A WIGGLER MAGNET FOR THE UK FREE ELECTRON LASER PROJECT

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A WIGGLER MAGNET FOR THE UK FREE ELECTRON LASER PROJECT

M. Poole, R. Bennett, R. Walker

To cite this version:

M. Poole, R. Bennett, R. Walker. A WIGGLER MAGNET FOR THE UK FREE ELEC- TRON LASER PROJECT. Journal de Physique Colloques, 1984, 45 (C1), pp.C1-325-C1-328.

�10.1051/jphyscol:1984166�. �jpa-00223722�

JOURNAL DE PHYSIQUE

Colloque C1, suppl6ment

a u n o

1,

Tome 45,

janvier

1984

page

Cl-325

A WIGGLER MAGNET FOR T H E UK F R E E ELECTRON L A S E R PROJECT

M.W.

Poole,

R . J .

Bennett and

R.P.

Walker

Science

&

Engineering Research CounciZ, Daresbury Laboratory, Daresbury, Warrington WA4 4AD, U.K.

Resume

-

La conception d'un onduleur ii aimants permanents e t de longueur 5 m e s t d e c r i t e . S a c a r a c t 6 r i s t i q u e n o u v e l l e e s t d ' a v o i r q u a t r e s e c t i o n s i d e n t i q u e s avec des e n t r e f e r s c o n t r b l e s sGpar6ment. On p r e s e n t e l e s r e s u l t a t s d e s mesures avec d e s p i e c e s c o b a l t - t e r r e s r a r e s a i n s i que l a methode d'assemblage dans l ' a i m a n t p e r m e t t a n t une r e d u c t i o n d e s e r r e u r s s u r l a t r a j e c t o i r e du f a i s c e a u d ' e l e c t r o n s .

Abstract. The d e s i g n of a

5

m long permanent magnet wiggler

i s

d e s c r i b e d , a novel f e a t u r e of which

i s i t s

c o n s t r u c t i o n i n f o u r i d e n t i c a l s e c t i o n s with independently c o n t r o l l a b l e gaps. The r e s u l t s of magnetic t e s t s on t h e r a r e e a r t h c o b a l t blocks a r e p r e s e n t e d and a method of a r r a n g i n g them i n t h e magnet t o reduce e r r o r s i n t h e e l e c t r o n beam t r a j e c t o r y

i s

p u t forward.

1. I n t r o d u c t i o n . C o n s t r u c t i o n of apparatus f o r t h e UK f r e e e l e c t r o n l a s e r (FEL) p r o j e c t I l l , i s n e a r i n g completion i n c l u d i n g t h e wiggler magnet w i t h i n which t h e i n t e r a c t i o n between e l e c t r o n beam and r a d i a t i o n f i e l d t a k e s p l a c e . A uniform wiggler ( c o n s t a n t peak f i e l d and p e r i o d ) and a p l a n e geometry has been chosen t o a l l o w a wide range of experiments t o c h a r a c t e r i s e FEL performance, i n c l u d i n g t h e demonstration of a wide tuning range, h o p e f u l l y from about 2-20 pin i n t h i s c a s e .

Design p r i n c i p l e s r e l e v a n t t o such p e r i o d i c systems have been reviewed p r e v i - o u s l y [21. In g e n e r a l r e d u c t i o n of t h e magnet p e r i o d

i s

b e n e f i c i a l f o r FEL g a i n , however t h i s l e a d s t o i n c r e a s e d f i e l d inhomogeneities s o t h a t t h e magnet d e s i g n must be c a r e f u l l y optimised. A s h o r t p e r i o d ( t y p i c a l l y

<

100

mm)

a l s o becomes d i f f i c u l t t o r e a l i s e with an electromagnet because of t h e r a p i d l y i n c r e a s i n g c u r r e n t d e n s i t y requirement, and f o r t h i s reason most FEL experiments 131 now u t i l i s e permanent magnet technology based on r a r e e a r t h c o b a l t (REC) m a t e r i a l .

2. Magnet design. The magnet comprises two p a r a l l e l a r r a y s of p e r i o d i c a l l y mag- n e t i s e d REC blocks t h a t g e n e r a t e a s i n u s o i d a l l y varying t r a n s v e r s e f i e l d on t h e median p l a n e with amplitude given i n a two dimensional approximation by 121:

-= ^-3

s i n E x/M Bo = 2 Br

n/M (1 - e h o ) e ' o

with B = remanent f i e l d M = no. of blocks p e r p e r i o d h = magnet p e r i o d

hr = block h e i g h t g = magnet gap EO = f i l l f a c t o r

A c h o i c e of

4

square c r o s s s e c t i o n blocks per p e r i o d h a s been made t o a l l o w maximum f l e x i b i l i t y i n s e l e c t i o n of t h e i r subsequent l o c a t i o n i n t h e a r r a y s . In o r d e r t o allow f o r dimensional t o l e r a n c e s on t h e blocks a small c l e a r a n c e gap w i l l be l e f t between each block, allowing t h e p e r i o d t o be maintained a c c u r a t e l y over t h e whole l e n g t h . The combined e f f e c t of t h e r e d u c t i o n i n block s i z e and f i l l f a c t o r , E = 4h/ho,

i s

t o reduce t h e peak f i e l d almost l i n e a r l y by 0.74% p e r 0.1 mm of gap.

