NMR MAGNET DESIGN FOR IMAGING

HAL Id: jpa-00223612

https://hal.archives-ouvertes.fr/jpa-00223612

Submitted on 1 Jan 1984

HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

NMR MAGNET DESIGN FOR IMAGING

J. Williams, W. Punchard, E. Bobrov, R. Pillsbury, Jr., R. Weggel, L.

Neuringer, R. Haberkorn, H. Segal

To cite this version:

J. Williams, W. Punchard, E. Bobrov, R. Pillsbury, Jr., et al.. NMR MAGNET DESIGN FOR IMAG- ING. Journal de Physique Colloques, 1984, 45 (C1), pp.C1-691-C1-694. �10.1051/jphyscol:19841140�.

�jpa-00223612�

J.E.C. W i l l i a m s , W.F.B. Punchard, E.S. Bobrov, R.D. P i l l s b u r y , J r . , R . J . Weggel, L. J . N e u r i n g e r , R. Haberkorn and H.R. S e g a l *

Massachusetts Institute of TechnoZogy, Cambridge, Massachusetts, U.S.A.

*IBM Instrwments, Danburzj, Connecticut, 7J.S.A .

RGsumd - Les p r i n c i p e s a p p l i q u e s d a n s l a c o n c e p t i o n d ' u n aimant supracon- d u c t e u r d ' u n d i a m s t r e i n t d r i e u r d e 60 cm e t d ' u n champ d e 1 , 5 T e s l a s o n t exposCs d a n s c e r a p p o r t , e n p r b t a n t p a r t i c u l i i 2 r e m e n t a t t e n t i o n aux e f f e t s d e s i m p e r f e c t i o n s d e bobinage s u r l'homog6nbit8 e t a u c h o i x d e s s p d c i f i c a - t i o n s du f i l c o n d u c t e u r e n c e q u i c o n c e r n e l a s t a b i l i t E , l e s c o n t r a i n t e s e t l a p r o t e c t i o n c o n t r e l e "quench".

A b s t r a c t - The p r i n c i p l e s u s e d i n t h e d e s i g n of a 60 cm b o r e , 1 . 5 T s u p e r - c o n d u c t i n g magnet a r e d i s c u s s e d w i t h p a r t i c u l a r r e f e r e n c e t o t h e e f f e c t s of winding i m p e r f e c t i o n s on homogeneity and t o t h e c h o i c e of w i r e s p e c i f i - c a t i o n s i n t e r m s of s t a b i l i t y , s t r e s s and quench p r o t e c t i o n .

INTRODUCTION

The F r a n c i s B i t t e r N a t i o n a l Magnet L a b o r a t o r y i s c o n d u c t i n g a program of r e s e a r c h i n t o NMR imaging. A p a r t of t h a t program i s t h e c o n s t r u c t i o n of a s e r i e s of s u p e r - c o n d u c t i n g imaging magnets of which t h e s y s t e m d e s c r i b e d h e r e i s t h e f i r s t .

The b a s i c o p e r a t i n g s p e c i f i c a t i o n s o f t h e p r e s e n t magnet s y s t e m a r e : F i e l d

Bore Homogeneity

C e n t r a l 1 . 5 T

Peak 2.5 T

60 cm a t room t e m p e r a t u r e 5 ppm o v e r 25 cm d . s . v .

Both m e c h a n i c a l and e l e c t r i c a l shimming w i l l b e used t o r e f i n e t h e f i e l d . Mechanical shimming p r o v i d e s s i x d e g r e e s of freedom w i t h which two a x i a l o r d e r s and f o u r r a d i a l o r d e r s of e r r o r c a n b e compensated.

E l e c t r i c shimming w i l l i n c l u d e t h e f i r s t f o u r a x i a l g r a d i e n t s and t h r e e r a d i a l g r a d i e n t s , x , xz and x2-y2 ( t o g e t h e r w i t h t h e i r complements).

WINDING OUTLINE

The chosen o u t l i n e of t h e t h r e e s e c t i o n s i s t h e r e s u l t of a number of d e s i g n s i n each of which a compromise was s o u g h t between w i r e l e n g t h , c o i l s e t l e n g t h , winding d i f f i c u l t y and s e n s i t i v i t y of homogeneity t o winding e r r o r s . S p e c i f i c a l l y t h e o r d e r of p r i o r i t y was, homogeneity, s h o r t c o i l s e t l e n g t h , minimum w i r e l e n g t h , winding d i f f i c u l t y . F i g u r e 1 shows t h e major p a r a m e t e r s o f t h e c o i l s e t .

