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EVALUATION OF EXPERIENCE WITH
SUPERCONDUCTING MAGNETS OPERATED AT 1.8 K
P. Seyfert, J. Lottin
To cite this version:
P. Seyfert, J. Lottin. EVALUATION OF EXPERIENCE WITH SUPERCONDUCTING MAG- NETS OPERATED AT 1.8 K. Journal de Physique Colloques, 1984, 45 (C1), pp.C1-633-C1-635.
�10.1051/jphyscol:19841128�. �jpa-00223598�
JOURNAL DE PHYSIQUE
Colloque CI, supplCment au no 1, Tome 45, janvier 1984 page Cl-633
EVALUATION OF EXPERIENCE WITH SUPERCONDUCTING MAGNETS OPERATED AT
l a 8 K
P. S e y f e r t and J.C. ~ o t t i n *
Centre dfEtudes NucZ6aires de GrenobZe, 8 5 X, 38041 GrenobZe Cedex, France 'centre dfEtudes NucZ6aires de SacZay, 91191 Gif-sur-Yvette Cedex, France
R6sume - A l a demande du comite d ' o r g a n i s a t i o n de MT8, nous avons organise une t a b l e ronde sur l e s u j e t ci-dessus. Cet a r t i c l e a pour b u t de resumer l e s diverses c o n t r i b u t i o n s presentees pendant l a seance.
Abstract : A t t h e request o f t h e MT8 o r g a n i z i n g committee we have arranged a panel discussion on t h e above s u b j e c t a t t h e conference. This paper i s an attempt t o summarize t h e various c o n t r i b u t i o n s presented.
Our p l a n f o r t h e panel has been t o p l a c e emphasis on h i g h performance helium I1 cooled magnet systems t h a t have already been p u t i n t o f u l l operation. To date several medium sized solenoids and a c c e l e r a t o r d i p o l e s have come t o t h i s stage. S i x speakers, each d i r e c t l y i n v o l v e d i n a p r o j e c t o f t h e k i n d considered here, had been i n v i t e d t o r e p o r t on t h e i r p r a c t i c a l experience.
I n a d d i t i o n t h r e e i n v i t e d experts p a r t i c i p a t e d i n t h e panel: G. Claudet from CEN- Grenoble, H. Kobayashi from Nihon U n i v e r s i t y , Tokyo and S.W. van S c i v e r from Univer- s i t y o f Wisconsin. Their work has made them known f o r long as strong proponents of t h e helium I 1 c o o l i n g mode f o r h i g h performance magnets. They a s s i s t e d t h e c h a i r - man t o c l a r i f y t h e various p o i n t s brought up d u r i n g t h e discussion which was o f course open t o t h e whole audience.
I n what f o l l o w s we s h a l l f i r s t b r i e f l y review t h e i n t r o d u c t o r y presentations t h e speakers gave o f t h e i r systems and then r e c a p i t u l a t e successively t h e cryogenic and e l e c t r i c a l aspects i n operation o f t h e systems.
THE MAGNETIC SYSTEMS
The two 10T t e s t f a c i l i t i e s l o c a t e d a t CEN-Saclay /1,2/ (speaker P. Genevey) and SNCI-Grenoble /1,3/ (speaker J.C. V a l l i e r ) are almost i n d e n t i c a l i n design and cons- t r u c t i o n : A solenoid assembly wound from d i f f e r e n t l y graded m o n o l i t h i c NbTi conduc- t o r s i s immersed i n a pressurized helium I 1 b a t h and generates a v e r t i c a l l y o r i e n t e d f i e l d up t o 10T w i t h i n a 325 rnm diameter bore. Enhancement o f b o t h NbTi c r i t i c a l p r o p e r t i e s and c o i l s t a b i l i t y has been looked f o r . The h i g h f i e l d c o i l s are charac- t e r i z e d by r e l a t i v e l y deep (1.8-2.1 mm) c o o l i n g channels and bare conductor surface.
The d i p o l e systems a t LBL-Berkeley /4,5/ (speaker M.V. Hassenzahl), KEK /6/ (speaker H. Hirabayashi ) and CERN ( p r i v a t e communication, speaker M. Morpurgo) a1 1 use cryo- s t a t designs based on t h e two r e s e r v o i r p r i n c i p l e . The helium I1 vessels are l a r g e enough t o accomodate magnets of 1 .lm (KEK) t o 1.4m (LBL) l e n g t h and 0.27111 (KEK) t o 0.75111 (CERN) diameter. While LBL and KEK have been using t h e i r systems i n support of t e s t programs w i t h d i p o l e s wound from close-packed NbTi cables M. Morpurgo has b u i l t and s u c c e s s f u l l y t e s t e d an 8T d i p o l e using a hollow NbTi composite conductor i n t e r n a l l y cooled by stagnant pressurized helium I 1 ( u n i t l e n g t h o f h y d r a u l i c p a t h about 60m).
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19841128
C1-634 JOURNAL DE PHYSIQUE
E s s e n t i a l p a r t o f t h e Tore Supra Model C o i l T e s t F a c i l i t y /7/ (speaker J. P a r a i n ) +s an immersion c o o l e d h i g h l y s t a b i l i z e d NbTi c o i l connected v i a a 7m l o n g p i p e t o a h e l i u m I 1 c r y o g e n i c c i r c u i t .
G e n e r a l l y speaking i t appeared t h a t a l l systems, n o t o n l y t h e l a s t mentioned, have been meant t o a c e r t a i n e x t e n t as a s t e p i n t h e development o f a new c o o l i n g t e c h n i - que towards p r o j e c t s on a l a r g e r s c a l e : T o r o i d a l f i e l d c o i l s f o r tokamaks, h i g h f i e l d a c c e l e r a t o r d i p o l e s o r h i g h f i e l d h y b r i d magnet f a c i l i t i e s .
CRYOGENIC ASPECTS OF MAGNET OPERATION
I n a1 1 p l a c e s c o o l down from ambient t o l i q u i d h e l i u m temperature has been c a r e f u l l y c o n t r o l l e d so as t o a v o i d harmful temperature g r a d i e n t s w i t h i n t h e magnets. That i s t h e reason f o r i t s r e l a t i v e l y l o n g d u r a t i o n (24-72 h r ) . The process has almost always been i n i t i a t e d b y f o r c e d c i r c u l a t i o n o f c o l d h e l i u m gas ( c y c l e gas o f r e f r i - g e r a t o r o r a u x i l i a r y l o o p w i t h l i q u i d n i t r o g e n s u p p l i e d c o o l e r o u t s i d e t h e c r y o s t a t ) and completed by d i r e c t use o f two phase helium.
By c o n t r a s t c o o l down o f t h e Tore Supra model c o i l has been achieved b y means o f h e a t c o n d u c t i o n across spacers which s u p p o r t t h e c o i l c o n t a i n i n g vessel i n s i d e a s t r o n g s t a i n l e s s s t e e l c a s i n g c o o l e d t o 4.5K.
C o o l i n g t h e magnets f r o m 4.2K t o 1.8K t u r n e d o u t t o l a s t between 1 h r (KEK d i p o l e ) and 24 h r (10T f a c i l i t i e s ) depending on t h e r e f r i g e r a t i o n power o f t h e i n s t a l l e d c o l d s i n k and t h e q u a n t i t y o f l i q u i d h e l i u m c o n t a i n e d i n t h e He I 1 v e s s e l . The d e s i g n o f a l l c o l d s i n k s i s a JT stage o p e r a t i n g between l i q u i d h e l i u m a t 4.2K and t w o phase h e l i u m a t 1.7-1.9K. Feedback c o n t r o l needle v a l v e s a r e c u r r e n t l y i n use t o m a i n t a i n e i t h e r p r e s s u r e o r l i q u i d l e v e l c o n s t a n t i n s i d e t h e c o l d s i n k . Ref- r i g e r a t i o n power o f s e v e r a l w a t t s (2-5W) a t 1.8K has been f o u n d s u f f i c i e n t f o r most p r a c t i c a l purposes, excepted a t LBL where up t o 40W a r e a v a i l a b l e .
Heat leakage i n t o t h e h e l i u m I 1 vessel c o u l d almost always be reduced t o t y p i c a l l y 2W a t 1.8K.
E l e g a n t ways t o c a r r y o u t p r e c i s e c a l o r i m e t r i c measurements o f h e a t r e l e a s e d b y magnets t o t h e h e l i u m I 1 b a t h were i n d i c a t e d b y t h e Tore Supra group /7/ and by LBL /5/.
R e l i e f v a l v e s have g e n e r a l l y been used t o c o n t r o l s a f e t y o f t h e magnet c o n t a i n i n g h e l i u m I 1 vessels. I n case o f o v e r p r e s s u r e (A 3 b a r ) t h e y p r o v i d e a communication channel t o t h e b a t h o f h e l i u m I k e p t a t atmospheric pressure. A l l c r y o s t a t s have been equipped w i t h a t l e a s t one s m a l l diameter (1-3 cm) m o s t l y w e i g h t loaded valve.
