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SHIELDING THE a.c. MAGNETIC FIELDS IN BITTER-TYPE MAGNETS
N. Alekseevskii, G. Fuchs, M. Gliński, V. Gostishchev
To cite this version:
N. Alekseevskii, G. Fuchs, M. Gliński, V. Gostishchev. SHIELDING THE a.c. MAGNETIC FIELDS IN BITTER-TYPE MAGNETS. Journal de Physique Colloques, 1984, 45 (C1), pp.C1-71-C1-74.
�10.1051/jphyscol:1984115�. �jpa-00223622�
S H I E L D I N G THE a . c MAGNETIC F I E L D S IN B I T T E R - T Y P E MAGNETS
N.E. A l e k s e e v s k i i , G. F u c h s , M. G l i n s k i and V. G o s t i s h c h e v
International Laboratory of High Magnetic Fields and Low Temperatures, Prdchnika 95, 53-529 Wroclaw, Poland
Résumé - Nous présentons les résultats des examens des coefficients d'effi- cacité pour plusieurs types d'écrans pour la composante alternative du champ magnétique engendré par des aimants du type de Bitter. Les meilleurs effets obtenus sont ceux pour l'écran en In avec RRR = 10s et pour un solenoïde à une seule couche de V3Ga court-circuité avec un fil de Cu. Dans les deux cas la composante alternative diminuait de 10 fois dans un champ de 14 T.
A b s t r a c t - We p r e s e n t t h e r e s u l t s of s h i e l d i n g e f f i c i e n c y i n v e s t i g a t i o n s of v a r i o u s s h i e l d s f o r a . c . component of magnetic f i e l d generated by B i t t e r magnets. The best e f f e c t s were obtained f o r s h i e l d s made of In w i t h
RRR = 105 and f o r s i n g l e - t u r n V3Ga s o l e n o i d shorted by Cu w i r e . In both cases the a . c . component was decreased about t e n times i n f i e l d o f 14 T.
High power, w a t e r - c o o l e d B i t t e r magnets are s t i l l a popular source of high i . e . , above 10 T, magnetic f i e l d s . T h e i r a p p l i c a t i o n , as an i n n e r p a r t , i n h y b r i d systems / ! / , i n d i c a t e s t h e i r c o n s t a n t u s e f u l n e s s i n g e n e r a t i o n of high magnetic f i e l d s . A . c . components o f magnetic f i e l d coming from power s o u r c e , e . g . m o t o r - g e n e r a t o r , and a l s o mechanical v i b r a t i o n s coming from c o o l i n g water make d i f f i c u l t and o f t e n i m p o s s i b l e t o c a r r y o u t t h e s u b t l e experiments l i k e quantum o s c i l l a t i o n s or c a l o r i - m e t r i c measurements.
Of t h e whole spectrum of a . c . components t h e most harmful are the l o w - f r e q u e n c y ones. In e l e c t r o m a g n e t s working a t our Laboratory t h e b i g g e s t a m p l i t u d e has a com- ponent w i t h t h e frequency of 3.3 Hz. Of c o u r s e , i t i s p o s s i b l e t o reduce t h e a . c . component i n supply systems using passive or a c t i v e f i l t e r s / 2 , 3 / or by a p p l y i n g ad- d i t i o n a l compensating c o i l w o r k i n g i n a l o c a l i z e d feedback system / 4 / . In many cases however a cheaper, s i m p l e r and more e f f e c t i v e i s s h i e l d i n g of t h e sample i n working area of t h e s o l e n o i d . Such s h i e l d i n g , i . e . , decreasing of t h e a . c . component a m p l i - tude a c t i n g on t h e sample and measuring leads could be done both by screens made of r e s i s t i v e m a t e r i a l s w i t h high c o n d u c t i v i t y and by t h e i r combinations w i t h supercon- d u c t i n g m a t e r i a l s w i t h high c r i t i c a l f i e l d s / 5 , 6 / .
