DESIGN OF THE DIPOLE MAGNETS FOR ORBIT CORRECTION IN LEP

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DESIGN OF THE DIPOLE MAGNETS FOR ORBIT CORRECTION IN LEP

Ph. Lebrun

To cite this version:

Ph. Lebrun. DESIGN OF THE DIPOLE MAGNETS FOR ORBIT CORRECTION IN LEP. Journal de Physique Colloques, 1984, 45 (C1), pp.C1-305-C1-308. �10.1051/jphyscol:1984161�. �jpa-00223717�

JOURNAL DE PHYSIQUE

Colloque C1, suppl&ment au n o 1, Tome 45, janvier 1984 page C1-305

DESIGN OF T H E DIPOLE MAGNETS FOR O R B I T CORRECTION IN LEP

Ph. L e b r u n

LEP Division, C. E. R. N., 121 1 Geneva 23, SwitzerZand

R6sume

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A b s t r a c t

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INTRODUCTION

Closed o r b i t d i s t o r t i o n s i n c i r c u l a r a c c e l e r a t o r s and storage r i n g s increase w i t h t h e number o f components, and hence t h e s i z e o f t h e machine. The l a t t i c e o f LEP c o n t a i n s about 4700 bending and focusing magnets [ l l ; random f i e l d e r r o r s o r misalignments produce d i s t o r t i o n s o f t h e closed o r b i t s , which may exceed t h e a p e r t u r e and l i m i t t h e performance o f t h e machine. An e f f i c i e n t o r b i t c o r r e c t i o n system i s , therefore, es- s e n t i a l t o speed up commissioning, o b t a i n e a r l y c i r c u l a t i n g beams, and 1 a t e r optimize o p e r a t i n g conditions. This system i n v o l v e s beam p o s i t i o n monitoring, data transmis- s i o n and processing [21, and c o n t r o l o f the power supplies [ 3 1 feeding t h e c o r r e c t i o n d i p o l e magnets. Although o n l y the l a t t e r are discussed here, t h e i r design i s con- s i s t e n t w i t h t h e g l o b a l requirements o f t h e system.

BASIC REQUIREMENTS

The t o t a l number o f c o r r e c t i o n d i p o l e s r e s u l t s from the d e s i r e d q u a l i t y o f o r b i t cor- r e c t i o n : a n a l y t i c a l c a l c u l a t i o n s [41 show t h a t about t h r e e c o r r e c t o r s p e r b e t a t r o n wavelength g i v e a good c o r r e c t i o n q u a l i t y . I n LEP, t h i s amounts t o about 260 inde- pendent c o r r e c t i o n d i p o l e s i n each transverse plane. For each magnet type, a l a r g e s e r i e s i s r e q u i r e d so t h a t c a r e f u l o p t i m i z a t i o n o f t h e design i s expected t o r e s u l t i n s i g n i f i c a n t o v e r a l l economy.

The magnets must be s t r o n g enough t o c o r r e c t t h e closed o r b i t g l o b a l l y and compensate l o c a l l y the d i s t o r t i o n s produced by quadrupole misalignments o f 1 mm a t maximum beam

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984161

CI-306 ^JOURNAL DE PHYSIQUE

Table 1

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energy. Moreover, t h e c o r r e c t i o n d i p o l e s w i l l be used t o produce l o c a l bumps f o r scanning t h e a p e r t u r e a t i n j e c t i o n energy. The t i g h t e s t c r i t e r i o n d e f i n e s t h e r e - quirements on f i e l d i n t e g r a l [ 2 1 l i s t e d i n Table 1.

The maximum f i e l d inhomogeneity acceptable i n t h e u s e f u l r e g i o n s o f t h e apertures, deduced from the f i e l d t o l e r a n c e i n t h e main d i p o l e magnets, i s 0.75 %. Larger values can be accepted a t h i g h e x c i t a t i o n s o f t h e magnets, mainly used f o r scanning bumps.

Furthermore, t h e s e t t i n g e r r o r s must be small enough t o have n e g l i g i b l e c o n t r i b u t i o n s t o t h e r e s i d u a l o r b i t , r e s u l t i n g i n a maximum absolute e r r o r o f 2 10-5 T m. Combining both requirements r e s u l t s i n t h e g l o b a l t o l e r a n c e s C51 o f Fig. 2; a good f i e l d qua1 i- t y i s r e q u i r e d throughout t h e o p e r a t i n g range, i n c l u d i n g a t low e x c i t a t i o n s where one has t o cope w i t h n o n - l i n e a r e f f e c t s i n the magnetic p r o p e r t i e s of t h e s t e e l .

MCHA 88 0.036 102 400 t 59 t 19

The gaps o f t h e c o r r e c t i o n d i p o l e s must be s u f f i c i e n t t o accommodate the LEP vacuum chamber, equipped w i t h l e a d s h i e l d i n g and bakeout i n s u l a t i o n , and a l l o w i n g f o r f a b r i - c a t i o n and alignment tolerances. L o n g i t u d i n a l space being l i m i t e d i n LEP, t h e l e n g t h o f t h e magnetic c i r c u i t s has been s e t a t 0.4 m, r e s u l t i n g i n r a t h e r s h o r t magnets where end e f f e c t s become important.

Magnet type Number

Maximum f i e l d i n t e g r a l ET m l

Gap [mml

Length o f magnetic c i r c u i t [mml H o r i z o n t a l u s e f u l a p e r t u r e [mml V e r t i c a l u s e f u l a p e r t u r e [mml

The c o r r e c t i o n d i p o l e s w i l l be i n d i v i d u a l l y e x c i t e d by b i p o l a r power s u p p l i e s through cables running along t h e machine tunnel. Environmental c o n d i t i o n s d u r i n g o p e r a t i o n of LEP w i l l be c h a r a c t e r i z e d by s t r o n g i o n i z i n g synchrotron r a d i a t i o n from t h e c i r c u l a t - i n g beams and a i r temperatures ranging up t o 38°C.

MCV 176 0.019 200 400 f 35 t 33

MCH 168 0.029 102 400 f 59 t 19

MAGNETIC CIRCUITS

MCVA 88 0.031 200 400 f 46 f 33

Once t h e l e n g t h o f t h e magnets has been f i x e d , optimal design c o n s i s t s i n f i n d i n g a d i s t r i b u t i o n o f e x c i t a t i o n c o i l s and magnetic yoke prod11 -7g t h e d e s i r e d f i e l d l e v e l and q u a l i t y , w h i l e m i n i m i z i n g t h e transverse dimensions, .td hence the c o s t o f the magnets. Several p o t e n t i a l l y s u i tab1 e geometries have been i n v e s t i g a t e d [ 6 1 and t h e i r r e l a t i v e e f f i c i e n c i e s compared on t h e b a s i s o f common requirements (Fig. 1). The U- shaped s o l u t i o n , r e q u i r i n g h a l f t h e conductor mass o f t h e others, appears t o be t h e most economical. F i e l d homogeneity i n the u s e f u l a p e r t u r e i s achieved by a p p r o p r i a t e shimming o f t h e p o l e pieces, a d j u s t e d by measurements on model magnets (Fig. 21.

Due t o simple geometry and slow ramping rates, i t was f i r s t thought t o use p l a t e s o f commercially pure ( "Armco" ) i r o n o r c o n s t r u c t i o n s t e e l . However, such m a t e r i a l s ex- h i b i t c o e r c i v i t i e s o f 100 A m - 1 and above, g i v i n g r i s e t o remanence i n t h e magnet

C O N V E N T I O N A L WINDOW-FRAME , WINDOW-FRAME, U-SHAPED

D I P O L E RACETRACK C O I L S B E D S T E A D C O I L S

Fig. 1

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at half operture

- - - / - - B i i T r n l - + L -_---

001 0 0 2 0 03

Fig. 2

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The choice o f s i l ic o n s t e e l imposes a

Fig. 4

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1 aminated construction. For t h e sake o f s c a l e economy, o n l y two types o f magnetic c i r c u i t s have been designed, one common t o MCV and MCVA, and t h e o t h e r common t o MCH and MCHA magnets

(Figs. 3 and 4). Each c i r c u i t i s made o f two stacks o f L-shaped laminations, glued t o g e t h e r w i t h epoxy r e s i n . The mechanical p r e c i s i o n r e q u i r e d by t h e d e s i r e d f i e l d q u a l i t y i s ensured by t h e c l o s e to1 erances on t h e punched 1 aminations (0.02 mm) and on t h e glued stacks, h e l d together by means o f an e x t e r n a l A1 - a l l o y frame and s t a i n l e s s s t e e l t i e - b o l t s which guarantee t h e p r e c i s e a p e r t u r e o f t h e magnet gaps.