The chosen i d e a l d e s i g n g e n e r a t e s no g r a d i e n t s below e i g h t h o r d e r . However, m i s - placement of t u r n s o r of whole c o i l s w i l l g e n e r a t e e r r o r s . T a b l e 1 l i s t s t h e a x i a l g r a d i e n t e r r o r f i e l d s i n ppm a t 1 2 . 5 cm r a d i u s r e s u l t i n g from a 1 mm d i s p l a c e m e n t of t h e o u t e r c o i l s w i t h r e s p e c t t o t h e c e n t e r c o i l . For t h e odd o r d e r g r a d i e n t s t h e movements a r e a n t i s y m m e t r i c and f o r t h e e v e n o r d e r g r a d i e n t s t h e y a r e symmetric.

It w i l l b e n o t i c e d t h a t t h e e f f e c t of c o i l misplacement g e n e r a l l y d e c r e a s e s w i t h ' ~ e s e a r c h s u p p o r t e d by IBM I n s t r u m e n t s , Danbury, CT.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19841140

Cl-692 JOURNAL DE PHYSIQUE

o r d e r . However, b e c a u s e t h e r e i s no 5 t h o r d e r a x i a l shim c o i l , t h e e f f e c t of d i s - placement i s c r i t i c a l i n t h a t c a s e .

The e f f e c t o f t h e misplacement o f i n d i v i d u a l t u r n s i s s m a l l . However, a n e r r o r i n t u r n placement which p r o p a g a t e s t h r o u g h t h e r a d i a l winding b u i l d w i l l g e n e r a t e s i g n i f i c a n t e r r o r s . A p a r t i c u l a r winding e r r o r t h a t h a s been c o n s i d e r e d i s t h e fol- lowing.

Using round w i r e , a n " a u t o c y c l i c " winding method a l l o w s v e r y a c c u r a t e c o n t r o l of winding p i t c h / I / . The method p l a c e s t u r n s i n a p l a n e w i t h a s h a r p t r a n s i t i o n r e g i o n from t u r n t o t u r n . I n s u c c e s s i v e l a y e r s t h i s t r a n s i t i o n r e g i o n may p r e c e s s around t h e azimuth and a v e r a g e o u t . However, it c a n happen t h a t t h e t r a n s i t i o n r e m a i n s s t a t i o n a r y , r e s u l t i n g i n a h i g h s p o t i n t h e winding. The g r a d i e n t s due t o s u c h a winding i m p e r f e c t i o n h a v e been c a l c u l a t e d f o r t h e c o i l s e t shown i n F i g u r e 1. The r e s u l t s o f t h e c a l c u l a t i o n a r e l i s t e d i n T a b l e I1 up t o t h i r d o r d e r .

F i g . 1 - Winding o u t l i n e o f main c o i l s and s u p e r c o n d u c t - i n g a x i a l shim c o i l s . (The s u p e r c o n d u c t i n g r a d i a l s h i m c o i l s a r e wound on a c y l i n - d e r s u r r o u n d i n g t h e a x i a l shims. )

No g r a d i e n t above 1 ppm a t 1 2 . 5 cm i s g e n e r a t e d above t h i r d o r d e r . However, i t i s s e e n t h a t f o u r low o r d e r g r a d i e n t s exceed 1 ppm. A l l g r a d i e n t s c a n b e c o r r e c t e d t o l e s s t h a n t h e o p e r a t i n g s p e c i f i c a t i o n s by t h e e l e c t r i c qhims b u t m e c h a n i c a l , o r o t h e r shimming would b e needed t o c o r r e c t t h e t h i r d o r d e r r a d i a l g r a d i e n t t o l e s s t h a n 1 ppm. I n o r d e r t o a c h i e v e s u c h homogeneity, i t i s t h e r e f o r e n e c e s s a r y t o minimize t h e a c c u m l a t i o n of t u r n misplacement t h r o u g h o u t t h e b u i l d . F u r t h e r m o r e , t h e low e r r o r s l i s t e d i n T a b l e I1 a r e o b t a i n e d o n l y by a p p r o p r i a t e a z i m u t h a l o r i e n t a t i o n of t h e t h r e e c o i l s of t h e s y s t e m .

TABLE I TABLE I1

HARMONIC ERRORS CAUSED BY END GRADIENTS CAUSED BY ACCUMULATED RADIAL

COIL MISPLACEMENT DISPLACEMENT OF THE TRANSITION TURN OF

GRADIENT ppm/mm a t TYPE OF DISPLACE- THE AUTOCYCLIC W I N D I N G

12.5 cm MENT

z - 511.5 Odd symmetry*

z 2 - ^113.6 Even symmetry*

23 - 4.476 0

z4 6,129 E

z5 2.545 0

GRADIENT ERROR n m

ppm a t 12.5 cm

x 2 4 . 3 1 1

22 - 4 . 8 2 0

x2 5 . 8 2 2

x3 1.1 3 3

*By odd symmetry i s meant a movement of e a c h e n d c o i l of 1 nun i n t h e p o s i t i v e z d i r e c t i o n . By even symmetry i s meant a

movement o f each e n d c o i l of 1 mm away n i s t h e o r d e r and m t h e d e g r e e of t h e from t h e s y s t e m m i d p l a n e . A s s o c i a t e d Legendre p o l y n o m i a l

pz

The c h o i c e of w i r e c h a r a c t e r i s t i c s depends on c o n s i d e r a t i o n s of s t a b i l i t y , s t r e s s and quench p r o t e c t i o n . These i n t u r n depend on o p e r a t i n g c u r r e n t which i s d i c t a t e d by c r y o g e n i c c o n s i d e r a t i o n .

