Measurements on two Nb superconductive RF cavities

Measurements on two Nb superconductive RF cavities

Volume ?~, number 3 PHYSICS LETTERS • m e n t of m a g n e t i c f l u x w a s d e t e r m i n e d b y s m a l l p i c k - u p c o i l s , e . g . ...

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Volume ?~, number 3

PHYSICS

LETTERS

• m e n t of m a g n e t i c f l u x w a s d e t e r m i n e d b y s m a l l p i c k - u p c o i l s , e . g . , u n d e r G. A s e r i e s of f l u x p a c k e t s can b e s w e p t - i n a t G s u c c e s s i v e l y . The p r o c e d u r e can b e r e g a r d e d a s a f l u x p u m p . The t r a n s p o r t of the f l u x a l o n g the f i l m i s a c c o m p l i s h e d b y s i m u l t a n e o u s , c o n t i n u o u s s w i t c h i n g of s u p e r c o n d u c t i n g m a t e r i a l into t h e n o r m a l s t a t e at t h e l e a d i n g edge of t h e s p o t and of n o r m a l m a t e r i a l into t h e s u p e r c o n d u c t i n g s t a t e a t t h e t r a i l i n g e d g e , h e a t b e i n g a b s o r b e d and e m i t t e d , r e s p e c t i v e l y . When no m a g n e t i c c o u p l i n g to d i s s i p a t i v e c i r c u i t s e x i s t s t h e s e a m o u n t s of h e a t c a n c e l , but if s u c h circuits are present the transition at the leading e d g e t a k e s p l a c e a t d e p r e s s e d v a l u e s of t h e m a g netic field, whereas at the trailing edge a transition at increased field strength might occur. In t h e e x t r e m e , i d e a l i s e d c a s e of a s t r o n g r e a c tion ( e . g . , b y F o u c a u l t c u r r e n t s in a s u i t a b l e n o r mal conducting plate underneath the film) the cycle g i v e n in fig. 3 i s a p p r o a c h e d . T h e m a x i m u m a m o u n t

MEASUREMENTS

ON

TWO

Nb

1 September 1962

T/~th ---~-T

Fig. 3. of w o r k to b e e m i t t e d i s g~H c2, b a t h p e r unit of switched volume. The v a l u a b l e h e l p of M r . P . B r o u w e r with the experiments is gratefully acknowledged.

References 1) I.M.Templeton, J. Sci. Instr. 32 (1955i 314. 2) P . S . A d m i r a a l , private communication.

SUPERCONDUCTIVE

RF

CAVITIES

L. R I N D E R E R , J . R U F E N A C H T University of Lausanne, Switzerland and A. SUSINI CERN, Geneva

Received 8 August 1962

Introduction S t a f f o r d 1) h a s r e c e n t l y d i s c u s s e d t h e f e a s i b i l i t y of a c r y o g e n i c p r o t o n l i n e a r a c c e l e r a t o r . A r i s i n g out of h i s p r o p o s a l to s t u d y s u c h a m a c h i n e m i n t e r e s t in t h e r f p r o p e r t i e s of s u p e r c o n d u c t i n g b o d i e s h a s s u d d e n l y d e v e l o p e d . The r e s u l t s given in r e f s . 2) and 3) a r e r e l a t e d to P b and Sn c a v i t i e s a t f r e q u e n c i e s r a n g i n g f r o m one to s e v e r a l k M c / s . U n f o r t u n a t e l y , in t h e r a n g e of f r e q u e n c i e s e m p l o y e d in p r o t o n l i n e a r a c c e l e r a t o r s (200 to 300 M c / s o r even l o w e r 4)) no d i r e c t m e a s u r e m e n t s s e e m to e x i s t with t h e e x c e p t i o n of t h o s e c a r r i e d out a t t h e U n i v e r s i t y of L a u s a n n e 5,6) on l e a d c a v i t i e s with t h e a i m of c o n s t r u c t i n g a f r e q u e n c y s t a n d ard. So f a r a s t h e a u t h o r s know, Nb c a v i t i e s h a v e never been tested, despite the great interest pres e n t e d b y t h i s m e t a l due to i t s m e c h a n i c a l p r o p e r t i e s w h i c h a r e v e r y d i f f e r e n t _. o m t h o s e of l e a d .

T h i s p a p e r d e s c r i b e s two Nb c a v i t i e s tuned to 270 M c / s a n d t h e r e s u l t s of t h e m e a s u r e m e n t s .

Description of the cavities The two c a v i t i e s , m e c h a n i c a l l y i d e n t i c a l , a r e of t h e c a p a c i t y - l o a d e d c o a x i a l t y p e a s it a p p e a r s in fig. 1. T h e y c o n s i s t of p i p e s a n d p l a t e s of s i n t e r e d n i o b i u m ( p u r i t y 99.98%) j o i n e d t o g e t h e r b y m e a n s 144 g 2

t i_:= Fig. 1.