EXCITATION COILS

The choice o f c u r r e n t d e n s i t y i n magnet windings u s u a l l y r e s u l t s from m i n i m i z a t i o n o f c a p i t a l and i n t e g r a t e d o p e r a t i o n costs. The l a t t e r can be neglected i n t h e case o f c o r r e c t i o n magnets, expected t o have a low r.m.s. e x c i t a t i o n l e v e l . For t h e magnets i n t h e machine a r c s (MCV and MCH), t h e d e f i n i t i o n o f c u r r e n t d e n s i t i e s r e s u l t s from t h e cab1 i n g l e n g t h s (up t o 1900 m) imposed by t h e l a y o u t and t h e 1 im i t e d v o l t a g e (de- s i g n value 120 V) a v a i l a b l e a t the power supply terminals: s a t i s f y i n g these c o n f l i c t - i n g requirements leads t o low design currents. I n t h i s range, m i n i m i z i n g the t o t a l c o s t o f t h e system a l s o minimizes t h e mass o f conductor i n t h e c a b l e and e x c i t a t i o n c o i l [61. This c o n d i t i o n leads t o t h e s e t s o f c h a r a c t e r i s t i c s i n Table 2, based on c u r r e n t l y a v a i l a b l e cross-sections o f copper w i r e s and cables. The use of aluminium conductor has n o t been considered due t o the 1 ack o f commercially a v a i l a b l e products.

C1-308 JOURNAL D E PHYSIQUE

Table 2

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The moderate c u r r e n t d e n s i t i e s i n t h e MCV and MCH c o i l s r e s u l t i n low power

densities.Calculations 161 and meas- thermal llmlt olr cooled to11

urements on models have shown t h a t the b u l k thermal c o n d u c t i v i t y o f t h e c o i l s allows them t o be cooled from t h e i r s u r f a c e only. Moreover, t h e low abso- ^I5O 1 u t e values o f power d i s s i p a t i o n sug- gested a i r cooling. A surface thermal r e s i s t a n c e o f about 0.15 K W - 1 was measured on model c o i l s f i t t e d w i t h black anodized f i n n e d a1 uminium pro- f i l e s glued on t h e i r o u t e r surface. ^{l o o} For t h e MCV and MCH magnets, t h i s per- m i t s n a t u r a l convection i n a i r t o keep the maximum c o i l temperatures be1 ow 70°C i n t h e worst o p e r a t i n g condi- t i o n s . MCVA and MCHA magnets, l o c a t e d i n t h e a c c e l e r a t i o n r e g i o n s o f LEP, ⁵⁰ a r e stronger b u t can accept higher c u r r e n t d e n s i t y because o f s h o r t e r cables, r e s u l t i n g i n h i g h e r power d i s - s i p a t i o n which r e q u i r e s water c o o l i n g (Fig. 5).

Copper w i r e o f round cross-section, o ¹ 2 3 L 5 6

i n s u l a t e d w i t h a polyamide-imide ena- Fig. 5

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der vacuum w i t h an epoxy r e s i n . I n s u l a t i o n t o ground i s ensured by g l a s s - f i b r e epoxy c o i l formers and g l a s s - f i b r e tape.

Magnet type

Number o f t u r n s i n c o i l

Nominal w i r e diameter [mml

Maximum e x c i t a t i o n c u r r e n t [A]

Maximum power d i s s i p a t i o n C W I E l e c t r i c a l r e s i s t a n c e o f c o i l a t 20°C [Q]

ACKNOWLEDGEMENTS

B. Jean-Prost and J.L. Servais c o n t r i b u t e d t o t h e design and c o n s t r u c t i o n o f t h e mo- del magnets, l a t e r measured by G. Brun. S t i m u l a t i n g discussions w i t h J.P. Gourber, 6. Guignard, L. Resegotti and C. Wyss are g r a t e f u l l y acknowledged.

MCV 2420

1.6 2.5 160

21.2

111 RESEGOTTI L., Inv. paper t o t h i s Conference (1E2-01).

[ 2 1 BORER J. e t a1

.,

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L31, ISCH H.W., Proc. . 5 t h I n t . PC1 Conf., Geneva (1982) 278.

[43 GUIGNARD E., CERN 70-24 (1970).

151 GUIGNARD G., CERN LEP-THL83-38

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C61 LEBRUN Ph., LEP Note 332 (1981)

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MCH 2050

1.5 2.5 150

20.4

MCVA 1980

1.9 5.0 320

12.3

MCHA 1300

1.9 5.0 210

8.1