RF superconductivity for accelerators--Is it a hollow promise?

IEEE TRANSACTIONS ON MAGNETICS, VOL. MAG-15, NO. 1, JANUARY 1979 15

R F SUPERCONDUCTIVITY FOR ACCELERATORS -- IS IT A HOLLOW PROMISE? * M . Tigner

SUMMARY

This review will at tempt to offer a r e a l i s t i c and coherent overview of the failures and par t ia l suc- cesses in harnessing rf superconductivity of niobium to pa r t i c l e acce l e ra t ion . Work in materials, prepara- t ion p rocesses , hea t t ransfer , cav i ty s t ruc tures a n d vacuum e l e c t r o n i c phenomena are discussed and p u t in perspect ive. An a t t empt i s made t o draw lessons from these observations and to ou t l ine the t asks and oppor- t un i t i e s fo r t he fu tu re .

INTRODUCTION

Since the ear ly 60 ' s and t h e measurements of Banford a n d Stafford ' and of Wilson e t a12, we have known t h a t i t i s p o s s i b l e t o s u s t a i n r f e l e c t r i c f i e l d s of many millions of volts per meter in large volumes by the expenditure of a few watts of r f power. Since those early days of inflated hopes and expecta- tions, almost twenty years ago now, we have become considerably sadder and, I hope, a l i t t l e w i s e r .

I t i s my in t en t ion he re t o eva lua te , c r i t i ca l ly , the accomplishments of this period in practical terms and t o t r y and draw from th is eva lua t ion ind ica t ions about profitable future approaches and appl icat ions a n d about needs for researcti and development. I wi l l argue that before we can expect a r e a l i z a t i o n of the ful l potent ia l of r f superconduct ivi ty for accelerators a two pronged approach will be necessary. O n the one hand we need t o s e l e c t a very few acce lera tor appl icat ions where t h e e x i s t i n g s t a t e o f t h e a r t i s compet i t ively advantageous with respect to a l ternat ive existing, technologies. Having selected these appl ica- t i o n s and accepted the l imitat ions of the exis t ing s t a t e of the a r t , t he cons t ruc t ion and put t ing in to useful, continuous operation of the devices must be aggressively pursued. Success in this will provide the motivational and economic support necessary to continue on t h e second f r o n t , namely, the discovery a n d invention o f the new ideas, techniques and mater- i a l s t ha t w i l l be . r equ i r ed t o make devices approaching the ideal in performance. To some extent these two avenues are a l ready being t ravel led.

GENERAL BACKGROUND

The theory of rf superconductivity and exper- mental progress u p t o a year ago a r e d e a l t wi h i a number of readi ly avai lable review a r t i c l e ~ ! ' ~ * ~ > ~ y ~

Below the superconducting transit ion temperature, t h e condensed Cooper pa i r s of e lec t rons can car ry current without diss ipat ion. They do, however, have i n e r t i a so t h a t f i e l d s must be present inside the con- ductor t o make the pa i r s car ry an a l t e rna t ing cu r ren t . These f i e lds w i l l d r ive the other charges present and thereby engender dissipation, even in the ideal case. The phenomena are described by the approximate rule

Manuscript received September 28, 1978.

Work supported by the National Science Foundation *Laboratory of Nuclear Studies, Cornel1 University,

I thaca, New York 14853.

The f i r s t term, the BCS par t of the res is tance, takes into account the processes in an idea l mater ia l . The Rres term represents al l of the non-ideal i t ies of the surface a n d i s , i n g e n e r a l , a composite of terms depending on f , the operat ing f requency, T the opera- t ing temperature of the sur face , and many o ther 7 ,9 ,10 physical and chemical parameters of the surface. I t cannot be calculated from f i r s t p r i n c i p l e s . Once the material parameters are known,8the f i r s t term can be computed from bas ic p r inc ip les . The residual e f f e c t s , coming from a wide var ie ty of sources , are l e s s well understood although some progress has been made. As we shal l see , these res idual res is tances do not limit the performance of devices now being constructed.

suppor tab le f ie ld i s a l so impor tan t . I f on ly the ideal superconductor properties were involved, one would expect to 'be able to reach a surface magnetic f i e l d of about the thermodynamic c r i t i c a l f i e l d a t l e a s t . I n macroscopic samples of several type I super onductors, rf f ields in excess of B a r e pos- s i b l e 71 . In acce lera t ing type cav i t ies o f Nb1*>l3 and Nb3Sn14y15, the f i r s t f l u x p e n e t r a t i o n c r i t i c a l f i e l d , B c l , has been exceeded. The achievement of these f ie lds in extended accelerat ing devices would correspond to 10's of MV/m. Thus, a s i s t h e c a s e with the surface losses, very good performance i s within the fundamental capability of rf superconduc t iv i ty .

