JAERI-Conf 95-021

2a3-3
MEASUREMENT OF MAGNETIC FIELD CEN1ER OFTilE QUADRUPOLES AND SEXTUPOLESFOR TilE SPRING-S STORAGE RING
Jun-ichi OHNISHI, Minoru KAWAKAMI and Sakuo MATSUI
SPring-8, Kamigori-cho, Ako-gun, Hyogo Pref., JapanTABLE Iabstract

MAIN PARAMETERS FOR TilE SR MAGNIITSThe quadrupole and sextupole magnets for theSPring-8 storage ring were made the measurement of themagnetic field center with a rotating coil one by one todefine the position of the fiducial points above the upper
part of yokes. The measurement accuracy was estimatedto be about 10 Jlm. This paper describes themeasurement system, method and error estimation.

DipoleB88

Number

Gap I Bore Diameter (mm) 64
Length (m)2.8

B(T), B'(T/m), B"(T/m2)
I{A}

0.679
1232

Quadrupole Sextupole
Sa-Sc
Qa-Qf
336
480
92
85
0.35, 0.41, 0.3, 0.450.51, 0.97
420
17.5300
550

1. Introduction
The SPring-8 storage ring has a circumference of1436 m with a Chasman-Green lattice of 48 unit cells
and contains 88 dipole, 480 quadrupole and 336
sextupole magnets. Their mechanical design andfabrication was started in 1991 and finished all in thisspring by Hitachi, Sumitomo Heavy Industries and
Mitsubishi Electric Corp. Field Measurements have been
made in the SPring-8 site for all of about nine hundreds
magnets. It took nine months from September 1994 toJune 1995 for the measurement of the quadrupoles andsextupoles. The magnetic lengths and multipole fieldstrengths were measured for all magnets and the positionof the magnetic field center also measured for thequadrupoles and sextupoles. A long flip coil was usedfor the measurement of the dipoles and a rotating coil forthe quadrupoles and sextupoles.
Main parameters of the storage ring magnets islisted in table 1. They are normal ones with water cooledcoils. Magnet yokes are made of laminated silicon steelof a thickness of 0.5 mm. Manufacturing accuracy wasachieved within ±40 Jlm in bore diameter and minimum
gap for the quadrupoles and ±50 Jlm for the sextupoles.
All magnets have two machined fiducial points foralignment at the upper part of the yokes in front and rear.
Fig. 1. shows the fiducial points for the quadrupoles. The
fiducial points consist of a plane with a hole. Thefabrication accuracy of the fiducial plane is ±0.04 mm in
the height from the magnet median plane and 0.04 mm
in the degree of parallelization to the median plane.
Since the horizontal position of the fiducial hole ischanged with tilt of the magnet, position accuracy of thehole is dependent on the parallelization of the fiducialplane. The target position for alignment is in a height of
500 mm from the median plane and the position
accuracy of the fiducial hole corresponds to ±0.13 mm.
Five or seven of the quadrupoles and sextupoles areplaced on one girder and aligned along a straight line.
Alignment accuracy is required to be within ±50 Jlm in agirder so that the manufacturing accuracy of the fiducial
points of each magnet is not sufficient. Therefore, We
had to measure the position of the fiducial points withmore accuracy during the field measurement.
2. Measurement System and Method
A rotating coil system was used for the fieldmeasurement of the quadrupoles and sextupoles. Fig. 2.
shows the side view of the measurement system. Twomeasurement probes were used and both have measuring
coil windings in a cylinder made by FRP with a length of1960 mm and a diameter of 80 mm. Fig. 3. shows crosssectional views of the both probes. Probe A is used forthe measurement of the main field component and probe
B is a type of a bucking coil which cancels out thequadrupole component and used for the multipole fieldsof quadrupole magnets. The measurement cylinders are-203-( r---..'-...., \II<------.\+---+---+++

I I+--;:r-+\'---"_//_/++"-'.:_/J/

fiducial pointFig. 1. A top view of the quadrupole magnet. Fiducialpoints are located on top of yokes, which consist ofplanes with a hole (cp20) parallel to the median plane.

JAERI-Conf 95-021

Fiducial Points..

(a)LaserBearing
Coil Clamp/(b)CCDCamera

Basement'2Fig. 2. (a} A side view of the rotating coil system. Four arrows indicate fiducial points. We place aCCD camera and basement (b) on each fiducial point and measure the position of the laser beam.
supported with bearings at the both sides and can bepulled out during the exchange of magnets or probes onleft side in the figure. The coil is rotated at about 30
r.p.m. The output from the coil is integrated with a PDI
5025 {METROLAB)by a constant angle (ex. 2rc/32 rad.).
The main and multipole components of the magnetic
fields can be calculated by the Fourier transformation.
The multipole components can be separated into normal
and skew component against the direction of the main
field.
In case of quadrupoles, a deviation between arotating axis of the probe and a magnetic field center
induces a dipole component in the output signal. The
ratio of the both components is proportional to thedeviation as the follow equations.

