A HIGHLY AUTOMATED MEASURING SYSTEM FOR THE LEP MAGNETIC LENSES

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A HIGHLY AUTOMATED MEASURING SYSTEM FOR THE LEP MAGNETIC LENSES

O. Pagano, P. Rohmig, L. Walckiers, C. Wyss

To cite this version:

O. Pagano, P. Rohmig, L. Walckiers, C. Wyss. A HIGHLY AUTOMATED MEASURING SYSTEM

FOR THE LEP MAGNETIC LENSES. Journal de Physique Colloques, 1984, 45 (C1), pp.C1-949-C1-

952. �10.1051/jphyscol:19841193�. �jpa-00223670�

A H I G H L Y AUTOMATED MEASURING SYSTEM FOR THE L E P MAGNETIC LENSES

0. Pagano, P. Rohmig, L. Walckiers and C. Wyss LEF Division, C. E.R. N., 1211 Geneva 23, SwitzerZand

REsum6

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Les q u a d r y 6 l e s e t sextup8les du LEP seront mesur6s par l a m6thode harmoni que. Un systeme de c o n t r d l e compl6tement autonome g a r a n t i t des mesures p r 6 c i s e s e t rapides de l a force, de l a q u a l i d du champ e t de l ' a x e magn6tique des l e n t i l l e s . L'alignement des aimants s u r l e banc de mesure e s t t616command6 grdce

a

l ' e m p l o i de niveaux Glectroniques e t d'un rayon l a s e r comme l i g n e de r6f6rence. Ce d e r n i e r e s t employ6 pour l e positionnement des mires de r 6 f 6 - rence des quadrup6les selon 1 'axe magn6tique. Cet expos6 d 6 t a i l l e 1 'a p p a r e i l - 1 age 61 ectronique e t mecanique e t l e u r s performances.

A b s t r a c t

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The quadrupole and sextupole magnets o f LEP w i l l be measured by the harmonic c o i l method. I n order t o achieve precise, r e l i a b l e and r a p i d measure- ments o f t h e strength, f i e l d q u a l i t y and magnetic a x i s o f t h e lenses, a f u l l y autonomous, microprocessor c o n t r o l l e d system has been developed. Remote con- t r o l o f t h e alignment o f the magnets on t h e i r measuring bench has been b u i l t - i n , thanks t o t h e use o f e l e c t r o n i c l e v e l gauges and o f a l a s e r beam as r e f e r - ence axis. The l a t t e r i s used f o r p o s i t i o n i n g t h e reference t a r g e t s o f t h e quadrupoles w i t h respect t o t h e i r magnetic axis. Design and performance o f t h e mechanical and e l e c t r o n i c equipment are presented.

I

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INTRODUCTION

The LEP magnet system [11 c o n t a i n s about 1300 quadrupole and sextupole magnets t o provide t h e r e q u i r e d f o c u s i n g o f t h e beams. The main c h a r a c t e r i s t i c s o f these lenses a r e shown i n Table 1.

The supply of t h e magnets w i l l take p l a c e over a p e r i o d o f about 34 months a t a t y p i - c a l r a t e o f 40 u n i t s p e r month. The r e c e p t i o n t e s t s a t CERN i n c l u d e t h e measurements o f t h e s t r e n g t h and f i e l d q u a l i t y o f each magnet t o v e r i f y t h a t t h e tolerances impos- ed by beam o p t i c s I 2 1 a r e respected. The r e l a t i v e p r e c i s i o n r e q u i r e d f o r these meas- urements i s a few p a r t s i n

lo4.

Furthermore, t h e reference t a r g e t s o f t h e quadrupoles must be a l i g n e d t o i 0.05 mm w i t h respect t o t h e measured magnetic axis, since they w i l l serve as reference monuments i n t h e LEP a r c s and hence d e f i n e the c e n t r a l o r b i t o f t h e beams.

Table 1

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Main c h a r a c t e r i s t i c s o f t h e LEP magnetic lenses TY pe

MQ quadrupol e MQA quadrupol e MSD sextupol e MSF sextupol e

Maximum g r a d i e n t

9.7Tm-l 10.9 Tm-l 180 T r 2 180 T w 2

I r o n l e n g t h 1550mm 1950 mm

730 mm 370 mm

I n s c r i b e d c i r c l e

0

125 mm 125 mm 150 mm 150 mm

T o t a l mass 2800 kg 4000 kg 920 kg 500 kg

Quantity 520 2 88 256 248

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19841193

Cl-950 JOURNAL DE PHYSIQUE

I 1

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MEASURING METHOD

The harmonic c o i l method [3, 41 has been chosen because the beam o p t i c s requirements on f i e l d q u a l i t y are expressed as tolerances on t h e systematic and random harmonic components o f the f i e l d [21, and t h e misalignment between t h e magnetic a x i s and the a x i s o f t h e r o t a t i n g measuring c o i l s i s d i r e c t l y measured [3, 41.