In addi t ion to the sur face losses , the maximum

PRESENT L E V E L OF ACHIEVEMENT

By now a bewildering array of superconducting accelerating devices have been made from various ma te r i a l s , u t i l i z ing a wide range of forming, polishing, cleaning and f inal preparation procedures. The mainstream of a c t i v i t y has focussed on s t ruc- t u r e made from niobium and t o some extent lead plated copper. Since i t i s my purpose t o emphasize the p rac t ica l and the search for near - term payoffs, I 'will emphasize Nb based devices. They have received the most at tention, have given the best performance, and have t h e most wel l e laborated lore for successful preparation.

A. Technology Base

Before discussing levels of performance presently achieved in devices, a br ief review of the scient i f ic and technological base upon which these devices res t i s i n order . I n t h i s I include, besides the basic superconductivity phenomenon i t s e l f , provenance of mater ia l , construct ion and processing procedures, rf structures, trouble shooting procedures and ins t ru- mentation, knowledge of the basic surface physics and chemistry of Nb and knowledge about vacuum elec- t r o n i c phenomena which affect device performance.

a ) Provenance of Material

V i r t u a l l y a l l of the work done so f a r i s w i t h so cal led reactor grade Nb. Several companies* provide the mater ' ial in ingot, bar, plate, sheet, and tube form. As f a r a s i s known, the mater ia l s from the 'various suppliers perform equally well. Impuri- t i e s of the common elements range from a few ppm t o a few hundred ppm. Since s ignif icant ly purer mater ia l

* Wah-Chang , Kaweki -Beryl co, Aremco

0018-9464/79/0100-0015$00.75 0 1979 IEEE

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i s not readi ly avai lable , i t i s not known whether substantial device performance gains m i g h t be had i n tha t d i r ec t ion . As some devices have achieved very high levels of performance, i t i s l i k e l y t h a t p r e s e n t p u r i t i e s a r e s u f f i c i e n t .

b ) Construction Procedures

Cavi t ies ave been fabr ica ted by machining from sol id mater ia l t3 without any joints, by combinations

drawing and welding,C1k and b r o l l i n g and welding Nb of hydroforming, mac n l n g and welding,21 by deep

explosion bonded t o copper.16 All of these methods have been successful to a degree and show t h a t Nb i s a f l ex ib l e ma te r i a l . Welding has been la rge ly car - r ied out by the e lec t ron beam method b u t has a l so been done successful ly by t h e TIG process in a glove box t h a t can be evacuated and f i l l e d w i t h Argon. While successful current carrying joints have been made by welding, operating them a t high f i e l d s r e - qu i res UHV f i r i n g a t above 18 g!yt9 Demountable j o i n t s have been developedz2¶ which can carry some r f cu r ren t and are su i tab le for spec ia l ized use

c) Processing Procedures

Crucial to what success has been achieved are the cur ren t ly used surface treatment procedures. While mechanical polishing finds some use in finishing

ishes are normally good enough t o s t a r t w i t h . A f t e r fabr icat ion the s t ructures are degreased thoroughly. Ul t rasonic ag i ta t ion of the bath has proved useful. The final preparation sequence varies from laboratory to l abo ra to ry b u t some combination of electro- pol ishing24, chemical polishingz1 ox ol ishingZ5 and UHV f i r i n g a r e commonly u s e d 1 6 ~ ~ 7 : ~ ~ $ depending upon the type of device.

machined surfaces or commercial p l a t e f i n -

d ) RF S t ruc tures

The exploi ta t ion of r f superconduct ivi ty for acce lera tors has inspired the development of r f s t ructures pecul iar ly adapted t o the task as well as the adaptation of conventional structures for cooling in helium. For low frequencies, heavily loaded struc- t u r e s , such a s t he he1 i x , an old t ime favori te , a n d a new inven t ion , t he e l egan t sp l i t r i ng s t ruc tu re31 have been used. A t UHF b o t h the a l ~ a r e z ~ ~ and re- entrant, klystron, type of cavity have been b u i l t . A t microwave frequencies special ver ions in the i r i s loaded waveguide with both circular33 and rectang- u l a r l 6 symmetry have been designed to make use of sheetmetal techniques. A bar loaded waveguide has a l s o been made for S-band.46 All of these structures have been made t o work a t r e s p e c t a b l e l e v e l s , a t l e a s t i n s ing le ce l l vers ions , a n d a r e s t i l l under active development. The hel ix has proven t o be ve ry d i f f i - c u l t , a1 though possible t o control, because of i t s mechanical weakness and so will probably have a l imited future .