119

Volume 2, number 3

PHYSICS

LET TERS

1 September 1962 Unannealed cavity

Annealed cavity

Resonant frequency 273 M c / s 264 M c / s Characteristic impedance 63 ~ 63 ~2 Vacuum 3 x 10-3 T o r r 3 x 10-3 T o r r Q at n o r m a l t e m p . 1000 800 Q at liquid a i r temp. 1400 2000 Q at 4.2OK ~ 106 8.7 × 106 Q of a s i l v e r e d b r a s s copy of the c a v i t y 2500

Fig. 2. of e l e c t r o n i c b o m b a r d m e n t . One c a v i t y i s m a d e of u n a n n e a l e d Nb; in the o t h e r the m e t a l h a s b e e n a n n e a l e d at 1200°C in v a c u u m f o r 1 h r . The s u r f a c e s have b e e n c h e m i c a l l y p o l i s h e d b y i m m e r s i o n in a b a t h of a m i x t u r e of H2SO 4 a n d HNO 3. The r e s u l t i s i l l u s t r a t e d in fig. 2 ( e n l a r g e m e n t 300 t i m e s ) .

M e a s u r e t~'zents The q u a l i t y f a c t o r Q h a s b e e n m e a s u r e d by the 3 db m e t h o d f o l l o w i n g t h e a r r a n g e m e n t of fig. 3. T h e c a v i t y i s w e a k l y c a p a c i t i v e l y - c o u p l e d to the generator through an approximately half-wave t r a n s m i s s i o n l i n e . R e a d i n g the rf v o l t a g e on the 50 Ohm t e r m i n a t i n g r e s i s t o r g i v e s the v o l t a g e Vo at the end of the t r a n s m i s s i o n l i n e .

ili H :T[ i:~ Ii I,Ill Fig. 3. Then the v o l t a g e at the c a p a c i t y of the c a v i t y i s of the s a m e o r d e r a s Vo ( m o r e s p e c i f i c a l l y it e q u a l s 1 Vo if the p r o b e s a r e e x a c t l y equal and the m i x e r input i m p e d a n c e i s e x a c t l y 50 Ohm). D i v i d i n g by the c h a r a c t e r i s t i c i m p e d a n c e one o b t a i n s the c u r r e n t f l o w i n g into the c a v i t y . The l e v e l of the c a v i t y v o l t a g e i s m e a s u r e d by a s t a n d a r d h e t e r o d y n e m e t h od. The c o u p l i n g i s a d j u s t e d b y c h a n g i n g the p e n e t r a t i o n of the p i c k - u p p r o b e s into t h e cavity. In o r d e r to d a m p a n y m e c h a n i c a l v i b r a t i o n s , the c a v i t i e s have b e e n e n c a s e d in a s a c k of sand. The m e a s u r e m e n t s w e r e a s f o l l o w s : 120

Due to the limitation of the measuring equipment the current into the cavity could be made to rise only to approximately 8 mA. In the unannealed cavity the Q was independent of the current; in the other one the Q appeared to decrease as soon as the current exceeded a few mA. At 8 mA the resonance curve shown in fig. 4 was recorded. When the current was decreased again, the Q returned to the high value.

IO 8

6 4 2

500

4600

Fig. 4.

Conclusion It a p p e a r s t h a t the i n t e r n a l (and a l s o e x t e r n a l ) s t r e s s e s on the m e t a l have p e r h a p s a h a r m f u l i n f l u e n c e on the q u a l i t y f a c t o r . The r e d u c t i o n of the Q with i n c r e a s i n g c u r r e n t c a n n o t be a t t r i b u t e d to the m a g n e t i c f i e l d of the c u r r e n t i t s e l f , s i n c e in t h i s c a s e s e v e r a l t h o u s a n d A m p e r e s would be r e q u i r e d to q u e n c h the s u p e r c o n d u c t i v i t y . The r o u g h n e s s of the s u r f a c e i s p e r h a p s r e s p o n s i b l e f o r that. The s a m e p h e n o m e n o n w a s o b s e r v e d in lead c a v i t i e s , even at a c u r r e n t m u c h l o w e r than 8 m A ; i n s e v e r a l c a s e s the Q r e m a i n e d at the l o w e r v a l u e when the c u r r e n t was once m o r e d e c r e a s e d . References 1) A . P . Bauford and G.H.Stafford, Plasma Phys. J. 3 (1961) 287. 2) E. Maxwell, Adv. Cryogenic Engineering 6 (] 960) 154. 3) M.S. Khaikin, Pribori i Tekh. Experimenta 3 (] 961) 104. 4) J. Pottier, Note CEA no. 195, february 1957. 5) J.Ruefenacht, J. math. phys. appliqu~es Vol. XIII, 1 (1962) 99. 6) J.Ruefenachtand L.Rinderer, Superconductingcavities at 280 Mc/s, to be publishedin Proe. of eighth int. conf. on low temperature physics, London, september 1962.