Long c o i l s measure d i r e c t l y t h e i n t e g r a l s o f t h e f i e l d values, whereas a p a i r o f s h o r t c o i l s i s used t o assess end e f f e c t s and l o c a t e t h e magnetic axis. For t h e qua- drupoles, t h e o u t e r r a d i u s o f t h e r o t a t i n g c o i l s i s equal t o t h e 1 i m i t o f t h e u s e f u l a p e r t u r e (59 mm). To measure the c o n t r i b u t i o n o f the 20-pole component t o w i t h i n 10-4, t h e r o t a t i o n o f t h e c o i l s should be c i r c u l a r t o w i t h i n 1.3 pm and t h e angular encoder d e f i n i n g t h e sampling i n t e r v a l s (see below) p r e c i s e t o 10'5 rad. By u s i n g a compensating c o i l ( r a d i i Cext, C i n t , Nc t u r n s ) connected i n o p p o s i t i o n t o t h e main measuring c o i l ( r a d i i

bxt,

M i n t 3 NM t u r n s ) i n order t o s a t i s f y

NM (M:,~

-

^M! t ) = Nc (c: t

-

CVnt) both f o r n = 1 ( t o cancel t h e e r r o r s due t o poor r o t a t i o n 7 and n = 2 ?suppression o f t h e main harmonic), t h e requirements on p u r i t y o f r o t a t i o n , encoder p r e c i s i o n and i n t e g r a t o r l i n e a r i t y are appreciably reduced. I n the Set o f p o s s i b l e s o l u t i o n s , Nc/Nm = 2 i s a good compromise between s e n s i b i l i t y t o t h e sextupole term and c o i l manufacture, and M i n t = 0 guarantees a p r e c i s e measurement o f t h e magnet strength.

I 1 1

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SYSTEM LAYOUT

The system has been designed w i t h t h e aim o f performing r a p i d l y and p r e c i s e l y t h e r e c e p t i o n t e s t s o f a magnet, i.e. automatic pre-alignment on the bench, l o c a t i o n o f t h e magnetic axis, s e r i e s measurements and alignment o f t h e reference t a r g e t s . To achieve f l e x i b i l i t y i n t h e schedule and l o c a t i o n o f t h e measurements, each system i s autonomous. R e l i a b i l i t y and easy maintenance a r e provided f o r by a modular f a y o u t a t a l l l e v e l s (Fig. 1). A CBM 8032 general purpose microcomputer c o n t r o l s t h e t h r e e sub-systems handling, r e s p e c t i v e l y , t h e a c t u a l magnetic measurements, t h e remote- c o n t r o l l e d alignment and the s e t t i n g o f t h e power supplies o f t h e magnets.

I V

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MAGNETIC FIELD MEASUREMENTS

The harmonic c o i l s , supported by an a p p r o p r i a t e r o t a t i n g s t r u c t u r e described below, achieve one f u l l rev01 u t i o n i n each d i r e c t i o n . During r o t a t i o n , t h e v o l t a g e induced a t t h e c o i l s ' t e r m i n a l s i s i n t e g r a t e d and sampled a t 256 e q u a l l y spaced i n t e r v a l s .

I

^Disk

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P r ~ n t e r

1

I , ,

, ^,

-

s t a t u s Measurement manager

Al~qnment controller Power supply controller Data check and FFT

-1 I

Measurement controller

I

I

---

L _ - _ J

Motor Co~ls Absolute angular encoder

Fig. 1

-

,*, ^p,

^drrvers

Laser Quadrant Electronic photodiodes Level gauges

Stepplng motors

.ayout o f t h e data treatment and c o n t r o l

16 bits DAC

IT'

Magnet's power supply

e3

System

tors. The s i n g l e values a r e then added up and t h e r e s u l t i n g f l u x curve submitted t o harmonic analysis. A f u r t h e r v e r i f i c a t i o n o f t h e measurement q u a l i t y i s c a r r i e d o u t by comparing the harmonic c o e f f i c i e n t s measured w i t h t h e s i n g l e c o i l t o those given by t h e compensating c o i l arrangement ( t h e corresponding i n t e g r a t e d voltages a r e sampled simultaneously). A f i l e c o n t a i n i n g the harmonic c o e f f i c i e n t s i s then created, t a k i n g t h e main f i e l d harmonic from t h e s i n g l e c o i l measurements and t h e h i g h e r ones from t h e compensating c o i l s , and made a v a i l a b l e t o t h e bench c o n t r o l l e r . The l a t t e r performs t h e c a l c u l a t i o n s t o assess t h e s t r e n g t h and f i e l d q u a l i t y o f t h e measured magnet. A complete measurement, i n c l u d i n g t h e v e r i f i c a t i o n s and F o u r i e r a n a l y s i s des- c r i b e d above, takes about 20 seconds.