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e ) Basic Surface Physics and Metallurgy

A l a rge amount of work of a more or l ess bas ic nature has been car r ied ou t to da te in an e f f o r t t o understand the behavior of rf superconducting surfaces on a microscopic level. I t has heightened the s e n s i t i v i t y of device bu i lders to sur face c leanl i - ness and homogeniety and the important role of the oxides. An appreciat ion of that work would requi re a rev iew in i t se l f . Access the x tens ive l i t e ra - t u r e can be had through refsSg and ti4 concerning the r o l e of oxygen enrichment in surface layers.

f ) Trouble Shooting Methods

One of the most important developments of recent years has been the r ea l i za t ion t ha t most breakdowns, i . e . , t r a n s i t i o n s t o t h e sormal s ta te whi le the r f f i e l d i s o n , o c c u r a t well localized spots. Instrum- en ta t ion for loca t ing these spots eas i ly has been indispensible in e lucidat ing breakdown mechanisms 16,27 and in repair ing defect ive structure^.^^ The pr in- c i p l e i s s i m p l e . A network of low mass r e s i s t o r s i s placed on or near the device body and temperature r i s e s due to e lec t ronic hea t ing or t o breakdown a r e seen d i r e c t l y or by second sound propagation with pulse timing used t o l o c a t e t h e f a u l t . Also very useful have been the methods fo r de t ec t ing e l ec t ron ic a c t i v i t y d i r e c t l y by co l lec t ing e lec t rons on probes and by measuring X-rays with counters or photographic methods, 28

g ) Vacuum Electronic Phenomena

For some time i t has been known t h a t f r e e e l e c - trons are caught u p i n t he f ' e ld s i n s ide t he cav i t i e s and can cause wall heating7yJ8 and surface damage where they s t r ike , a s well as more subt le problems such as t h exc i ta t ion of o ther modes of the s t r u ~ t u r e . ~ ~ The source of the e lectrons can be fiel 'd emission or secondary electron multiplication, that i s , m u 1 t i pactoring , and perhaps bremsstrahlung fol- lowed by photo emission. I t i s not always c l e a r which mechanism i s a t work in any given instance. By means o f t h e o r e t i c a l c a l c u l a t i o n s 7 ~ ~ 7 ~ ~ 9 and measurement^^^ i t has been found t h a t one point multipactoring due t o t ransverse e lec t r ic f ie lds , in concer t wi th the r f magnetic f ie ld near the ou ts ide cav i ty wal I s , i s an important source of wall heating in L and S-band cavi t ies operat ing a t or near the i r maximum f i e l d s . This i s an important discovery, the exploitation of which has just s t a r t e d .

B . Device Performance

Figure 1 displays, in pract ical terms, our pre- sen t capabi l i t i es for bu i ld ing la rge sca le devices . The poin ts p lo t ted show the maximum acce lera t ing fields achieved under cw conditions versus the wall power per meter required to support an acce lera t ing f i e l d of 1 M V per meter , the surface res is tance used being tha t app ropr i a t e t o t he maximum operating f i e l d . Thus, the actual rf power per meter for a given device a t i t s maximum f ie ld opera t ing po in t i s found by mult iplyin? the number given in the f igure by ( E a in M V / m ) . The operating frequency of the device i s i n d i c a t e d by the band designat ion, X , S, etc . Squares indicate a mult i -cel l accel- e ra t ing device , c i rc les a s ing le ce l l a n d t r i a n g l e s the low f i e l d l o s s e s f o r a se lec t ion of s ing le ce l l accelerating devices. Sources of the data are in- dicated by superscript numerals adjacent the points. Each point represents a real accelerating device with beam holes and pe r t a ins t o i t s ope ra t ion i n the fundamental accelerating mode.

* Points plotted are representative rather than

comprehensive. Separator devices are n o t included.

1

A

L

n 0 0

S

o5

LOW FIELD SINGLE CELL

HIGH FIELD SINGLE CELL

HIGH FIELD MULTI. C E L L

8 10 12 14 16 18 20 22 24 26 Ea-MV/m-Max-CW OPERATION

i 28

F ig . 1 Wal l losses per meter per (MV/m) V.S. max. a c c e l e r a t i n g f i e l d s a t v a r i o u s f r e q u e n c i e s f o r Nb s t r u c t u r e s o f v a r i o u s d e s i g n s .

2

EVALUATION OF CURRENT Nb D E V I C E PERFORMANCE

The abso lu te va lues o f t he pa ramete rs and t he t r e n d s e v i d e n t i n t h e d a t a c a n be e x p l a i n e d o n l y p a r t i a l l y i n more fundamental terms. A more com- p l e t e a n d q u a n t i t a t i v e u n d e r s t a n d i n g i s s t i l l i n t h e f u t u r e .