A magnet p e r i o d of 65

mm

was chosen following e x t e n s i v e computations t o o p t i m i s e t h e FEL g a i n

I l l .

The g a i n was shown t o s c a l e somewhat f a s t e r than l i n e a r l y with l e n g t h however reasons of c o s t , space a v a i l a b i l i t y and t e c h n i c a l complexity determined a maximum f e a s i b l e l e n g t h of

5 m.

With 76 p e r i o d s t h e a c t u a l l e n g t h

i s 4.94 m.

The f i e l d s t r e n g t h

i s

a d j u s t e d by changing t h e gap between t h e a r r a y s . The s e n s i t i v i t y ,

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984166

C

1-326 JOURNAL DE PHYSIQUE

4.8%

p e r

mm,

r e q u i r e s t h a t t h e gap

i s

c o n t r o l l e d with high p r e c i s i o n . For neglig- i b l e d i f f r a c t i o n l o s s e s a t t h e maximum o p e r a t i n g wavelength a minimum gap of 21

mm

has been d e f i n e d , i n c l u d i n g allowances f o r chamber wall t h i c k n e s s and t o l e r a n c e s . A maximum gap of 200

mm

has been s p e c i f i e d a t which p o i n t t h e peak f i e l d

i s

l e s s t h a n

0.1 mT and t h e magnet may be c o n s i d e r e d . t o be "switched o f f " .

To

complete t h e c h o i c e of main parameters a g e n e r a l program, PMU3D [ 4 1 , h a s been used t o compute three-dim- e n s i o n a l f i e l d d i s t r i b u t i o n s . Considerations of FEL g a i n determine a maximum permit- t e d f i e l d e r r o r of 0.2% over t h e e l e c t r o n beam h a l f - a p e r t u r e comprising beam width ( 3

m m ) ,

wiggle amplitude (0.3 m m ) and alignment t o l e r a n c e s (1

mm)

and a s shown by Fig.1 t h i s l e a d s t o a block width of 70 mm. With such a width t h e peak f i e l d , 0.46T,

i s

a l s o c l o s e t o

i t s

two-dimensional

l i m i t ,

shown by t h e d o t t e d l i n e i n Fig.1.

Block wid1h.w Imml \\

'0-4 . io 40 !b i.3 . I&, otstance tiom REC black (mmi

Fig.1 V a r i a t i o n of f i e l d s t r e n g t h and Fig.2 S t r a y f i e l d s o u t s i d e t h e

homogeneity with block width. volume of REC m a t e r i a l .

I n such a magnet t h e s t r a y f i e l d s a r e of some importance s i n c e t h e r e

i s

no mag- n e t i c s c r e e n i n g m a t e r i a l included. From t h e d i s t r i b u t i o n s p r e s e n t e d i n Fig.2

i t

can be seen t h a t t h e f i e l d reduces t o

<

0.1 mT a t 60

mm

from t h e o u t e r f a c e s of t h e REC blocks i n t h e t r a n s v e r s e d i r e c t i o n s and t o about 1 mT a t a s i m i l a r d i s t a n c e from t h e magnet end f a c e a l o n g t h e beam a x i s . This d a t a has been used t o determine t h e accep- t a b l e proximity of mild s t e e l components w i t h i n t h e magnet e n g i n e e r i n g s t r u c t u r e .

An e a r l y d e s i g n d e c i s i o n was t h a t t h e magnet would be c o n s t r u c t e d and t e s t e d i n f o u r s e c t i o n s i n o r d e r t o ease t h e handling d i f f i c u l t i e s . This has been achieved by a novel scheme which allows each s e c t i o n t o be o p e r a t e d f u l l y independently t h u s extending t h e range of experimental p o s s i b i l i t i e s . The arrangement shown i n Fig.3a p r e s e r v e s t h e 65 mm p e r i o d b u t i n t r o d u c e s a l o c a l p e r t u r b a t i o n i n t h e f i e l d , shown i n Fig.3b. A s each s e c t i o n

i s

terminated i n

a

h a l f block s u p e r p o s i t i o n and symmetry p r i n c i p l e s can be used t o show t h a t no n e t displacement o r divergence

i s

i n t r o d u c e d i n t h e e l e c t r o n t r a j e c t o r y . Tracking s t u d i e s showed t h a t t h e r e was no s i g n i f i c a n t

-20.1

10 20 30 40

F i g . 3 ( a ) Arrangement of REC blocks n e a r ^Z(mm)

t h e i n t e r f a c e between 2 s e c t i o n s Fig.3(b) The r e s u l t i n g f i e l d e r r o r .

change t o t h e p a t h l e n g t h and s o

it

was concluded t h a t t h e r e would be n e g l i g i b l e e f f e c t on FEL performance. The f o u r s e c t i o n s a r e each composed of 19 p e r i o d s and have i d e n t i c a l design.