I n t e g r a t o r and angular encoder

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The i n t e g r a t o r i s based on t h e use o f a h i g h - q u a l i t y

.--- --- ---

i n p u t ampTi7ier connected t o a vol tage-to-frequency c o n v e r t e r (max. frequency 1 MHz) feeding a 3 2 - b i t b i n a r y counter. The h i g h e s t r e s o l u t i o n i s 0.1 pVs p e r b i t a t an i n - p u t d r i f t l o w e r than 5 pV.

To f i n d t h e magnetic a x i s o f t h e quadrupoles t o w i t h i n 0.02 mm, t h e angles a t which t h e i n t e g r a t e d voltages are sampled must be p r e c i s e t o w i t h i n 2 For t h i s pur- pose, a 1 5 - b i t absolute encoder i s used; t o achieve t h e nominal p r e c i s i o n , any r a d i a l l o a d on t h e encoder s h a f t must be avoided.

Measuring c y l i n d e r s

-

Hollow c y l i n d e r s , which should i d e a l l y show no sag t o guarantee

EijnSt5fit-E6iT-gCoKtry,

support the two l o n g c o i l s and t h e two s h o r t end c o i l s used f o r t h e measurements. A g l a s s - f i b r e epoxy tube (1.37 m long,

0

= 140 mm) i s used t o measure t h e sextupoles, and ~ e v l a r a epoxy tubes (2.48 m and 3.0 m long, respec- t i v e l y ,

0

= 115 mm) have been chosen f o r the MQ and MQA quadrupoles ( t h e r a t i o ten- s i l e s t r e n g t h / d e n s i t y o f impregnated K e v l a r i s 2.6 times t h a t o f impregnated E-gl ass). Special t o o l s had to, be developed t o overcome the poor machinabil i t y o f Kevlar. The g l a s s - f i b r e and K e v l a r tubes a r e manufactured u s i n g t h e f i l a m e n t winding technique.

The measuring c y l i n d e r s a r e loaded w i t h counterweights a t b o t h ends t o minimize t h e i r sag which can so be k e p t below 0.02 mm f o r even t h e l o n g e s t ones.

The c o i l s a r e made w i t h m u l t i s t r a n d wires p u t under tension a t one end, supported on t h e c y l i n d e r s a t p o i n t s spaced a t maximum by 0.7 m t o l i m i t t h e w i r e sag t o 0.02 mm and a v o i d v i b r a t i o n problems. Copper w i r e has been used up t o now ( s t r a n d diameter 0.07 mm, t e n s i o n f o r c e 2.8 N/strand)

,

b u t tungsten w i r e (go1 d p l ated, s t r a n d diameter 0.05 mm, t e n s i o n f o r c e 4.5 N/strand), i s being t e s t e d because i t shows a sag f o u r times smaller than t h a t of copper wire.

The measuring c y l i n d e r s are supported near t o t h e magnet ends by non-magnetic, f r i c - t i o n l e s s a e r o s t a t i c j o u r n a l bearings C51, which ensure a r o t a t i o n c o n c e n t r i c i t y o f b e t t e r than 0.01 mm.

V

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REMOTE-CONTROLLED MAGNET ALIGNMENT

Sensors

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I n t h e past, t h e v i r t u a l reference a x i s has been d e f i n e d by a telescope and TZyT6?-Hobson t a r g e t s , b u t t h e a1 ignment procedures were l e n g t h y and prone t o human e r r o r . By u s i n g a l a s e r beam a c t i n g on quadrant photo-diodes, a magnet can be automa- t i c a l l y aligned. A p r e c i s i o n o f t h e v i r t u a l a x i s o f f 0.01 mm a t 4 m d i s t a n c e from t h e source can be obtained by s h i e l d i n g t h e outgoing beam from a i r movements over a l e n g t h o f 400 mm, and by a c a r e f u l design o f t h e support o f t h e l a s e r tube (HeNe, 1.5 mu).

Level measurements are c a r r i e d o u t w i t h e l e c t r o n i c l e v e l s achieving a p r e c i s i o n of 10-5 w i t h i n a s e t t i n g time o f a few seconds.

P o s i t i o n i n g mechanisms

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The magnets a r e displaced on commercially a v a i l a b l e a i r

fL7iTf-6Ziri~~Z-Tii7~7ilm

thickness 0.03 mm) t o ensure a h i g h r e s o l u t i o n (0.01 mm) p o s i t i o n i n g . Consequently, t h e alignment o f t h e magnets i n t h e h o r i z o n t a l plane i s

Cl-952 JOURNAL DE PHYSIQUE

p r a c t i c a l l y f r i c t i o n l e s s (provided t h e compressed a i r i s o i l -f r e e ) , so t h a t t h e t h r e e d r i v i n g stepping motors and g u i d i n g mechanisms need o n l y t o withstand t h e forces a r i s i n g from the i n e r t i a o f a magnet and the r e s i d u a l slope o f t h e s l i d i n g surfaces.