The l o w f i e l d l o s s e s a r e v e r y good. D e s p i t e t h e a l m o s t t w o o r d e r s o f m a g n i t u d e r a t i o o f o p e r a t i n g f r e - q u e n c i e s r e p r e s e n t e d i n t h e p l o t , t h e a c t u a l s u r f a c e r e s i s t a n c e s i m p l i e d a r e a l l i n t h e nanoohm range. Thus, the ach ieved losses i n r e a l d e v i c e s r e f l e c t t h e l o w r e s i d u a l r e s i s t a n c e r a t h e r t h a n t h e BCS r e s i s t - ance. A t h i g h f i e l d s t h e l o s s e s a r e a t l e a s t an o r d e r o f m a g n i t u d e h i g h e r t h a n a t l o w f i e l d s . It i s g e n e r a l l y b e l i e v e d t h a t h i g h f i e l d e f f e c t s on t h e r e s i d u a l r e s i s t a n c e p l a y n o i m p o r t a n t r o l e i n t h i s b e h a v i o r b u t t h a t , b e c a u s e o f i t s e x p o n e n t i a l depen- dence on surface temperature, i t i s t h e BCS component t h a t i s r e s p o n s i b l e . The BCS r e s i s t a n c e has a depen- dence on the su r face magne t i c f i e ld as we l l as on t he t e m p e r a t ~ r e . 3 ~ Perhaps t h i s e f f e c t m a n i f e s t s i t s e l f a t t h e h i g h e s t f r e q u e n c i e s w h e r e we a r e g e t t i n g c l o s e t o b u l k c r i t i c a l f i e l d s . 2 6 H e a t i n g i s p r o b a b l y s t i l l t h e d o m i n a n t e f f e c t a t p r e s e n t l e v e l s o f p e r f o r m a n c e . P u t t i n g t h e BCS t e m p e r a t u r e a n d f i e l d dependences i n t h e rf d i s s i p a t i o n t o g e t h e r w i t h t h e h e a t t r a n s f e r which i s gove rned by t he t he rma l conduc t i v i t y , wa l l t h i c k n e s s and kap i t za res i s tance be tween wa l l and

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bath, , one can f ind, ad hoc, s m i - q u a n t i t a t i v e a g r e e - men t w i th some d a t a a t X-band.56 For example, under ce r ta in c i r cums tances , a decrease i n w a l l t h i c k n e s s c a n i n c r e a s e t h e a c h i e v a b l e f i e l d l e v e l b y t h e p r e - d i c t e d amount and the achieved f i e l d l e v e l i s c o n s i s - t e n t wi th t h e c a l c u l a t i o n . The parameters which d e s c r i b e t h e s e c i r c u m s t a n c e s a r e n o t c l e a r . A t S-band, assemblages o f c a v i t i e s h a v i n g a f a c t o r o f t w o d i f f e r e n t w a l l t h i c k n e ~ s e s l ~ * ~ ~ , a c h i e v e a b o u t t h e same maximum f i e l d s . T h e r e a r e c o m p l i c a t i n g fac to rs wh ich p revent the emergence o f a c l e a r p i c - t u r e a t o u r p r e s e n t s t a t e o f k n o w l e d g e : t h e s o u r c e s o f h e a t v a r y i n d i f f e r e n t f r e q u e n c y and geometry regimes, t h e r m a l c o n d u c t i v i t i e s v a r y o v e r t h e s u r f a c e s , e . g . , a t w e l d s , t h e k a p i t z a r e s i s t a n c e may p l a y a r o l e i n some c i rcumstances and not i n o t h e r s , t h e Tc v a r i e s Over t h e s u r f a c e , 43y44 l o c a l h e a t i n g r a t h e r t h a n genera l heat ing governs the breakdown, loca l breakdown 25 t h e s u p e r f l u i d h e a t t r a n s p o r t may p l a y a r o l e .

L ikewise the genera l tendency to lower b reakdown f i e l d s a t l o w e r f r e q u e n c i e s c a n b e e x p l a i n e d o n l y i n vague terms a t t h e moment. T h e r e a r e s e v e r a l f a c t o r s wh ich p robab ly p lay a r o l e h e r e : t h e f i e l d a t w h i c h a g i v e n o r d e r o f m u l t i p a c t o r i n g will occur i s p r o p o r - t i o n a l t o f r e q u e n c y , - l o w e r f r e q u e n t y s t r u c t u r e s h a v e l a r g e r s u r f a c e a r e a s a n d a r e h a r d e r t o h a n d l e a n d process, so t h a t t h e p r o b a b i l i t y o f h a v i n g d e f e c t i v e o r p o o r l y p r o c e s s e d a r e a s i s l a r g e r . 4 7 The lower f r e q u e n c y s t r u c t u r e s t e n d t o have deeper and longer w e l d s , a l s o i n c r e a s i n g t h e p r o b a b i l i t y o f d e f e c t s . I n d i s c u s s i n g t h e h i g h e r f i e l d f o r h i g h e r f r e q u e n c y t rend, one impor tan t caveat must be observed: a t X-band, t h e m u l t i - c e l l s t r u c t u r e d a t a p l o t t e d a r e f o r two c e l l s t r u c t u r e s . The o n l y t r u e m u l t i - c e l l d a t a a re f rom an X- band separator mode136 where a f i e l d o f 74mT was recorded, and a 32 c e l l a c c e l e r a t o r u n i t j u s t b e i n g p u t i n t o o p e r a t i o n , 3 7 n o d a t a b e i n g a v a i l - a b l e a t t h e moment. The s e p a r a t o r f i e l d i s r o u g h l y e q u i v a l e n t t o t h e l o w e s t m u l t i - c e l l X-band p o i n t p l o t t e d .