Up t o a l e v e l at which t h e c u r r e n t s i n i n d i v i d u a l c o n d u c t o r s i m p a i r t h e homogeneity, t h e c h o i c e of o p e r a t i n g c u r r e n t i s d i c t a t e d by t h e c r y o g e n i c s . Low c u r r e n t o p e r a t i o n e n s u r e s s m a l l l e a d s and narrow l e a d t u b e s . I n t u r n , t h e narrow l e a d t u b e s g i v e r i s e t o low h e a t l e a k a g e i n t o t h e 4 . 2 K r e g i o n . I n t h e p r e s e n t d e s i g n an o p e r a t i n g p o i n t of 2 3 4 A h a s beefi chosen.The c o r r e s p o n d i n g c e n t e r f i e l d i s 1 . 5 T and t h e peak

f i e l d i s 2.5 T. To p r o v i d e margin and p o s s i b l e upgrade a d e s i g n c e n t e r f i e l d of 2 . 0 T h a s been used.

The c h o i c e of w i r e s p e c i f i c a t i o n was based on t h r e e c r i t e r i a : s t a b i l i t y , s t r e s s and quench. These a r e reviewed i n o r d e r .

I.. S t a b i l i t y

Because t h e winding is r e s i n - i m p r e g n a t e d , f o r o r t h o d o x r e a s o n s o f mechanical s t a b i - l i t y , an a d i a b a t i c c r i t e r i o n of s t a b i l i t y i n t h e s u p e r c o n d u c t o r must b e used.

A s i m p l e c r i t e r i o n of Iwasa / 2 / f o r s t a b i l i t y h a s been used. It assumes t h a t a po- t e n t i a l debonding e n e r g y o f es e x i s t s p e r u n i t s u r f a c e a r e a of t h e w i r e . T h i s t r a n s - forms t o a t h e r m a l energy e v p e r u n i t volume of w i r e which c a n r a i s e t h e w i r e tem- p e r a t u r e . For s t a b i l i t y t h i s energy d e n s i t y must n o t exceed e h , t h e e n t h a l p y r e - q u i r e d t o a t t a i n t h e c u r r e n t s h a r i n g t e m p e r a t u r e . The c u r v e s of ev a s a f u n c t i o n of w i r e d i a m e t e r and f o r v a r i o u s v a l u e s of es a r e shown i n F i g u r e 2. The v a l u e o f ev

d e c r e a s e s a s w i r e d i a m e t e r i n c r e a s e s . Also i n F i g u r e 2 a r e shown c u r v e s of e h , t h e e n t h a l p y p e r u n i t volume r e q u i r e d t o r e a c h t h e c u r r e n t s h a r i n g t e m p e r a t u r e , shown p l o t t e d a s f u n c t i o n s of w i r e d i a m e t e r and copper-superconductor r a t i o .

F i g . 2 - Curves of e v , t h e p o t e n t i a l e n e r g y d i s s i p a - t i o n p e r u n i t volume, e n t h e e n t h a l p y d e n s i t y t o c u r r e n t s h a r i n g tempera- t u r e and c u r v e s of con- s t a n t c o p p e r s t r e s s as f u n c t i o n s of w i r e d i a - m e t e r w i t h c o p p e r t o

s u p e r c o n d u c t o r r a t i o and s u r f a c e debonding e n e r g y a s i n d e p e n d e n t p a r a - m e t e r s .

Wire Diameter (cm.)

Measurement of e s f o r w i r e i n s u l a t e d w i t h Formvar and bonded t o a p p r o p r i a t e epoxy y i e l d s s u r f a c e debonding e n e r g i e s o f a p p r o x i m a t e l y 0.05 m J ~ m - ~ . T h e r e f o r e i f t h e r e were no o t h e r c r i t e r i a of s i g n i f i c a n c e a n a p p r o p r i a t e d e s i g n p o i n t f o r es=O.l m J ~ m - ~

( a l l o w i n g a margin) and copper-superconductor r a t i o = 4 , would be a d i a m e t e r of 0.148 cm. However, o t h e r c o n s i d e r a t i o n s a r i s e a s f o l l o w s .