3. Fmgineering design. C o n s i d e r a t i o n s of matching t h e e l e c t r o n beam emerging from t h e wiggler i n t o a spectrometer l e d t o t h e adoption of a v e r t i c a l wiggling plane.

This has t h e advantage t h a t movement of t h e block a r r a y s t a k e s p l a c e h o r i z o n t a l l y , t h u s s i m p l i f y i n g t h e s u p p o r t s t r u c t u r e . The main s u p p o r t system comprises a p a i r of 6 m "I-beams", s e t i n t o t h e f l o o r . The f o u r s e c t i o n s a r e mounted on s h o r t e r sub- assemblies, allowing a c c e s s from beneath t h e magnet i n t h e v i c i n i t y of t h e s e c t i o n i n t e r f a c e s . Two alignment t a r g e t s

w i l l

be a v a i l a b l e on each s e c t i o n t o allow survey- i n g of t h e complete 5

m

magnet and s c r e e n s can be i n s e r t e d i n t o t h e e l e c t r o n and l a s e r beams a t i n t e r v a l s along t h e l e n g t h and viewed from below by t e l e v i s i o n cameras.

The d r i v e system f o r each s e c t i o n

w i l l

comprise a s m a l l v a r i a b l e speed DC motor with a double-ended o u t p u t s h a f t a t t a c h e d t o a p a i r of double a c t i n g l e a d screws by f l e x i b l e couplings. Use of 50:l worm and wheel g e a r i n g and l i n e a r encoders allows t h e magnet gap t o be s e t t o an accuracy of 10 pm. To c o u n t e r a c t t h e l a r g e non l i n e a r a t t r a c t i v e f o r c e between a r r a y s [51, up t o 1 tonne a t minimum gap, s e t s of compact d i s c s p r i n g s have been i n c o r p o r a t e d t o o p e r a t e a t s m a l l gaps ( < 4 0 mm).

4. Permanent magnet blocks. A t o t a l of 600 blocks of s i z e 15.9 x 15.9 x 70.0

mm3

and 16 h a l f blocks were r e q u i r e d . The m a t e r i a l chosen was t h e H i t a c h i Hicorex-22A grade of samarium c o b a l t (Sm Cog) with a s t a n d a r d range of remanent f i e l d of 0.85

-

0.95 T. V a r i a t i o n s of magnetization s t r e n g t h and d i r e c t i o n from block t o block a r e p a r t i c u l a r l y s e r i o u s i n an FEL a s t h e r e s u l t i n g e r r o r i n t h e e l e c t r o n t r a j e c t o r y re- duces t h e o v e r l a p with t h e photon beam and hence t h e gain. The p r o p e r t i e s of e a c h block have t h e r e f o r e been measured a c c u r a t e l y using t h e technique d e s c r i b e d i n r e f .

[51. Figure 4 shows t h e d i s t r i b u t i o n s of magnetisation e r r o r s f o r t h e f u l l s i z e d blocks. The f u l l range of M x and Mz v a l u e s corresponds t o e r r o r s i n t h e magnetisa- t i o n d i r e c t i o n of about +0.4O and f1.3O r e s p e c t i v e l y . The

My

d i s t r i b u t i o n

i s

asym- m e t r i c with a mean of 0.934 T and a f u l l spread of f1.9%, w i t h i n t h e s p e c i f i e d 5 . 5 % . The spread

is

s i m i l a r i n magnitude t o t h a t obtained by o t h e r workers [61 u s i n g a d i f f e r e n t measurement technique, although t h e shape of t h e d i s t r i b u t i o n d i f f e r s .

Fig .4 Measured d i s t r i b u t i o n s of magnetization i n t h e x, y,z d i r e c t i o n s . To a s s e s s t h e e f f e c t of such e r r o r s on t h e e l e c t r o n t r a j e c t o r y a simple t r a c k - i n g program has been used