The d i f f e r e n c e i n p e r f o r m a n c e b e t w e e n s i n g l e c e l l and m u l t i - c e l l s t r u c t u r e s o f t h e same des ign a n d f r e q u e n c y a r e b e l i e v e d t o b e d u e i n l a r g e p a r t t o t h e l e s s s e v e r e m u l t i p a c t o r i n g e n c o u n t e r e d i n s i n g l e c e l l s , o w i n g t o t h e s l i g h t l y d i f f e r e n t f i e l d p a t t e r n s , a n d i n p a r t t o t h e r e l a t i v e ease o f c l e a n i n g and t r e a t i n g t h e s m a l l e r u n i t s .

i n performance among d i f f e r e n t examples o f t h e same des ign and f requency? Noth ing more d ramat ica l l y d e m o n s t r a t e s t h a t t h e r e a r e s t i l l p a r a m e t e r s w h i c h a r e n o t u n d e r o u r c o n t r o l a n d a r e t h e r e f o r e n o t understood. Even t h e b e h a v i o r o f a s i n g l e u n i t r e p e a t e d l y s u b j e c t e d t o an " i d e n t i c a l " p r o c e s s i n g c y c l e c a n show f a c t o r s o f 2 o r 3 i n l o s s and peak f i e l d , n o t n e c e s s a r i l y c o r r e l a t e d , f r o m c y c l e t o cycle. These cycles remove small amounts o f m a t e r - i a l . Are we r e v e a l i n g d i f f e r e n t b a d s p o t s i n t h e m a t e r i a l as we process? Do t h e p r o c e s s i n g s o l u t i o n s have inhomogeneous composition o r i n t e r n a l t e m p e r a - t u r e g r a d i e n t s , a r e p h y s i c a l o r c h e m i c a l i n h o m o g e n i e - t i e s on t h e s u r f a c e r e s p o n s i b l e f o r n u c l e a t i n g b a d ox ide growth i n a s t o c h a s t i c f a s h i o n ? A r e r e s i d u e s o f t h e p r o c e s s i n g f l u i d s c l i n g i n g t o t h e s u r f a c e m o r e o r l e s s w i t h e a c h c y c l e ? A r e t h e r e p h y s i c a l a n d chem- i c a l e f f e c t s we d o n ' t know about?

D e s p i t e t h i s r a t h e r n e g a t i v e r e c i t a l we shou ld n o t l o s e s i g h t o f t h e f a c t t h a t w e ' v e done r a t h e r w e l l a t X-band. I t a k e t h i s a s a s t r o n g i n d i c a t i o n t h a t we now unders tand many o f t h e i m p o r t a n t p a r a - m e t e r s . C l e a r l y t h e f i r s t o r d e r o f b u s i n e s s i s t o unders tand why s t r u c t u r e s s c a l e d down f rom X-band t o l ower f requenc ies do no t ope ra te up t o X-band s tan - dards. The u n d e r s t a n d i n g o f t h e d i f f e r e n c e b e t w e e n c u r r e n t X-band per fo rmance and the idea l and, to

F i n a l l y , c a n we e x p l a i n t h e r a t h e r l a r g e s c a t t e r

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some extent , of the f luctuat ions in individual uni t performance belong t o the fu ture .