beam axis because measuring coils are longer than themagnets. In the measurement of the field center, only anaverage position is measured and the direction of magnet
axis cannot be measured. However, the direction ofmagnet axis does not affect the beam if it is not verylarge.
The measurement procedure of the field center isfollowing: First, a sample magnet is pre-aligned on therotating coil system in about 0.1 mm. In the firstmeasurement after the initialization process, thedeviation of the field center from a rotating axis of thecoil is measured. The stage where the magnet is locatedcan be driven with motors with accuracy of several
microns. Actually, the field center is made coincidence

dx=«llln/cll2nxR/2, dy=«llls/cll2nx R/2
clll and cll2 indicate the maximum flux of dipole andquadrupole fields which goes through the coil. Suffix nand s denotes normal and skew components. R means areduce raidus of the coils. In case that a quadrupole ismeasured with coil 1 of probe A, maximum flux is 0.24
Vs at maximum current level and output voltage is about±0.75 V. The resolution of the integrator is 4 x 10-6 Vs
in five times amplification because a V -F converter of500 kHz I 10 V is used for voltage integration.
Consequently, the resolution of position measurement of
the magnetic center corresponds to 0.2 J.Lm. Actually
measurement stability was about 2 J.Lm due to coilvibration and others.
The rotating coil measures an average field along-204-

CoUI :12Tunll
CoU2::UTunll

Colli : 50 .......
Coi12 :50 Tumaprobe Aprobe BFig. 3. Cross-sectional views of lhe rotating coil probes. Probe A isfor the measurement of the main term and probe B for multipolefields.

JAERI-Conf 95-021

II••

1 50

••

...: .. • 0~
c:..

QM

SMlL :~11-')1, rml : ~4J.,' ..S.~~· :rm~

TABLE II
Es11MATIONS OF MEASUREMENT ERRORS OF 1liE MAGNETIC CENi"ERy(gm)x (l,!m)J: ~6Um l

Cl

~

100

1z·+··1 :ii;

50ii

1·+··+···l····i .j ... ,.... ; ...

j0'-'--'-.........._J........I..........._.L.....l...........llldiLIIILJ
0-80-160·24016080240dx (IJm)

Ul

150~
c:

..

lCl::E01 j j

1 ~!

!·····L..i............ ~.....i_;·····L....L.....i..••••;••••• t··.. L•....

100~.0.i, •••••. \ li

1 !

Ez"

=50........... j •••••

l i~ It····+··
i-80

080

160240dy (llmlFig. 4. Histogram of the position of the magnet fiducial
points measured with the rotating coil system. Dx and dy
indicate horizontal and vertical deviations.
with the rotating axis of the coil by moving the magnetwith the stage. After that, the main term and multipole
field strengths are measured in some current levels.
Finally, the position of the fiducial points on top of themagnet is measured. The method is as follows.
The rotating coil system has two fiducial points justabove the bearings on the both sides. A laser beam goes
through above the four fiducial points and its positions
are measured by locating the target with CCD camera oneach fiducial point by turns. The accuracy of these
position measurements is about five micron meters in thefarthest point from the laser source because of thefluctuation of the laser beam. This system is also used
for the magnet alignment in the ring and being reported
in another paper of this meeting.
In addition, the tilt of the magnet has to be measured
at the same time because the position of the laser beam islocated in a height of 500 mm from the median plane of
the magnet as already mentioned. The tilt is measured
with an electrical level instrument TAL YVEL (RANKTAYLOR- HOBSON). The TAL YVEL had to belocated in the same position on the fiducial plane everytime because the roughness of the surface was not very
good. This positioning of the TAL YVEL could obtain
the reproducibility of 10-15 j..l.rad between the position
measurements and magnet alignment.
3. Result and Analysis
Each quadrupole and sextupole was made themeasurement of position of the fiducial points. Fig. 4.

Instability of coil rotation
Reproducibility of coil calmp
Position stabilty of the fiducialpoints
Measurement of magnet tilt
position measurement withthe laser-camera system
Temperature effectTotal

2252336959

shows the histogram of the measured position of thefiducial points. The fluctuation of the position deviations
is within ±140 j..l.m for horizontal direction and ±50 j..l.mfor vertical. This fluctuation indicates the fabrication oneof the fiducial points though a measuring error of about
10 j..l.m is included as mentioned later and it is found thatthe magnitude is agreement with the specification of thefabrication accuracy. The reason why the averages do
not equal to zero is that some kinds of offset exists infabrication and measurement such as position error of
the fiducial point of the rotating coil, coil sag and so on.
Now, it is important to estimate the accuracy of themeasurement because all magnets are aligned in the ringaccording to each measured position of the fiducialpoints. Table 2 lists the estimation of the measurement
errors of the field center. The magnitude of errors
indicated in the table is about half of the maximum andmay correspond to the standard deviation. The first,fourth and fifth error sources listed have been mentioned
in a previous section. The second source means thereproducibility of the coil position in clamping on thebearings. This reproducibility was confirmed bymeasuring the field center with two probes for every
magnet. The third means that a slight tilt of the bearing
supports with the fiducial points is changed with time
and the points are moved. Though the tilt of the supports
was being monitored and the position of the fiducial
points corrected, this order of error was remained. The
sixth is the effect due to the temperature difference
between the apparatus and the magnet generated by thechange of the room temperature.
Total of these errors is estimated to be 9 j..l.m in thestandard deviation for both horizon tal and verticaldirections.
4. Conclusion
Strong quadrupole and sextupole magnets are usedin an electron storage ring with low emittance beams andbeam dynamics are very sensitive to their alignment.
Accordingly, the alignment accuracy of 50 j..l.m is
required to the SPring-8 storage ring. This value iscomparable to the dimensional accuracy of usuallyfabricated magnets and it is impossible to make fiducial
points mechanically with more accuracy. Therefore, it issignificant and essential to position the fiducial points bymeasuring the magnetic field center.

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