2. S t r e s s

The e f f e c t s of hoop t e n s i o n ( t o g e t h e r w i t h some a x i a l compression) a r e twofold. (1)

Cl-694 JOURNAL DE PHYSIQUE

The r e s i n m a t r i x o f t h e winding i s s t r a i n e d and p o t e n t i a l e n e r g y i s a v a i l a b l e f o r r e l e a s e i n debonding; (2) The winding expands r a d i a l l y and c o n t r a c t s a x i a l l y (and s l i g h t l y i n b u i l d ) . I n o r d e r t o l i m i t t h e s e e f f e c t s t o a c c e p t a b l e l e v e l s i t was d e c i d e d t o s p e c i f y a t e n s i l e s t r e s s i n t h e copper of 1 0 k s i (69 MPa). L i n e s of c o n s t a n t copper s t r e s s a r e i n c l u d e d i n F i g u r e 2 , c o r r e s p o n d i n g t o d e s i g n c o n d i t i o n s of 300 A, 3 . 3 T peak f i e l d and a mean winding r a d i u s of 0.39 m. The r e q u i r e m e n t of a c o p p e r s t r e s s n o t e x c e e d i n g 1 0 k s i i n c r e a s e s t h e w i r e d i a m e t e r t o 0.179 cm f o r a copper-to-superconductor r a t i o o f 4.

At t h e same t i m e t h e p e r m i s s i b l e s u r f a c e energy of d i s b o n d i n g i s r a i s e d t o 0.22 m~cm-'. The c h o i c e o f copper s u p e r c o n d u c t o r r a t i o a l s o depends on quench c h a r a c t e r i s t i c s .

3 . Quench P r o t e c t i o n

The p r e d i c t i o n of quench p r o p a g a t i o n i s b a s e d on work of K e l l y and W i l l i a m s ( t o be p u b l i s h e d . ) It assumes t h a t quenching s t a r t s a t one end of t h e c e n t e r of t h e t h r e e s e c t i o n s . Because t h a t s e c t i o n h a s o n l y 10 l a y e r s , b u t 380 t u r n s p e r l a y e r , t h e t i m e f o r quenching t o a f f e c t a l l l a y e r s i s i g n o r e d , a n d an e s s e n t i a l l y one d i m e n s i o ~ a 1 and u n d i r e c t i o n a l quench p r o p a g a t i o n i s modelled. The c a l c u l a t i o n i s performed by a s i m p l e code w r i t t e n f o r a P e r s o n a l Computer and i n c l u d e s s o l u t i o n of t h e t h r e e c o u p l e d c i r c u i t s o f which t h e s y s t e m c o n s i s t s .

The t i m e o f p r o p a g a t i o n of n o r m a l i t y between a d j a c e n t t u r n s was e x t r a p o l a t e d from measurements on s m a l l w i r e s u s i n g a n e x p r e s s i o n shown by experiment t o b e a f a i r a p p r o x i m a t i o n under a d i a b a t i c c o n d i t i o n s .

Because t h e o p e r a t i n g c u r r e n t of t h e magnet i s much l e s s t h a n t h e c r i t i c a l c u r r e n t and b e c a u s e even t h e d e s i g n peak f i e l d i s o n l y 3.3 T, t h e p r o p a g a t i o n t i m e d o e s n o t v a r y g r e a t l y w i t h f i e l d . The v a r i a t i o n o f p r o p a g a t i o n t i m e w i t h c u r r e n t i n t h i s p a r t i c u l a r c a s e i s a p p r o x i m a t e l y e x p r e s s e d a s ,

t o a n a c c u r a c y of 5% between 80 A and 300 A , a s compared w i t h e x t r a p o l a t i o n from ex- p e r i m e n t .

The quench c u r v e p r e d i c t e d by t h e code i s shown i n F i g u r e 3. The peak t e m p e r a t u r e i s 90 K,and t h e decay t i m e t o l / e of i n i t i a l c u r r e n t i s 8 s . T h i s s u p p o r t s t h e c h o i c e o f a 4-to-1 c o p p e r - s u p e r c o n d u c t o r r a t i o .

Time ( s e c ) REFERENCES

F i g . 3 - C u r r e n t and t e m p e r a t u r e a s a f u n c t i o n o f t i m e r e - s u l t i n g from a quench s t a r t i n g a t one end of t h e c e n t e r c o i l . The end c o i l s do n o t quench.

1. W i l l i a m s , J . E . C . , N e u r i n g e r , L . J . , Bobrov, E . S . , Weggel, R., Ruben, D.J. and H a r r i s o n , d . G . , Rev. S c i . , I n s t . 11 ( 1 9 8 1 ) , 649.

2. I w a s a , Y., F.B.N.M.L. Memorandum.