151,

using f i e l d v a l u e s f o r a complete s e c t i o n c a l c u l a t e d with PMU3D [41. I t was found t h a t a random s e l e c t i o n of blocks gave r i s e t o e r r o r s a t t h e end of a s e c t i o n of t y p i c a l l y 1

mm

i n t h e X p o s i t i o n and 0.2

mm

i n Y, which was considered unacceptable. The r e l a t i v e l y low energy of t h e e l e c t r o n beam i n t h i s c a s e (- 50 MeV) makes

i t

p a r t i c u l a r l y s e n s i t i v e t o magnetisation e r r o r s . A proced- u r e f o r a r r a n g i n g t h e blocks I n a s p e c i f i c o r d e r t o compensate t h e e r r o r s has been devised. Blocks a r e s e l e c t e d s o a s t o minimize t h e cumulative e r r o r i n ( M x 2 + ~ y 2 ) , t a k i n g i n t o account t h e d i f f e r e n t o r i e n t a t i o n s . ' Figure 5a d i s p l a y s a t y p i c a l t r a j e c t o r y o b t a i n e d with a random s e t of blocks and t h e g r e a t improvement obtained from o r d e r i n g t h e same blocks by t h i s procedure can be seen i n Fig.5b.

5. Magnet c o n s t r u c t i o n . Between two i n d i v i d u a l blocks t h e magnetic f o r c e s c a n r e a c h 235 N when i n c o n t a c t 151 and

it is

t h e r e f o r e e s s e n t i a l t o t a k e extreme c a r e when handling what

i s

a l s o a b r i t t l e m a t e r i a l . A l l c o n s t r u c t i o n work must t a k e

Cl-328 JOURNAL DE PHYSIQUE

0 80

-

1 o m -

2 -

E x 040

-

0 40-

0

0-

-

-500 0 500

r ( m m l

Fig.5 E l e c t r o n beam t r a j e c t o r y i n a magnet s e c t i o n with ( a ) a random s e l e c t i o n of b l o c k s , ( b ) t h e same blocks arranged i n a s p e c i f i e d o r d e r .

p l a c e i n a c o n t r o l l e d , c l e a n environment. A f t e r magnetic t e s t s t h e blocks, indiv- i d u a l l y i d e n t i f i e d , a r e glued i n t o t h e i r s t a i n l e s s s t e e l h o l d e r s i n one of t h e f o u r m a g n e t i s a t i o n o r i e n t a t i o n s determined by t h e procedure d e s c r i b e d i n S e c t i o n

4.

I n d i v i d u a l h o l d e r s a r e used f o r e a s e of handling and t o a l l o w replacement of a block i n c a s e of damage. Bonding s u r f a c e s a r e thoroughly cleaned and a s p e c i a l j i g used t o mount, p r e c i s e l y a l i g n , and clamp a block d u r i n g c u r i n g ; t h e a p p a r a t u s

i s

shown i n Fig.6. A low temperature n r a l d i t e epoxy

i s

used and blocks a r e t y p i c a l l y cured i n an oven f o r about one hour a t 50-60°C.

Fig.6 REC block, holder and bonding j i g

Assembly of block a r r a y s has a l s o r e q u i r e d u s e of a s p e c i a l f i x t u r e because of t h e magnetic f o r c e s . P a r a l l e l guide r a i l s , dowelled h o l e s and removeable l o c a t i o n p i n s allow t h e block h o l d e r s t o be a c c u r a t e l y p o s i t i o n e d and t h e n b o l t e d a f t e r being moved i n t o p l a c e by o p e r a t i o n of a screw on a c a r r i a g e system. A f t e r completion of

the

a r r a y assemblies t h e f o u r magnet s e c t i o n s

w i l l

be b u i l t up with t h e i r mechanisms and c o n t r o l s and each s e c t i o n w i l l have

i t s

magnetic f i e l d a c c u r a t e l y mapped with a Hall p l a t e system.

Acknowledgements. D e t a i l e d e n g i n e e r i n g design has been c a r r i e d o u t by R. Punter, c o n s t r u c t i o n by 8. Taylor, M. Thirsk and o t h e r s and magnetic t e s t s by P.S. Williams.

The encouragement of D . J . Thompson, G. Saxon and t h e UK FEL p r o j e c t l e a d e r , S.D. Smith (Heriot-Watt U n i v e r s i t y )

i s

a l s o acknowledged.

References

[ I ] POOLE M.W. e t a l , I E E E h a n s . N u c ~ . S c i . NS-30, NO.4 (1983) 3091.

[ 2 ] POOLE M.W. and WALKER R.P., IEEE -Trans. Magnetics Mag-1 7, N0.5 (1981) 1978.

[ 3 ] For a review of c u r r e n t experiments see: J.de Physique, Colloque C1, Supplement No.2, Tome 44 (Feb.1983).

[41 FAN M.W., POOLE M.W. and WALKER R.P., Daresbury Laboratory I n t e r n a l Report DL/SCI/TMZgA (1 981 )

.

[ 5 ] WALKER R.P., POOLE M.W., and TAYLOR, D.G., t h e s e proceedings.

[61 LUCIO A. e t a l , Brookhaven National Laboratory Report BNL 33161, submitted t o Nucl. Instrum. Meth. Phys. Res.