FUTURE DIRECTIONS IN Nb D E V I C E DEVELOPMENT

The evidence gathered to date indicate strongly that multipactoring a n d l in i i ta t ions in hea t t ransfer a re respons ib le for the re la t ive ly poor performance of the lower frequency structures. This has been suspected for some time b u t we were unable t o see how the secondary electron multiplication process could opera te a t the high f ie lds achieved, even though we could see the e lectrons. The new element i s , t h e understanding of the one p o i n t multipactoring which depends on the interplay of the ra ther small t rans- v e r s e e l e c t r i c f i e l d w i t h the l a rge r f magnet ic f ie ld a t t h e o u t e r wall .27y30,7 To t e s t t hese i deas t he technology base needs t o be expanded t o include methods for suppressing multipactoring and for in - c r eas ing , ma te r i a l ly , t he hea t t r ans fe r ab i l i t y of the s t ruc tures . Methods for suppressing multipac- tor ing b surface coatings such a s TiN, Ti a n d Rh38 and NbNL16 have been inves t iga ted for some time without great success. The u t i l i t y of helium processing for get t ing through high order mult ipactor barr iers suggests t h a t adsorbed or frozen-on gas l aye r s a r e s ign i f icant cont r ibu tors t o the secondary emission c o e f f i c i e n t so t h a t su h coatings may be i r r e l e v a n t . Recent work shows t h a t 2 9 the coat ings t r ied so f a r have secondary emission coefficients greater t h a n one i f t hey a r e exposed t o a i r . Successful appl ica- t ion of coatings would appear t o r equ i r e some s o r t of in s i tu deposi t ion and cleaning of the surfaces. The appreciat ion of the importance of one point mult ipl icat ion gives us another handle by suggesting that cavi ty shaping may allow us t o a l l e v i a t e t h e problem.27 One should n o t be too optimistic, given the past h is tory of this business . The problems seem t o have an onion l i k e s t r u c t u r e : no sooner i s one layer of d i f f i c u l t y overcome than one sees another r i g h t behind i t . I f we are ab le t o overcome the 2 n d order mult ipactor ing barr ier we a l l now seem t o be u p aga ins t , we may find thermal breakdown due t o defec ts or f ie ld emission problems right behind.

increase of surface area and decrease in wall thickness need development. Some success along these l i n e s has already been reported.26~39 Considerable work wil l be necessary t o learn how t o bu i ld fu l l s t ructures with very thin wal ls of suff ic ient r igi- d i t y and enhanced surface area, should this step turn o u t t o be c r u c i a l .

The behavior of existing devices gives other in- d ica t ions of areas of po ten t i a l p ro f i t i n s t ruc tu re work. The prevalence of fluctuations from uni t t o un i t and t h e f a c t t h a t breakdown occurs in well local ized places , suggests that large accelerat ing units should be b u i l t u p from smaller subuni ts which can be individual ly tes ted on a semi-automatic basis. In t h i s way high performance u n i t s can be se lec ted f o r assembly i n t o the f inal uni t whi le substandard uni t s can be repaired by recycle or rework of the bad spot . Uni ts that cannot be improved can be scrapped. Location and r e p a ' r of bad spots has been e f f e c t i v e a t low frequenciesJ3 b u t has been l e s s so a t microwave frequencies. This c l e a r l y i s an area where some emphasis should be p u t . Another area for p o t e n t i a l p r o f i t i s i n t h e e l i m i n a t i o n o f welds. Fabrication of mul t i - ce l l un i t s so l e ly by drawing or hydroforming would save on c o s t and reduce the pro- babi l i ty o f defec ts .

F ina l ly , weshould not f a i l t o take another lesson t h a t the accumulated experience has taught: the best s t ruc tu re i s t he s imp les t s t ruc tu re . The more complex a s t r u c t u r e i s t h e more cos t ly i t i s t o f a b r i c a t e and the more d i f f i c u l t i t i s t o process. I t i s h a r d t o

With regard t o thermal transport, simultaneous

over emphasize th i s po in t . An important aspect of the application of rf

superconductivity t o acce le ra to r s o f t en l e f t o u t i s t ha t t he re i s considerably more t o an acce lera tor than a low loss , h ig f ie ld cav i ty . The xperience with the recyclotron$l and the m i ~ r o t r o n ~ ~ show the central role played by modes of the s t ruc ture o ther than the fundamental which can be beam or mult i - pactor exci ted. A ra ther l a rge number of ad hoc damping probes with a l l their a t tendant construc- t ional and cryogenic complexity are absolutely necessary. Heavy damping of higher modes wil l a lso be necessary for other appl icat ions. Thus, in addi t ion to the fea tures l i s ted above , the s t ruc tures o f the fu ture wi l l have t o accommodate the higher mode damping ab i n i t i o .

ARE PRESENT CAPABILITIES USEFUL AND COF,IPETITIVE

The vertical arrows in Fig. 1 indicate performance l e v e l s we can expect today a t the f requencies marked. Are the re u ses fo r t he a r t a t t h i s p re sen t s t a t e i n which the advantages offered by superconducting devices are so compelling as to justify the obvious r i s k s ? There are ongoing projects for the

c o n s % ~ s b UHF. Some of the progress will be reported a t ion of heavy ion accelerators operating a t

this conference. Electron accelerators for low and intermediate energy physics have been under con- s t ruc t ion fo r some time.41 9 4 9 y 5 0 For a r e l a t i v e l y small acce le ra tor , espec ia l ly of the recirculat ing type , in which t h e f a c i l i t i e s c o s t w i l l be substan- t i a l compared t o t h e a c c e l e r a t o r , i t i s n o t c l e a r that conventional technology wouldn't be very compe- t i t i v e . Using well shaped c a v i t i e s a t S-band, one might obtain 75 NQ/m shunt impedance and so achieve an acce le ra t ing f i e ld of 2 M V / m f o r an expenditure of 53 K w h . The r f power f o r a 50 meter l inac giving 100 MeV would be 2-2/3 MW which can be supplied by two klystrons. Such a system will be very re l iable and, given the relatively low Q f o r the higher modes, r a t h e r l e s s l i a b l e t o beam breakup problems.

t i o n , one would c e r t a i n l y use the highestpossible frequency. As wi 1 1 be discussed momentarily, perhaps C-band would be most appropriate .

