e s e g o t t i LEP D i v i s i o n C ERN Geneva, S w i t z e r l a n d 843 A b s t r a c t : The s t e e l -c o n c r e t e c o r e s of t h e LEP d i p o l e magnets are b u i l t o f r e g u l a r l y spaced s o f t magn e t i c steel l a m i n a t i o n s , t h e s p a c e s b e i n g f i l l e d w i t h cement m o r t a r . Mortar s h r i n k a g e a t h y d r a t i o n i s prev e n t e d i n t h e t r a n s v e r s e p l a n e by t h e s t e e l l a m i n a t i o n s and t h e r e f o r e i n d u c e s compressive stresses i n t h e s t e e l and t e n s i l e s t r e s s e s i n t h e m o r t a r . The e f f e c t of comp r e s s i v e s t r e s s e s on t h e p e r m e a b i l i t y and t h e c o e r c i v it y of s o f t magnetic s t e e l h a s been measured on samples and t h e consequences on t h e magnetic c h a r a c t e r i s t i c s o f t h e c o r e s have been e v a l u a t e d . By a c o n t r o l l e d s t r a i ni n g o f t h e aged c o r e s i n a h y d r a u l i c d e v i c e , t h e t e ns i o n i n t h e m o r t a r i s b r o u g h t l o c a l l y t o exceed t h e r u p t u r e l i m i t . Thus, t h e compressive s t r e s s e s i n t h e s t e e l l a m i n a t i o n s a r e p a r t i a l l y r e l i e v e d and t h e l o s s o f bending s t r e n g t h i n t h e d i p o l e s i s reduced. The exc i t a t i o n c h a r a c t e r i s t i c s of t h e c o r e s a r e measured by means o f a f i x e d set of e x c i t a t i o n windings and measuri n g c o i l s i n t o which t h e c o r e s a r e moved a f t e r streqs r e l i e v i n g . These measurements i n c l u d e t h e determinat i o n of t h e e q u i v a l e n t s u r f a c e a r e a s of " f l u x loops" which a r e embedded i n t h e lower p o l e s and which w i l l p e r m i t t h e measurement o f t h e f i e l d i n t e g r a l on t h e c e n t r e l i n e d u r i n g LEP o p e r a t i o n . I n t r o d u c t i o n The LEP main r i n g magnet system [11 c o n t a i n s 3280 e q u a l d i p o l e c o r e s , e a c h 5 . 7 5 m l o n g , e x c i t e d i n s e r i e s : t h e vacuum chambers i n which t h e p a r t i c l e s w i l l c i r c u l a t e a r e l o c a t e d i n t h e i r 100 m h i g h a i r gaps. The magnetic f i e l d on t h e e q u i l i b r i u m o r b i t w i l l r i s e frcm 0.0218 T t o 0.1090 T from t h e beam i n j e c t i o n e n e rgy of 20 GeV t o t h e e n e r g y of 100 GeV. For an economic a l e x p l o i t a t i o n of t h e s t e e l , t h e f i e l d i n t h e r e t u r n yokes should b e more t h a n t e n times h i g h e r : t h i s e f f e c t i s a c h i e v e d p a r t l y by r e d u c i n g t h e t r a n s v e r s e dimens i o n s o f t h e s t e e l l a m i n a t i o n s and p a r t l y by s p a c i n g them l o n g i t u d i n a l l y a l o n g t h e beam o r b i t . The s p a c e s a r e f i l l e d w i t h cement m o r t a r , which i s a s t r o n g bondi n g a g e n t and e n s u r e s t h e mechanical r i g i d i t y of t h e c o r e s [2]. This s t r u c t u r e i s i l l u s t r a t ...
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