1538

, I

IEEE TRANSACTIONS ON MAGNETICS, VOL. 25, NO. 2, MARCH 1989

QUENCHING OF MULTISECTION SUPERCONDUCTING MAGNETS W I T H INTERNAL AND EXTERNAL SHUNT RESISTORS

A.A.Konjukhov, V.A.Malginov, V.V.Matokhin, V.R.Karasik Lebedev Physical I n s t i t u t e o f the Academy o f Sciences

USSR, 117924, Moscow 5-333 Leninsky prospect 53

Abst ract

Sel f -protect ion o f superconducting magnets by subdiv is ion w i t h us ing o f i n t e r n a l and external shunt r e z i s t o r s was invest igated. Temperature r i s e o f var ious places o f winding was measured by means o f soldered t o the conductor thermocouples. V e l o c i t y o f t he normal zone propagation along r a d i a l and a x i a l d i r e c t i o n s was obtained. I n i t i a l cu r ren t was var ied from 0.3 up t o 0.9 I,+= 1000 A. The effectiveness o f p ro tec t i on by subdiv is ion w i t h external shunts depends on the number and dimensions o f sections. I t increases by reducing o f s i n g l e sec t i on s ize. Being placed i n t o the magnet, shunts work as heater and d im in i sh the t ime o f quenching and maximum temperature o f the winding.

I n t roduc t i on

Operating a t h igh cu r ren t dens i t y superconducting magnets a re used i n acce le ra to rs ,e lec t r i ca l machines magnetic separators and o the r devices. When acc identa l quenching happens the stored energy can released i n small reg ion o f t he winding and the magnet w i l l be destroyed. Therefore superconducting magnets are needing a p ro tec t i on

During many years we inves t i ga ted p ro tec t i on b y subdiv is ion w i t h external shunt r e s i s t o r s 2*3'4 . I n t h i s paper experimental i n v e s t i g a t i o n o f superconducting magnets p ro tec t i on by means of external and i n t e r n a l shunts i s described. The idea about us ing i n t e r n a l shuts f o r magnets p ro tec t i on had being proposed by M.N.Wilson', but i t was rea l i zed f o r t he f i r s t time.

.

Experimental Procedure

Two superconducting magnets o f i d e n t i c a l geometry were i nves t i ga ted . The inne r diameter of winding was 560 mm, the outer one 640 mm, the l eng th was 78 mm. The magnet was wound o f 1.5 mm cable. The cable consis ted o f s i x mu l t i f i l amen ta ry composite Nb-Ti conductors, which was tw is ted around a cen t ra l copper conductor. I n s u l a t i o n o f the cable was made from two laye rs o f synthetic, Superconductor t o copper r a t i o was 2:3. C o i l s were impregnated w i t h epoxy. Magnet num.l (SM1) had on ly external shunts . Each laye r o f maqnet num.2 (SM2)

I R,Layers I y,Angle along turn I

0 - thermocodples.(+) 'ashunt-hea ter(S-€1) - voltage taps = - hester(H) F i g . l . Locat ion o f detectors i n the winding o f

t he superconducting magnet.

had add i t i ona l i n t e r n a l shunt. I n te rna l shunt were made from s ta in less s tee l s t r i p , placed i n i n t i m a t e heat contact with laye rs o f winding. The res is tance each the s t r i p i s equal 0.1 Om. The s t r i p s could connect t o taps o f t he correspoding section.

Quench was s t imulated w i t h the help o f small spot heater located on the cen t ra l t u r n o f the f i r s t l aey r . Temperature r i s e o f w ind in ig and normal zone propogation v e l o c i t y were measured by means o f soldered t o t h e cable thermocouples. F ig .1 shows the l o c a t i o n o f heaters and thermocouples i n the the winding. Experimental procedure has d i sc r i bed i n d e t a i l s i n re fe renceJ .

Results and Discussion

Choice o f p ro tec t i on technique i s determined by s i ze o f winding and i t s e f f e c t i v e t ransversa l ( i n r a d i a l and a x i a l d i r e c t i o n s ) heat conduc t i v i t y . I f l a t t e r i s s u f f i c i e n t l y low the s i ze o f normal zone i s much smaller than the dimensions o f winding i n corresponding d i rec t i ons . To prov ide r e l i a b l e p ro tec t i on i t i s neccesary t o d imin ish the dimension o f u n i t sect ion and t h a t o f res is tance o f external and i n t e r n a l shunts. Low res is tance external shunts cause the phenomenon o f "electromagnetic avalanche" which ensures almost un i form d i s t r i b u t i o n o f heat i n winding' . Working as heaiers i n t e r n a l shunts increase the normal zone propagation v e l o s i t y . I n shor t , e f f e c t i v e p ro tec t i on i s rea l i zed when the volume o f u n i t sect ion i s t he same order t h a t normal zone evolved i n quenching.

