Rf guns: bright injectors for FEL

Nucl ear I nst r ument s and Met hods i n Physi cs Resear ch A304 ( 1991) 285- 296 Nor t h- Hol l and 285 Sect i on V. Accel er at or t echnol ogy Rf guns : br i ght i nj ect or s f or FEL C. Tr avi er Labor at oi r e de ! ' Accél ér at eur Li néai r e, I N2P3 - CNRS and Uni ver si t é de Par i s- sud, 91405 Or say Cédex, Fr ance I nvi t ed paper Fr ee el ect r on l aser s ( FELs) r equi r e ver y br i ght el ect r on beams . Dur i ng t he past f ew year s, r f guns have demonst r at ed t hei r pot ent i al i t y t o pr oduce such br i ght beams . A r evi ew of t he on- goi ng wor l dwi de ef f or t t o devel op t hi s new t echnol ogy i s gi ven . Pr esent l i mi t at i ons and possi bl e i mpr ovement s ar e al so pr esent ed. Gener al desi gn consi der at i ons ar e i nt r oduced . 1 . I nt r oduct i on FELs r equi r e ver y good qual i t y el ect r on beams . A l ow emi t t ance i s necessar y t o obt ai n shor t wavel engt hs . A hi gh peak cur r ent pr ovi des a good gai n . These r equi r ement s ar e al r eady met by t he best convent i onal i nj ect or s ( dc gun + buncher ) f or t he i nf r ar ed spect r um. However , a need f or br i ght er beams i s r eal t o i mpr ove I R- FEL per f or mances and t o obt ai n shor t er wavel engt h l aser s . Ther ef or e, new i deas ar e necessar y si nce convent i onal i nj ect or s have near l y r eached t hei r l i mi t s [ 1] . I n 1983, Madey and West enskow [ 2] pr oposed t o put a t her mi oni c cat hode i n an r f cavi t y. Thi s new gun named " r f gun" or " mi cr owave gun" was used as a br i ght el ect r on sour ce f or t he Mar k I I I FEL. Meanwhi l e, Lee et al . [ 3] r epor t ed t hat ver y hi gh cur r ent densi t i es coul d be obt ai ned f r om semi conduct or phot ocat hodes . Then, Fr aser and Shef f i el d [ 4, 5] exper i ment ed t he use of such cat hodes i n an r f gun. The " f i r st demonst r at i on of a FEL dr i ven by el ect r ons f r om a l aser i r r adi at ed phot ocat hode" was made at St anf or d i n 1988 [ 6] . Si nce t hat t i me, many l abor at or i es have under t aken t he necessar y ef f or t t o desi gn and bui l d an r f gun, wi t h or wi t hout a phot ocat hode . Al most 30 pr oj ect s ar e i dent i f i ed i n t hi s r evi ew [ 7] . Fi g . 1 shows t he exponent i al evol ut i on of t he number of pr oj ect s over t he past ei ght year s . I t al so shows t he i ncr easi ng pr eponder ance of phot oi nj ect or s . Fi g . 2 shows t hat hal f of t hese r f gun pr oj ect s ar e devot ed f or FEL i nj ect or s . The ot her hal f i s devel oped f or di f f er ent pur poses such as TeV col l i der s, st or age r i ng i nj ect or s, accel er at or t est f aci l i t i es, et c. A r evi ew of t hese pr oj ect s i s pr esent ed. The most si gni f i cant ar e descr i bed. 30 ci l U w ari 25 THERMI ONI C RF GUNS LASER- DRI VEN RF GUNS El FI ELD EMI SSI ON RF GUNS 20 0- 15 O m w 10 m U_ z 5 83 86 87 88 89 YEAR Fi g . 1 . Cur r ent number of r f gun pr oj ect s . 84 85 90 er at i ng, el ect r ons can be ext r act ed. Fi el ds obt ai ned i n an r f cavi t y ar e much hi gher t han do f i el ds . Thi s pr oper t y i s f avour abl e si nce at l ow ener gy, el ect r on beams ar e subj ect t o space- char ge f or ces t hat l ead t o emi t t ance gr owt h . Ther e exi st t hr ee t ypes of r f guns accor di ng t o t he cat hode t hat i s used . 30 U w 25 am 20 a 15 O m w 10 m z 5 2. Rf gun pr i nci pl e The r f gun pr i nci pl e i s r at her si mpl e . A cat hode i s pl aced i n an r f cavi t y . When t he el ect r i c f i el d i s accel - N ® 83 84 86 87 88 89 YEAR Fi g . 2 Rf gun pr oj ect s dedi cat ed t o FEL . 