For a separator one would c e r t a i n l y choose superconductivity if the device were t o be used with a long pulse accelerator, just on power grounds, s ince the shunt impedance of the se a ra tor s t ruc ture i s r a t h e r low. The C E R N separator5y works. A second version would sure ly work b e t t e r and with higher performance levels.

the cw f ie ld level required must be high compared t o conventional standards and the t o t a l power must be kept low. Such an a p p l i c a t i o n i s a t hand. The success of e lectron-posi t ron col l iding beams in revealing new aspects of sub nuclear behavior has inspired many to contemplate an instrument capable of producing the neutral intermediate boson. This i s desired both for studying the nature of the weak i n t e r a c t i o n i t s e l f and as a copious source of other e lementary par t ic les . Such a machine wil l require e lec t ron beam energies of 50-100 GeV. A t present i t appears that the most prac t ica l form f o r such a machine would be a s torage r ing. The copious synchrotron radiation from such a n instrument will require accelerat in cavi ty vol tages in the range from 1 .7 to 3 .7 GV59~53. The nature of the beam dynamics in such a machine requires that the operating frequency be in the UHF o r L bands. The high peak currents in the beams requires a large s tored energy in the cavity a n d thus high f i e l d s . The sheer s ize o f the acce le ra t ion requi res the minimum possible power f o r e s t a b l i s h i n g t h e f i e l d s i f such a

I f one were t o o p t f o r a superconducting solu-

The most natural applications are those in which

19

d e v i c e i s t o be p r a c t i c a l . The appl ica t ion of r f superconductivity to these machines have been examined by several groups.53~54 One group concluded53 t h a t a f a c t o r of two overall savings in the c o s t of the instrument could be had by the use of superconducting r f . To produce several GV,'a cavity length of the order of a km or two i s needed. Clearly one would not launch such a project without a l a r g e s c a l e t e s t in an ex is t ing s torage r ing . The beginning of such t e s t s a r e be ing planned actively in Europe.55 I t w i l l be su rp r s ing i f s tud ie s a long t ha t l i ne a r e no t soon begun in t he U.S. I t should be noted that a cent ra l fea ture o f cav i t ies deve loped for th i s appl ica t ion must be a well engineered mechanism fo r t he damping of t h e beam exci ted modes. The extracted power must be removed t o a room temperature sink to prevent excessive refr igerat ion requirement and beam in- s t a b i l i t i e s . For one case studied,53 the beam ex- c i t ed l o s ses t o h ighe r modes i s about 2 MW t o t a l or about 2 kW per meter of structure. This repre- sen ts a substantial engineering challenge.

As an exe rc i se t o expose the economic potent ia l of the present state o f t h e a r t , one might ask for t he cos t of a very l a rge l inac bu i l t us ing ex is t ing technology. Figure 2 shows the cos t fo r a 100 GeV electron l inac bui l t us ing the technology developed a t Cornel1 f o r S and X-bands, projected t o frequencies between L and X-band. The cos ts o f the s t ruc tures and non-refrigeration i tems are based on current experience. The r e f r ige ra t ion cos t s a r e based on a compilat ion of recent refr igerator costs .56 I n addition, Fig. 2 contains the cos ts for poss ib le advance states of superconducting technology.

600 -

500 -

* 400 - I I- cn 0 V

300 -

200 -

100 -

PRESENT TECHNOLOGY HAS q . f "9

2 4 6 8

I I I I I I I I

IO ih 14 16 18 20 I

Emax f: 3

Fig. 2 Cost estimate opera t ing f ie ld wi th parameter.