1

SM-2

a k= 'ai 01 < 1 I 10 g

400 600 800 Current (A

Fig.2. Quench v e l o c i t y along cable d i r e c t i o n and t imes o f heat t r a n s f e r i n transverse axes.

F ig .2 shows the connection between quench v e l o c i t y along cable and i n i t i a l current . It a l so shows t imes o f heat t r a n s f e r t n and t , through i n s u l a t i o n between tu rns . Because o f t h e add i t i ona l f iber-g lass i n s u l a t i o n between laye rs o f winding

It fo l l ows from experimental date t h a t a normal zone expands three-dimensional ly u n t i l the t ime when zone boundaries meet each other a t opposite s ide o f s ide o f t u rn . Futher the zone expands i n two d i r e c t i o n s ( R and Z axes). When e f f e c t i v e t ransversa l heat conduc t i v i t y i s small, the lumped res is tance zone ( LRZ ) a r i ses i n normal p a r t o f winding about i n i t i a l quench po in t and almost t he a l l storage energy w i l l be released i n i t . Fig.3 and F ig.4 show maximum temperature and a c t i v e vo l tage d i s t r i b u t i o n s i n unprotect ing winding.

t R > tz .

0018-9464/89/0300-1538$01 .WO1989 IEEE

-~ -

10 20 0

S; 0,lOm Ne of sections

- t t -

-\; a

R, Layers 2, Purns

Fig.3. Maximun a c t i v e vo l tage d i s t r i b u t i o n i n un- p r o t e c t i n g winding o f the SM-1.

0 5 10 R. Lavers

Fig.4. Temperature d i s t r i b u t i o n a f t e r quench i n unprotect ing windings.

Subdividing the magnet i n t o some equal r a d i a l sect ions we can decrease the maximum temperature The value o f shunt res is tance ( R s & ) was choosen the same order t h a t res is tance o f sect ion ( .RSec ) a t 77 K. For instance, i n the case o f shunting the each laye r we take R S h = 0.005 Om. Subdiv is ion i s e f f e c t i v e p a t i c u l a r y when the of s i ze sect ion i n R-d i rect ion i s equal o r l ess o f s i ze L R Z ( Fig.5 ) .

400 M v

a 5 200 E a €-I E o

I

Number of sections

I - I 1 I I I

400 GOO 800 1000 Current ( A )

Fig.5. Hot spot temperature vs i n i t i a l cu r ren t a t d i f f e r e n t va r ian ts o f subdiv is ion.

r I I I n M

H I 1 I 400 600 800

Current ( A ) Fig.6. Hot spot temperature f o r d i f f e r e n t

techniques o f p ro tec t i on .

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Concening the a x i a l subdiv is ion i t i s p o s i i b l e t o apply another technique o f protect ion. This method i s t o use r e s i s t o r shunts as heaters . Note t h a t external shuts across each laye rs w i t h the same res is tance as the i n t e r n a l shunt one employed no pro tec t i on o f magnet. F ig .6 i l l u s t r a t e s t h i s .

Using the i n t e r n a l shunt increases many t imes the e f f e c t i v e propagation v e l o c i t y V,$J o f the normal zone. Fig.7 shows the p o s i t i o n o f normal zone versus t ime. The slope o f l i n e determines the v e l o c i t y V e d ~

This technique we can use not o n l y i n Z d i r e c t i o n . For example, we could placed few i n t e r n a l shunts i n each laye r t o quench d i f f e r e n t po in ts along superconductor i f the t u r n was very long.

R,Layers Z , % r n s -

Fig.7. Pos i t i on o f t he normal zone vs t ime i n SM-2 when i n t e r n a l shunt were used

D i f f e r e n t techniques o f p ro tec t i on by external

res is tance shunt and i n t e r n a l one ( Risk / Rrec = 20) prov ide var ious ways o f normal zone evaluat ion in magnet. It fo l l ows t h a t the cu r ren t decay must be d i f f e r e n t f o r above mentioned v a r i a n t p ro tec t i on . I n case h igh external res is tance shunts the cu r ren t decay i s determined main ly by heat ing o f L R Z . There i s another s i t u a t i o n when RlCO,,/ R,, = 1 o r R i b / R,, = 20 . The process o f cu r ren t decay may be d i v ided i n t o two pa r t s . In t he begining many o f po in ts o f quench are a r i sed i n winding. U n t i l t he normal zone occupied winding, t he t o t a l current i s p r a c t i c a l l y constant. Then cu r ren t ab rup t l y decreases due t o i n tens i ve d i s s i p a t i o n o f storage energy i n a l l volume o f winding. This confirmes by i d e n t i c a l cu r ren t decay a f t e r c e r t a i n t ime moment. See Fig.8 .

The ef fect iveness o f var ious p ro tec t i on technique are shown on Fig.9 .

h igh ( Rhc,L/ R*C = 20 and IOW ( RI.,L/ R,c = 1 )

800

4

-P

v

400 k V

0

Kj 0 1 2

-H;O,l(hn

0 1 2

Time ( 9 )

Fig.8. Tota l cu r ren t i n magnet (do t ted l i n e ) and currents i n sect ions i n SM-2 .

1540

1 I 1

580A

I

-P m I 1 I I I FI

a

€4 fi 100

0 ~~

10 20 z, Turns R, Layers

Fig.9. Temperature d i s t r i b u t i o n a f t e r quench o f 18-sections va r ian t o f SM-2.

I n order t o determine optimum res is tance o f i n t e r n a l shunt we appl icated two connection scheme o f t he s t r i p s i n s ix-sect ions magnet. I n t h i s case each sect ion o f the winding consisted o f three layers. A t m u l t i p l e connection the r a t i o RI&/ Rsec i s equal 2 and a t ser ies connection Rl,h/ R,, = 20.

res is tance i n t e r n a l shunt not o n l y i n compare w i t h h igh one bu t a l so w i t h t r a d i t i o n a l subdiv is ion o f winding by external shunts.

Fig.10 i l l u s t r a t e s advantages o f low

n M W I I I I I

e" 100

400 600 800

Current (A

resistancese o f heat i n t e r n a l shunts. Fig.10. Hot spot temperature f o r d i f f e r e n t

To understand t h i s s i t ua t i on , we analyzed specia l case when the coupl ing c o e f f i c i e n t s k are equal i n mu l t i sec t i ona l magnet '. It fo l l ows t h a t the l a t t e r may be replaced by uns immetr ica l ly d i v ided winding. Fol lowing the approach d i sc r i bed i n we w r i t e next combined equations

where L- inductance o f u n i t section, L,- inductance ( n-1 ) 'sect ions o f the r e s t p a r t o f magnet, M - mutual inductance between L,and L, , I,( t )- t o t a l cu r ren t i n magnet, I , ( t ) - cu r ren t i n the f i r s t section, I t ( t )- cu r ren t i n the r e s t p a r t o f magnet R,( t )- res is tance o f shunt, RI( t )- res is tance o f normal zone i n f i r s t sect ion.

A f te r some manipulat ion t h i s combined equetions

o f may be w r i t t e n i n term I , only . Se t t i ng k = M 2 / L,L, and assuming f o r l a rge n >> 1 L c L o ( inductance magnet ) we r e w r i t e ( 1 ) as

I n the begin ing o f quench R,( t ) << R, and assuming dR/dT= constant we may solve eq. ( 2 )

S i m p l i f i n g ( 3 ) f o r begining o f quench we get

( 5 )

According t o eq.( 5 ) t he smaller res is tance o f i n t e r n a l shunt t he higher power generation i n the s t r i p the sooner magnet quenchs i n normal s ta te . See Fig.10

Conclusion

P ro tec t i on method o f superconducting magnets w i t h h igh cu r ren t dens i t y i s connected w i t h i nsu la - t i o n heat conduction between winds and laye rs . When heat conduction i s h igh enough subdiv id ing o f t he winding and shunting each sect ion w i t h external r e s i s t o r secures normal operat ion mode o f the magnet. When heat conduction o f i n s u l a t i o n i s small i n t e r n a l shunts are needed.

References

[11 M.N.Wilson, Superconducting Magnets. Oxford: Clarendon Press, 1984, ch.9, pp.200-232

C21 V. R .Karasi k, N.V.Kri vo l utckaya, A. I .Rusi nov, "Analysis o f Electromagnetic Processes i n Sectioned Superconducting Solenoids", Proc. o f Lebedev Phys. I n s t . o f the Academy o f Scienses, 1980, v.121, pp.52-75 ( i n Russian).

[3] G.I.Agapov e t .a l . , "Set-up f o r I nves t i ga t i on o f Superconducting Magnets", Proc.of Lebedev Phys. I n s t . o f t he Academy o f Scienses, 1984, v.150, pp.111-123 ( i n Russian).

[4] V.A.Malginov, V.V.Matokhin, V.R.Karasi k, A.A.Konjukhov, "K ine t i cs o f Heat Processes under Quenching i n the Normal State", Proc.

o f Lebedev Phys. I n s t . o f t he Academy o f Scienses, 1984, v.150, pp.48-56(in Russian).