0168- 9002/ 91/ $03 . 50 © 1991 - El sevi er Sci ence Publ i sher s B. V. ( Nor t h- Hol l and) 85 90 V. ACCELERATOR TECHNOLOGY 28 6 C Tr avi er / Rf guns br i ght i nj ect or s f or FEL 2. 1 . Ther mi oni c r f gun El ect r ons ar e cont i nuousl y emi t t ed by a t her mi oni c cat hode but can onl y be ext r act ed and accel er at ed dur i ng hal f an r f cycl e. The el ect r ons, emi t t ed dur i ng t he accel er at i ng hal f cycl e, but wi t h a t oo l ar ge phase ( t ypi cal l y l ar ger t han 100' ) do not have enough ener gy t o r each t he cavi t y out put and ar e accel er at ed backwar d t o t he cat hode . The pul se t hat act ual l y comes out t he cavi t y i s ver y l ong ( about one f our t h of t he r f per i od) and has a ver y l ar ge ener gy spr ead . I t i s t hus necessar y t o pl ace behi nd t he gun a magnet i c bunchi ng syst em ( e . g . , an " a- magnet " ) wi t h ener gy sl i t s t o r educe t he pul se l engt h, i ncr ease, t he peak cur r ent and sel ect a gi ven spect r um. 2 . 2 . Laser - dr i ven r f guns A phot ocat hode pl aced i n an r f cavi t y i s i l l umi nat ed by a l aser whi ch del i ver s shor t pul ses . Ther ef or e as t hey l eave t he cat hode, el ect r ons ar e al r eady bunched . Besi des t he obvi ous advant ages ( hi gh f i el d, bunched beam) , l aser - dr i ven guns have ot her at t r act i ve f eat ur es . The pul se f or mat i s mor e f l exi bl e t han t hat of convent i onal i nj ect or s and t hat of t her mi oni c r f guns. I t depends essent i al l y on t he l aser pul se f or mat whi ch can be var i ed over a wi de r ange. Phot ocat hodes can del i ver much hi gher cur r ent densi t i es t han t her mi oni c cat hodes : mor e t han 400 A/ c m2 has been r epor t ed [ 3] . 2 . 3 Fi el d emi ssi on r f guns When pl aced i n a hi gh f i el d, shar p needl es can pr oduce el ect r ons by t he so- cal l ed f i el d emi ssi on pr ocess . Thi s pr i nci pl e can be used i n an r f gun. The di f f i cul t y i s t o f i nd st abl e oper at i ng condi t i ons . When hi gh cur r ent i s emi t t ed, t he needl e i s heat ed and easi l y dest r oyed. 3. Wor l dwi de r evi ew Si nce 1983, many l abor at or i es have l aunched R&D pr ogr ams on r f guns . Some ar e al r eady pr oduci ng exper i ment al r esul t s whi l e ot her s ar e st i l l at t he pr el i mi nar y desi gn st age. Revi ewi ng al l t hese pr oj ect s i s a di f f i cul t t ask, due t o t he f ast changi ng si t uat i on and t he l ack of l i t er at ur e f or t he most r ecent pr oj ect s . Thus, t he r evi ew pr esent ed her e i s pr obabl y i ncompl et e and f or some cases not up- t o- dat e . May t hose who wi l l not f i nd t hei r wor k r epor t ed her e or who wi l l f i nd i t r epor t ed i naccur at el y, be compr ehensi ve and bel i eve t hat t hey ar e not t he subj ect of a speci al " censur e" . Anot her di f f i cul t y met dur i ng t hi s r evi ew was t o pr oduce consi st ent l i st s of par amet er s mor der t o f ai r l y compar e t he di f f er ent pr oj ect s . Each aut hor uses hi s own uni t s and def i ni t i ons of t he par amet er s and of t en al l t he par amet er s ar e not gi ven f or t he same oper at i ng condi t i ons . The f ol l owi ng convent i ons ar e assumed . Desi gn dat a f or emi t t ance ar e al ways gi ven as f our t i mes t he r ms nor mal i zed emi t t ance . When an aut hor uses a di f f er ent def i ni t i on, necessar y adj ust ment s ar e made . Exper i ment al dat a ar e r epor t ed as gi ven i n t he publ i cat i ons . Si nce i n some cases r eal beams have much l onger t ai l s t han i deal beams and have not necessar i l y Gaussi an or uni f or m pr of i l e, t he " 4 r