I n a d d i t i o n l a r g e diameter (5-10 cm) s p r i n g loaded v a l v e s have been i n s t a l l e d i n t h e LBL f a c i l i t y and t h e two 10T f a c i l i t i e s w h i c h a l l c o n t a i n more t h a n 100 1 o f s t o r e d h e l i u m 11.
ELECTRICAL ASPECTS OF MAGNET OPERATION
The h i g h s t a b i l i t y o f t h e b a t h c o o l e d c o i l s i n t h e 10T f a c i l i t i e s and t h e Tore Supra model c o i l r e l i e s on good thermal c o n t a c t between conductor and he1 ium 11. Wetted conductor s u r f a c e s have t h e r e f o r e been l e t f r e e o f any i n s u l a t i n g c o a t i n g s . T h i s has r e p o r t e d l y caused no t r o u b l e i n o p e r a t i o n o f t h e magnets so f a r .
The d i p o l e s a t LBL, KEK and CERN a r e a l l u s i n g conductors whose o u t e r s u r f a c e s were e n t i r e l y covered w i t h i n s u l a t ' i n g m a t e r i a l . Only v e r y l i t t l e h e l i u m I 1 i s a b l e t o p e n e t r a t e i n t o t h e NbTi c a b l e s employed a t KEK and LBL (2-3% v o i d f r a c t i o n ) and t h a t c o u l d n o t p r e v e n t t h e s e d i p o l e s f r o m t r a i n i n g . The d i p o l e a t CERN i s u s i n g a h o l l o w NbTi conductor w i t h 19% o f i t s c r o s s - s e c t i o n occupied b y h e l i u m 11. It
showed almost no t r a i n i n g and sustained f a s t ramp r a t e s (+16A/s, -40/s) w i t h o u t quenching.
Only one group (10T f a c i l i t y a t SNCI Grenoble) has r e p o r t e d on t r o u b l e w i t h r e s i d u a l Joule heating from r e s i s t i v e sections i n t h e magnetic: A t nominal c u r r e n t a power o f 6W was found t o be d i s s i p a t e d from what was b e l i e v e d togbe the 40 conductor j o i n t s w i t h i n t h e o u t e r pancake c o i l o f t h e assembly (- 0 . 1 3 ~ 1 0 per j o i n t ) . According t o t h e group i t turned o u t t o o l a t e t h a t t h e conductor j o i n t technology a p p l i e d here was n o t very w e l l s u i t e d t o t h e helium I 1 c o o l i n g mode.
Quench behaviour o f t h e t h r e e d i p o l e s was described as being almost t r i v i a l : Only a small f r a c t i o n o f t h e s t o r e d energy i s d i s s i p a t e d w i t h i n t h e magnet and recovery time o f t h e system a f t e r a quench i s i n t h e order o f a few hours ( 2 h r a t LBL).
Quench behaviour o f the two 10T f a c i l i t i e s i s somewhat more spectacular though not dramatic a t a l l : During quench t e s t s release o f t y p i c a l l y 2 MJ w i t h power spikes i n t h e order o f probably 30-40 kW has been observed. Due t o t h e p e r s i s t e n t Joule heat- i n g coolant i s vaporized i n s i d e the c o o l i n g channels and a pressure r i s e generated which pushes some helium I 1 through t h e r e l i e f valve i n t o t h e helium I bath. As a consequence a momentary subcool i ng o f t h i s bath occurs. Normal operating c o n d i t i o n s
(1.8K) are r e s t o r e d a f t e r 28 hours.
CONCLUSION
Beyond the d e t a i l s brought up d u r i n g t h e meeting some p o i n t s o f general i n t e r e s t have become obvious through t h i s panel discussion:
A s i g n i f i c a n t number o f groups i n t h e superconducting magnet engineering community have found t h e pressurized helium I 1 c o o l i n g mode promising enough t o t r y i t s u t i l i z - a t i o n i n t h e development o f magnets f o r f u s i o n technology, o f h i g h f i e l d solenoids and a c c e l e r a t o r dipoles. A t present i n each o f these areas a f i r s t generation o f medium s i z e d magnets employing helium I 1 cooled NbTi conductors have been b u i l t and operated. A t t h a t stage convenient and r e l i a b l e s o l u t i o n s t o a l l t e c h n i c a l pro- blems have been shown t o e x i s t and success has g e n e r a l l y come up w i t h expectations.
I n several places e f f o r t s are already underway t o r e a l i z e f u r t h e r steps i n these developments and i n t e r e s t i n g new experience w i l l be gained w i t h i n t h e n e x t years.
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