the bore radius and the length of the magnet are 120mm, l4 mm and 60 mm, respectively. The effective length ~fthe steering magnet is measured to be 97 mm and the magnetic
field of 0.4 kG is achieved. The steering magnet with eight~polestructure is very advantageous tmder severe space limitation.
Acknowledgment
The authors would like to thank Prof. Akira Noda andProf. Makoto Inoue for their continuous encouragement andsupport, and all the other members of the acceleralor group ofNuclear Science Research Facility for their help and fruitfuldiscussions.
References[1) M. Inoue et al., "Commissioning of the 7 MeV Proton L~acatiCR Kyoto University", Bull. Inst. Chern. Res. Kyoto Uruv.,
Vol. 71, No. 1, pp. 57-61 (1993)[2) P. F. Dahl et al., IEEE Trans. Nucl. Sci. Vol. NS-20, No.3,pp. 688 (1973)[3) H. Ikegami et al., RCNP Annual Report 1976, ResearchCenter of the Nuclear Physics, Osaka Univ., pp. 137-147 (1976)[4) User's Guide for the POISSON /SUPERFISH group ofcodes, LA-UR-87-115, LANL (1987)- 102-