ms" emi t t ance may not be appr opr i at e t o descr i be t hem. For l aser - dr i ven guns, t he r mcr opul se l engt h i s t aken as 4a b , wher e a b i s t he r ms bunch l engt h . Rms def i ni t i on ( l a b ) i s adopt ed f or t her mi oni c guns because of t he pul se shape whi ch has a l ong t ai l and a ver y shar p f r ont end . I n t hi s case, l a b i ncl udes most of t he par t i cl es . When onl y t he char ge Q and t he bunch l engt h T ar e gi ven, t he peak cur r ent I i s cal cul at ed i n a conser vat i ve way assur i ng a uni f or m di st r i but i on ( I = Q/ T ) . Tabl e 1 l i st s t he di f f er ent pr oj ect s f or each t ype of r f gun . St at us of t he pr oj ect s and t hei r mai n pur pose ar e gi ven . I n t abl e 2, t he mai n par amet er s of each pr oj ect ar e gi ven . Accor di ng t o t he pr oj ect st at us, t hese par amet er s cor r espond t o exper i ment al or desi gn val ues . A mor e det ai l ed descr i pt i on of each pr oj ect can be f ound i n r ef . [ 1] . 3. 1 . St anf or d ( HEPL) - Duke ( DFELL) The f i r st t her mi oni c r f gun was bui l t at St anf or d Hi gh Ener gy Physi cs Labor at or y ( HEPL) by West enskow and Madey [ 2] t o ser ve as an i nj ect or f or t he Mar k I I I accel er at or dedi cat ed t o FEL st udi es . The desi gn wor k st ar t ed i n 1983 . The whol e syst em ( r f gun + l mac + wi ggl er ) was oper at ed as a l aser osci l l at or i n Sept ember 1985 . The syst em was t hen di sassembl ed and moved t o t he St anf or d Phot on Resear ch Labor at or y wher e i t was l asi ng agai n i n Sept ember 1986 . I n August 1987, a new moment um f i l t er whi ch i ncr eased t he peak cur r ent was i nst al l ed . I n Sept ember 1988, a new gun cavi t y al l owed a mor e st abl e oper at i on wi t h l onger r f macr opul ses ( 8 Ws) r epeat ed at a hi gher f r equency ( 15 Hz) . The t her mi oni c gun consi st s of a LaB6 cat hode pl aced i n an r f cavi t y oper at ed at 2857 MHz . The cavi t y i s f ol l owed by a t r anspor t syst em i ncl udi ng an " a- magnet " wi t h moment um f i l t er and sever al quadr upol es, as shown i n f i g. 3. I n or der t o cont r ol t he backbombar dment , a t r ansver se magnet i c f i el d at t he cat hode was added [ 8] . A summar y of t he gun' s mai n par amet er s and per f or mances i s gi ven i n t abl e 2 [ 9] . Af t er t he " ci - magnet " , 2 t o 3 ps el ect r on pul ses cor r espondi ng t o 20 t o 40 A peak cur r ent ar e obt ai ned wi t h emi t t ances as good as 41T mmmr ad i n t he ver t i cal pl ane . I n col l abor at i on wi t h Rocket dyne di vi si on of Rockwel l Cor por at i on, t he same gun was oper at ed as a C. Tr ai ner / Rf guns - br i ght i nj ect or s f or FEL 28 7 phot oi nj ect or and l ed i n November 1988 t o t he f i r st successf ul oper at i on of a phot oi nj ect or dr i ven FEL [ 6] . The LaB6 cat hode was i l l umi nat ed by a t r i pl ed El ect r on Laser Labor at or y ( DFELL) wher e i t schedul ed t o be oper at i ng agai n i n 1990 . was Nd : YAGl aser . Because t he l aser pul se was qui t e l ong ( 100 ps) , t he " a- magnet " was st i l l necessar y. Al t hough t he exper i ment was r un f or onl y a ver y shor t t i me, t hi s phot oi nj ect or gave bet t er r esul t s ( i n t er ms of cur r ent and emi t t ance) t han t he t her mi oni c gun [ 10] . The FEL 3. 2. Los Al amos Nat i onal Labor at or y per f or mances suggest r ms emi t t ances as good as 2m mmmr ad i n t he ver t i cal pl ane . The par amet er s and per f or mances ar e gi ven i n t abl e 2 [ 9] . The whol e syst em was t hen di sassembl ed agai n i n December 1988 t o be moved t o Duke Uni ver si t y Fr ee I n 1985, