GEOMETRY AND REMANENT FIELD SERIES MEASUREMENT OF THE LEP DIPOLE MAGNETS

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GEOMETRY AND REMANENT FIELD SERIES MEASUREMENT OF THE LEP DIPOLE MAGNETS

J. Billan, J. Gourber, K. Henrichsen

To cite this version:

J. Billan, J. Gourber, K. Henrichsen. GEOMETRY AND REMANENT FIELD SERIES MEASURE-

MENT OF THE LEP DIPOLE MAGNETS. Journal de Physique Colloques, 1984, 45 (C1), pp.C1-

953-C1-956. �10.1051/jphyscol:19841194�. �jpa-00223671�

Colloque C l , supplkment au n o I, Tome 45, janvier 1984 page Cl-953

GEOMETRY AND REMANENT F I E L D SERIES MEASUREMENT OF THE LEP DIPOLE MAGNETS

J . B i l l a n , J.P. Gourber and K.N. Henrichsen

LEP Division, C . E . R. N . , 1211 Geneva 23, SwitzerZand

RZsumg - Les performances magngtiques des 3304 d i p o l e s du LEP sont dgduites m s u r e s gQomQtriques prgcises de l ' e n t r e f e r e t du champ rgmanent de chaque

noyau d'aimant. Pour Q v i t e r banc de mesure e t alignement, deux c h a r i o t s rou- l a n t directement s u r l e p 6 l e i n f Q r i e u r sont u t i l isQs. Le premier, 6quipQ de c i n q sondes de proximi t6, d' un niveau 61 ectronique e t d'une c e l l u l e photo61 ec- t r i q u e couplQe ?I un faisceau l a s e r , mesure l ' e n t r e f e r . Le second c h a r i o t per- met de magn6tiser l e b l o c dont l e champ rgmanent e s t e n s u i t e mesur5 p a r une bobine tournante longue. L'ensemble de l a mesure e s t contr616 par un micro- ordinateur.

A b s t r a c t - The magnetic performance o f t h e 3304 d i p o l e magnets f o r LEP a r e -from systematic air-gap geometry and remanent f i e l d measurements. To

a v o i d having t o use a measuring bench and t o minimize alignment problems, two c a r r i a g e s r o l l i n g d i r e c t l y on t h e lower p o l e face are used. The f i r s t one, equipped w i t h f i v e p r o x i m i t y detectors, one e l e c t r o n i c i n c l i n o m e t e r and one p h o t o e l e c t r i c c e l l coup1 ed w i t h a l a s e r beam, measures t h e air-gap geometry.

The second one magnetizes t h e yoke, t o enable t h e measurement o f t h e remanent f i e l d by means o f a l o n g f l i p c o i l . The measurement i s f u l l y computer-con- t r o l l ed.

I - INTRODUCTION

The l a r g e e l e c t r o n p o s i t r o n c o l l i d e r (LEP) under c o n s t r u c t i o n a t CERN r e q u i r e s a l a r g e number (3304) o f low f i e l d d i p o l e magnets w i t h steel-concrete cores [I]. For t h i s production a t t h e r a t e o f s i x cores per day, complete magnetic measurements w i l l be performed o n l y on a s t a t i s t i c a l b a s i s (using every t e n t h magnet c o r e ) and t h e mag- n e t i c p r o p e r t i e s o f each c o r e w i l l be determined from systematic measurements o f i t s air-gap geometry and remanent f i e l d . This i s p o s s i b l e because t h e s t e e l yoke i s f a r from s a t u r a t i o n (3 % ampzre-turn drop i n the yoke a t maximum [ 2 1). It i s a1 so more economical f o r t h e f o l l o w i n g reasons:

- t h e r e i s no need f o r mounting t h e e x c i t a t i o n bars which a r e foreseen t o be i n - s t a l l e d on p a i r s o f cores p r i o r t o t h e i n s t a l l a t i o n i n t h e tunnel;

- these measurements can be combined w i t h t h e s t r a i g h t n e s s measurements which are needed f o r t h e v e r t i c a l and h o r i z o n t a l alignment o f t h e cores;

- t h e r e i s no need f o r a s e r i e s o f measuring benches.

The r e q u i r e d tolerances o f t h e magnetic f i e l d are [31:

- random d i s p e r s i o n o f t h e bending s t r e n g t h : 5 lo-'+ r.m.s.

- random v a r i a t i o n o f the f l u x d e n s i t y i n the useful gap a p e r t u r e : 2 r.m.s.