The approximate empir

I I Emax Emax f :6 f.9

E MeV/m

f o r a 100 GeV cw l i n a c vs. ~ ~ 1 0 - 9 ) x frequency as a

cal formula for the costs,

i s based upon cos t s fo r small s t r u c t u r e u n i t s : c

Ctot% L9000 f ( 7 9450 f 650)f ( 7 3000 + 1000) +

Proceeding from 1 . t o r . inside the large brack- e t s on the r .h . s . , we have the terms representing ( 1 ) the standard components, e.g., vacuum, instruments and con t ro l s , t unne l , p ip ing , r f d i s t r ibu t ion , sup- ports and alignment, quadrupoles, etc. , ( 2 ) the acce le ra t ing s t ruc tu re , ( 3 ) t he c ryos t a t , ( 4 ) t he r e f r i g e r a t o r . f i s the opera t ing f requenc in GHz, q i s the Q o f the s t ruc ture i n units of 108, E i s t he t o t a l beam energy in GeV and & i s t he ope ra t ing acce le ra t ing f i e ld i n M V / m . The crucial parameter turns out to be q . f . , q because that governs losses a t a fixed frequency and f because the r /Q f o r a constant geometry scales with f. For the pa r t i cu la r type of hardware picked,l6$13 q.f. = 9. On the low f i e l d s i d e t h e c o s t s r i s e because the structure length becomes excessive. On the h igh f ie ld s ide the c o s t s r i s e because the refr igerator cost becomes excessive. The top three curves show the present s t a t u s of th i s par t icu lar t echnique . In these cases we believe the operating Q t o be l imited n o t by the superconductivity b u t by the s t ruc ture to le rances achieved, coupled with the presence of j o i n t s , coupl ing ports , probes, e tc . . C-band i s favored because of i t s r e l a t i v e l y high operating f ield and r e l a t ive ea se of meeting construction tolerances as compared with X-band. I f we could increase q . f . by a f a c t o r of 4 , the cos t would be halved and C-band would probably s t i l l be the choice.

One might ask why we have n o t b u i l t small and very compact acce le ra to r s fo r i ndus t r i a l and medical uses using this technology. Perhaps a natural skep t i c i sm r ega rd ing t he r e l i ab i l i t y i s pa r t i a l ly t o blame. However, the biggest stumbling block i s t h e lack of closed 2-4Ok refr igerat ion devices with the r e l i a b i l i t y and ease of operation of home and industr ia l uni ts for higher temperatures . The ex is - tence of such units would open a host of new appl i - cat ions.

OTHER MATERIALS

Only passing reference has been made t o mater- ia l s o ther than niobium. Great s t r i d e s have been made in the use of lead plated copper s t r ~ c t u r e s . ~ ~ The performance of lead i s i n h e r e n t l y worse than Nb and Nb has turned o u t t o be qui te easy to work with. Thus, lead will probably be confined t o spec ia l ized appl icat ions. A l a rge number of other mater ia ls have been d i scussed i n t he l i t e r a tu re , e spec ia l ly the A-15 compounds. The technology of t h e i r u s e i s a t p re sen t so involved tha t eva lua t ing the i r u t i l i ty must await future developments.

CONCLUSIONS

Despite i t s r a t h e r long history, rf supercon- d u c t i v i t y s t i l l has a long way t o go t o prove i t s worth f o r a wide range of applications. I t appears t h a t t h e r e a r e a p p l i c a t i o n s f o r s c i e n t i f i c i n s t r u - ments where i t can make a s ignif icant impact . I t wi l l have u t i l i t y for common industr ia l devices only in the fu ture , i f ever . To bend the phenomenon t o our needs now and in the future , considerable advance in the engineering aspects of i t s use and i n i t s b a s i c development are necessary. The engineering advances will only come through the aggressive application of the art a t i t s p r e s e n t l e v e l t o instruments requiring

20

i t s pecul ia r advantages . I n advancing the engineering aspects through building large numbers of ident ical devices for a n accelerat ion appl icat ion, some more basic advances will emerge: in coping with the epidimology of a large number of supposedly iden- t i c a l u n i t s , some of the important parameters of the mater ia l s and preparation methods, parameters now only dimly perceived, will reveal themselves. T h u s , both extensive engineering work on devices having immediate payoffs and basic development of the art and science of superconductivity will be needed f o r r e a l i z a t i o n of the fu l l po ten t ia l o f the phenomenon. Those of us, t h e f a i t h f u l , who a r e s t r i v i n g t o make rf superconduct ivi ty our servant , have a long, hard road ahead of us before we have shown t h a t t h e promise i s r e a l .

ACKNOWLEDGEMENTS

I am great ly indebted to H. Padamsee f o r h i s ins t ruc t ion and correct ion in the course of preparing this review. Absolutely indispensible also were extensive information and thoughtful ly wri t ten comments from representat ives of many of the pr inci- pal i n s t i t u t i o n s engaged i n t h e f i e l d : A. Ci t ron, J . H a l b r i t t e r , W . Bauer, P . Kneisel of the Kernforschungszentrum, Karlsruhe; J. Turneaure of Stanford H E P L ; Lowell Bollinger of Argonne; A. 0. Hanson o f U. o f I l l i n o i s ; G. J . Dick of Cal. Tech.; H. Hahn of Brookhaven; H . P f i s t e r , Siemens A G ; and H . P i e l , Gesamthochschule Wuppertal.

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