Three o t h e r more powerful stepping motors are coupled t o t h e j a c k s f o r v e r t i c a l ad- justments.

Pre-alignment

-

Magnets must be p r e - a l i g n e d t o w i t h i n 0.5 mm w i t h r e s p e c t t o the f6G6FGflE:T-ixis so as t o be a b l e t o i n s t a l l t h e measuring c y l i n d e r s . A j i g c a r r y i n g two e l e c t r o n i c l e v e l s and a photo-diode i s p o s i t i o n e d on t h e t h e magnet's reference surfaces. A1 1 displacements a r e made a u t o m a t i c a l l y

,

except t h e angular mi sal ignment around t h e v e r t i c a l axis, which i s c o r r e c t e d by observing t h e r e f l e c t i o n angle on t h e photo-diode o f t h e incoming beam. The whole procedure takes l e s s than t e n minutes.

L o c a t i o n o f t h e magnetic a x i s

-

F o l l o w i n g t h e f i r s t magnetic measurement, t h e bench

co~fF6iTer-?oi~ifGs-fh~-6?Ps~t

between the t h e o r e t i c a l and t h e measured magnetic axes from t h e harmonic c o e f f i c i e n t s obtained from t h e s h o r t end c o i l s . This i n f o r m a t i o n i s used t o a d j u s t t h e magnet p o s i t i o n i n order t o cancel ( w i t h i n 0.02 mm) t h e measured o f f s e t . It may take up t o f o u r i t e r a t i o n s , each l a s t i n g about f i v e minutes, t o achieve t h e r e q u i r e d p r e c i s i o n i n magnet p o s i t i o n .

Once the magnetic a x i s c o i n c i d e s w i t h the a x i s o f the r o t a t i n g c o i l s , which i n t u r n i s i n t h e same v e r t i c a l plane as t h e l a s e r beam, t h e reference t a r g e t s o f t h e magnet a r e then manually a l i g n e d w i t h r e s p e c t t o t h a t beam.

V I

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OVERALL SYSTEM PERFORMANCE AND CONCLUSIONS

A p r o t o t y p e measuring bench has been used t o measure t h e prototype LEP l a t t i c e qua- drupole. Systematic e r r o r s due t o mechanical asymmetries i n the system were estimated by t u r n i n g t h e measuring c y l i n d e r end t o end and by s t a r t i n g t h e measurement a t d i f - f e r e n t angles. Expressed i n terms o f r e l a t i v e e r r o r on the g r a d i e n t a t t h e u s e f u l aperture, a p r e c i s i o n of -1 on t h e c o n t r i b u t i o n o f each harmonic c o e f f i c i e n t up t o n = 15 has been achieved. 8-pole and 16-pole terms c r e a t e d by an asymmetry i n the assembly o f t h e p r o t o t y p e quadrupole have been measured and found t o be i n e x c e l l e n t agreement w i t h t h e values p r e d i c t e d by t h e p e r t u r b a t i o n method 16, 71. The magnetic a x i s o f t h e quadrupoles can be found t o w i t h i n 4 0.01 mm.

Thanks t o t h e complete o n - l i n e data r e d u c t i o n and t o t h e automatization o f t h e a l i g n - ment, t h e time needed t o make a l l t h e r e c e p t i o n t e s t s o f a magnet i s reduced t o l e s s than h a l f a normal working day. Three benches, one f o r the two types o f sextupole and one f o r each type o f quadrupole, are now being assembled t o measure the 1300 magnetic lenses needed f o r LEP.

ACKNOWLEDGEMENTS

The c o n t r i b u t i o n s o f C. Bugnone f o r the mechanical assemblies, J. Souverain f o r the measuring c y l i n d e r s and G. Turcato f o r the e l e c t r o n i c s a r e g r a t e f u l l y acknowledged.

REFERENCES

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

121 GUIGNARD G., CERN LEP-TH/83-38 presented a t 1 2 t h I n t . Conf. on High-Energy Accelerators, Fermil ab, Batavia, I L (August 1983).

131 WYSS C., Proc. 5 t h I n t . Conf. on Magnet Technology, Rome (1975) 231.

141 WALCKIERS L., IEEE Trans. on Magnetics, MAG-17, No. 5 (1981) 1872.

151 POWELL J.W., Design o f a e r o s t a t i c b e a r i n m e Machinery Publishing, Brighton, U.K. (1970).

161 HALBACH K., Nucl. I n s t r . and Meth., 74 (1969) 147.

[71 WADLINGER A., Los Alamos LP-1449 (August 1982).