- random t i 1 t o f t h e median plane : 0.2 mrad r.m.s.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19841194

Cl-954 JOURNAL DE PHYSIQUE I 1 - AIR-GAP GEOMETRY MEASUREMENT

1 - P r i n c i p l e

The adopted s o l u t i o n i s t o i n t r o d u c e a c a r r i a g e i n s i d e the a i r gap, Fig. 1. It i s equipped w i t h a l l t h e necessary d e t e c t o r s as shown i n Fig. 2. F i v e p r o x i m i t y de- t e c t o r s measure t h e p o s i t i o n o f t h e c a r r i a g e i n s i d e t h e a i r gap and thus t h e h e i g h t

o f t h e a i r gap i n two points, one incii~ometer four-quadrant d i f f e r e n t i a l p h o t o c e l l allgment Jig measures t h e p o s i t i o n o f t h e c a r r i -

age w i t h r e s p e c t t o a l a s e r beam and reference an e l e c t r o n i c i n c l i n o m e t e r measures

t h e t i 1 t o f the carriage. T h i s t i 1 t steel i s compared t o t h e tilt o f t h e core ,aminatran measured by a second e l e c t r o n i c i n - proximity c l inometer placed on t h e a1 ignment detectors j _ig.

inclinometer

photocell As t h e c a r r i a g e r o l l s d i r e c t l y on t h e lower p o l e face (which, due t o carriage the presence o f p a i n t , i s n o t a

plane surface), a f l u c t u a t i o n o f t h e p o s i t i o n o f t h e c a r r i a g e i n t h e a i r gap i s p o s s i b l e ( w i t h i n f 1 mm).

This d e f i n e s t h e measuring range o f t h e p r o x i m i t y detectors.

concrete

The beam emitted by a l a s e r tube

- laser beam placed a t one end o f t h e core and

p o i n t i n g a f i x e d p h o t o c e l l a t t h e Fig. 1 - Schematic view o f t h e air-gap o t h e r end, d e f i n e s a f i x e d s t r a i g h t

geometry measurement system l i n e p o s i t i o n e d w i t h i n f 1 mn from each end, enabling t h e measurement of t h e h o r i z o n t a l and v e r t i c a l straightnesses o f t h e magnet a i r gap. These measure- ments determine t h e average a x i s o f t h e a i r gap and, r e l y i n g upon t h e h i g h punching p r e c i s i o n of t h e c o r e laminations, g i v e d i r e c t l y t h e c o r r e c t i o n s t o apply f o r t h e

Fig. 2 - I n t r o d u c t i o n o f t h e measurement c a r r i a g e i n t h e a i r gap o f t h e core

alignment i n t h e LEP tunnel. The r e q u i r e d measurement range of * ⁵ mm f o r t h e photo- c e l l mounted on t h e c a r r i a g e i s given by t h e sum o f p o s s i b l e e r r o r s i n l a s e r beam p o s i t i o n (f 1 mm max.), on c a r r i a g e p o s i t i o n i n t h e a i r gap (f 1 mm max.), and the s t r a i g h t n e s s o f t h e c o r e (f 3 mm max.). To g e t a good l i n e a r i t y (k 0.05 mm) w i t h i n t h a t range, a 50 mm diameter p h o t o c e l l was selected.

The e l e c t r o n i c i n c l i n o m e t e r s have a l a r g e measuring range: f 20 mrad so t h a t l e v e l - l i n g o f t h e magnet i s n o t required. T h e i r accuracy i s w i t h i n * ^0.04 mrad ( * 0.02 mrad i f c a l i b r a t e d d a i l y ) .

2 - The P r o x i m i t y Detectors

The main challenge was i n f a c t t o measure the e f f e c t i v e h e i g h t o f t h e magnetic a i r gap which, due t o t h e p e n e t r a t i o n o f t h e f l u x l i n e s i n s i d e t h e spacing between l a m i - n a t i o n s (Fig. 3) i s about 1.0 mm l a r g e r than t h e mechanical gap. A magnetic p r o x i m i t y d e t e c t o r (Fig. 4) has been s p e c i a l l y designed t o measure t h i s e f f e c t i v e gap. It con- s i s t s o f a U-shaped c i r c u i t which c l o s e s through t h e magnet p o l e v i a two a i r gaps.

These a i r gaps a r e determined by t h e measurement o f t h e self-inductance o f t h e c o i l s e x c i t e d by a 80 Hz s y n t h e t i z e d cur- rent. A l o c a l microprocessor c o n t r o l s t h e f i v e probes and permits a r e s o l u t i o n o f 0.001 mm and an accuracy o f f 0.005 mm.

The d i f f e r e n c e between magnetic and me- chanical gap height, h, depends on t h e thickness, e, t h e p i t c h , p, and t h e shape o f t h e c u t edges o f t h e laminations. The- o r e t i c a l computations, w i t h t h e two-di- mensional POISSON program i n i t s "elec- t r o s t a t i c " version, were i n good agree- ment w i t h experimental r e s u l t s u s i n g samples where o n l y one o f those t h r e e pa- rameters was v a r i e d a t a time. For small v a r i a t i o n s , Ap and Ae, around t h e nominal values o f D = 5.5 mm and e = 1.5 mm:

h = 1.02 + 0.35 Ap - ^{0.59 Ae i n mm. Fig. 3} - F l u x 1 in e c o n f i g u r a t i o n a t t h e o o l e face The lengthening o f t h e magnetic f l u x l i n e s , due

t o t h e deformation by punching o f t h e c u t edges o f t h e lamination, was measured and found t o be o f t h e o r d e r o f 0.05 mm p e r pole, i .e. 0.10 mm f o r t h e t o t a l a i r gap.

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Fig. 4 - Cross-section o f t h e p r o x i m i t y d e t e c t o r

A measurement d i s t a n c e between probe and p o l e face of 3 mm f 1 mm was chosen because below 2 mm, t h e probe s i g n a l i s modulated by t h e discontinuous s t r u c t u r e o f t h e magnet core. The l e n g t h (200 mm) o f t h e probe r e - s u l t s from a compromise between good mechanical s t a - b i l i t y and t h e choice o f a reasonable number o f meas- urement p o i n t s over t h e l e n g t h o f t h e magnet.

3 - Measurement Procedure and Performance

The p r e p a r a t i o n o f t h e measurement c o n s i s t s o f mount- i n g t h e g u i d i n g r a i l , f i x i n g t h e l a s e r tube a t one end o f t h e core, and i n t r o d u c i n g t h e c a r r i a g e i n s i d e t h e a i r gap w i t h i t s p h o t o c e l l r e t r a c t e d t o p e r m i t the a1 ignment o f t h e l a s e r beam w i t h respect t o t h e f i x e d p h o t o c e l l mounted a t t h e o t h e r end.

Moved by a stepping motor, t h e c a r r i a g e stops every

200 mm a t t h e 28 measuring p o i n t s , thus c o v e r i n g the

e n t i r e core length. The complete measurement i s

C1-956 JOURNAL DE PHYSIQUE

c o n t r o l l e d by a microcomputer which c a l c u l a t e s a t each p o i n t t h e e f f e c t i v e gap h e i g h t i n two transverse p o s i t i o n s (f 77 mm), t h e tilt o f t h e median plane and t h e p o s i t i o n o f t h e l a s e r beam on t h e photocell. A t t h e end o f t h e measurement, i t deduces -

- t h e average e f f e c t i v e gap h e i g h t which i s c o r r e l a t e d t o t h e magnetic f i e l d f o r a given e x c i t a t i o n ( w i t h i n f 0.01 mm, i.e. AB/B = +

- t h e mean e r r o r o f p a r a l l e l i s m o f t h e p o l e faces, which i s c o r r e l a t e d t o t h e f i e l d inhomogeneity i n the useful gap a p e r t u r e ( w i t h i n 1 a t k 77 mm);

- t h e average t i l t o f t h e median plane w i t h r e s p e c t t o t h e reference j i g ( f 0.1 mrad) and t h e t w i s t o f t h e core;

- t h e h o r i z o n t a l and v e r t i c a l p o s i t i o n s o f t h e l o n g i t u d i n a l core a x i s w i t h respect t o t h e a1 ignment r e f e r e n c e j i g s (f 0.1 mm).

111 - REMANENT FIELD MEASUREMENT

The s t e e l o f t h e core i s f i r s t magnetized up t o a l e v e l s i m i l a r t o t h a t reached by t h e magnet a t maximum e x c i t a t i o n . I n the absence o f t h e magnet c o i l s , t h i s magnetiz- a t i o n i s done by c l o s i n g the a i r gap w i t h a s h o r t magnetic block l e a v i n g a clearance o f o n l y 2 mm a t each p o l e face. The necessary e x c i t a t i o n c u r r e n t i n t h e c o i l , wound around t h i s block, i s thus s t r o n g l y reduced. A b l o c k l e n g t h o f 100 mm was found t o be s u f f i c i e n t .

Since t h e magnetization i s done w i t h asymmetrical c y c l e s

(0 - Bmax - 0 ) f a r from satura- t i o n , t h e remanent f i e l d increases w i t h t h e number o f c y c l e s t o reach a s y m p t o t i c a l l y i t s maximum value.

A t maximum e x c i t a t i o n , t h e rema- nent f i e l d i s reached i n f o u r cycles. F o r t h a t reason, two mag- n e t i z i n g blocks have been mounted on t h e same carriage, thus produc- i n g two c y c l e s a t each passage along the core. T h i s c a r r i a g e (Fig. 5) uses the same d r i v i n g system as t h e air-gap geometry measurement carriage.

A f t e r magnetization, a l o n g r o t a t - i n g c o i l i s i n s e r t e d i n t o the a i r gap f o r measuring t h e remanent f i e l d . The measurement accuracy i s b e t t e r than f 5 T.

Fig. 5 - The magnetizing c a r r i a g e and t h e measuring equipment Since t h e l e v e l o f magnetization reached i s about t w i c e t h e l e v e l i n phase I o f LEP

(60 GeV), a demagnetization i s necessary a f t e r measurement. This i s obtained by mak- i n g a passage o f t h e c a r r i a g e e x c i t e d by 50 Hz a.c. current.

REFERENCES

C11 GOURBER J.P., WYSS C., IEEE Trans. Nucl. Sci. NS-28, 3 , (1981) 2867.

[21 GOURBER J.P., BILLAN J., LAEGER H., PERROT A*, RESEGOTTI L., CERN LEP-MA/83-9 -

presented a t 1983 Part. Acc. Conf., Santa Fe, N.M. (March 1983).

[31 GUIGNARD G., CERN LEP-TH/83-38 - presented a t 1 2 t h I n t . Conf. on High-Energy

Acc., Batavia, I L (August 1983).