In t h i s work we p r e s e n t t h e r e s u l t s o f e f f i c i e n c y measurements o f both types o f s h i e l d s immersed i n l i q u i d h e l i u m , c a r r i e d o u t i n a 15 T B i t t e r magnet f o r t h e most d i f f i c u l t t o screen i n our magnets a . c . component - 8.3 Hz.
SHIELDS MADE OF HIGH PURITY RESISTIVE MATERIAL
High p u r i t y Al and In were used t o c o n s t r u c t i n v e s t i g a t e d s h i e l d s . Two s i m p l e s t c o n f i g u r a t i o n s , shown i n F i g . 1, were chosen. Their dimensions and p u r i t y c o e f f i - c i e n t s given by RRR a r e presented i n T a b l e l . T h e screen made of I n was placed i n t h i n - - w a l l e d brass tube t o be mounted on t h e measuring i n s e r t . Screening e f f i c i e n c y we d e f i n e by t h e q u a n t i t y s = (A - As) / A , where A and As are the amplitudes o f t h e a . c . component s e l e c t i v e l y measured by t h e probe c o i l placed a t t h e c e n t r e of t h e
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984115
JOURNAL DE PHYSIQUE
Fig. 1
-
Shields made o f h i g h p u r i t y metals: a-
Al, b-
A1 o r I n . Dimensions are given i n the Table.magnet w i t h and w i t h o u t shield, r e s p e c t i v e l y . The values o f c o e f f i c i e n t S f o r t h e 8.3 Hz a.c. component i n the f i e l d o f 14 T a r e given i n theTable1.AS can be seen, the A1 s h i e l d s o f b o t h c o n f i g u r a t i o n s a l l o w t o o b t a i n the screening e f f i c i e n c y o f the order o f 0.6 t 0.7
.
Even b e t t e r e f f e c t , S = 0.85 we obtained using a very h i g h p u r i t y I n s h i e l d o f t h e type shown i n Fig. l b , f o r the gap g = 3 mm.SUPERCONDUCTING SHIELDS,
By s l i t t i n g a h o l l o w superconducting c y l i n d e r along i t s a x i s and then by s h o r t i n g i t w i t h a r e s i s t i v e m a t e r i a l , one can, f o r a p p r o p r i a t e l y chosen time c o n s t a n t T = L/R
,
s h i e l d t h e low frequency a.c. f i e l d w i t h o u t i n f l u e n c i n g s i g n i f i c a n t l y t h e d.c. f i e l d penetration.
In
t h e p r e s e n t work two d i f f e r e n t superconducting s h i e l d s , shown i n Fig. 2 , a r e proposed.Version given i n F i g . 2a was r e a l i z e d u s i n g Nb3Sn tape w i t h thickness 0.02 mm and w i d t h 60 mm. Over a t h i n P e r t i n e x c y l i n d e r 10 l a y e r s o f such tape were wound and shorted over a 10 mm w i d t h by means o f Sn s o l d e r . The s h i e l d presented i n Fig. 2b
,
was made o f V3Ga cable of 0.5 mm diameter, c o n s i s t i n g o f 6 t w i s t e d mu1 t i f i l a m e n t w i r e soldered by In. From t h i s cable s i n g l e - l a y e r e d c o i l o f 43 t u r n s was wound. Both i t s end weresoldered by I n t o c o p p e r connectors b r i d g e d by copper wires. T h e i r number, hence the r e s i s t a n c e was e a s i l y changeable. For t h e i n v e s t i g a t e d c o i l R = 4 X 10-6 Q
\
Sn
solder - copper
wires
cable
Fig. 2
-
Shields made o f superconductors w i t h ohmic shunt:a
-
Nb3Sn tape+
Sn solder, V3Ga cable+
Cu wires.en i n Fig. 3, and i t s values f o r f i e l d o f 14 T
-
i n Table 1.Fig. 3
-
S h i e l d i n g e f f i c i e n c y c o e f f i c i e n t .9 versus magnetic f i e l d f o r : a-
V3Ga c o i l w i t h copper r e s i s t i v e shunt, b-
Nb3Sn w i t h Sn s o l d e r shunt.As seen, using V3Ga c o i l as a s h i e l d we obtained i n the f i e l d s up t o 14 T, s h i e l d i n g e f f i c i e n c y S = 0.9
.
For t h e Nb3Sn c o i l the screening e f f i c i e n c y drops r a p i d l y w i t h i n c r e a s i n g f i e l d , and f o r f i e l d o f 14 T i s about 0 . 3 .TABLE 1
D
-
o u t e r diameter i n mm, L-
l e n g t h i n mm, d-
thickness o f w a l l , g-
gap between h a l f - c y l i n d e r s , S-
s h i e l d i n g e f f i c i e n c yThe comparison o f c r i t i c a l c u r r e n t s obtained from measurements on s h o r t samples i n f i e l d o f 14 T, i . e . j, = 104 A/cm2 f o r Nb3Sn f o i l and j, = 2x104 A/cm2 f o r V3Ga cable, gives no e x p l a n a t i o n o f weak s h i e l d i n g by Nb3Sn c o i l . Probably t h e forces a c t i n g on t h e c o i l i n the magnetic f i e l d decrease s i g n i f i c a n t l y t h e value o f t h e c r i t i c a l c u r - r e n t . I t seems t h a t i n c r e a s i n g t h e mechanical s t r e n g t h o f t h e c o i l , a l l o w i n g f o r b e t t e r f i x i n ? o f the l a y e r s , should cause more e f f e c t i v e screening.
S f o r 14 T 0.6 t 0.7 0 . 6 e 0 . 7
0.85 0.3 0.9 High
conductivity r e s i s t i v e m a t e r i a l Superconducting
+
r e s i s t i v eM a t e r i a l RRRl A1 [2x103]
A1 [104]
I n L1051 Nb3Sn
V3Ga
C o n f i g u r a t i o n see F i g . l a s e e F i g . 1 b see F i g . l b see Fig. 2a see Fig. 2b
Other data d = 3mm g = 3 m m g = 3
mm
10 l a y e r s 43 t u r n s Dimensions
D 22 22 22 21 21
L
80 80 80 60 90
JOURNAL DE PHYSIQUE
DISCUSSION
Several ways o f s h i e l d i n g the a.c. components o f magnetic f i e l d s appearing i n B i t t e r magnets are presented. The s h i e l d i n g e f f e c t was achieved due t o e i t h e r h i g h conducti- v i ty o f very pure m t a l s a t low temperatures, o r superconductivity. S p e c i a l l y e f f e c - t i v e s h i e l d i n g was found f o r a screen c o n s i s t i n g o f two massive h a l f - c y l i n d e r s o f h i g h p u r i t y I n
,
w i t h a small gap between them, and f o r a V3Ga c o i l shorted by cop- per s t r i p .Using t h e A1 shield, shown i n Fig. l b we were a b l e t o measure i n our B i t t e r magnet thethermopoweroscillations accompanying t h e magnetic breakdown i n A1 /7/, i n f i e l d s up t o 14 T. A d d i t i o n a l advandage o f t h e r e s i s t i v e s h i e l d s w i t h a gap i s t h e i r "mag- n e t i c f i x i n g " i n a f i e l d , caused by mutual i n t e r a c t i o n between a.c. f i e l d o f t h e magnet and t h e screening f i e l d . The sample placed i n a sample h o l d e r and then i n the s h i e l d gap i s f i x e d j u s t i n t h e c e n t r e o f t h e magnet and i s i s o l a t e d from t h e me- c h a n i c a l v i b r a t i o n s which are the source o f noise. We took advantage o f t h i s f e a t u r e w h i l e measuring u n d e t e c t a b l e w i t h o u t s h i e l d very weak changes o f niagnetoresistance i n amorphous m a t e r i a l s , i n f i e l d s up t o 14 T.
REFERENCES
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6 (1946) 299./6/ GOLDSTEIN I.S., TOBIN P.J., Rev. Sci. I n s t r . , - 40 (1969) 1.
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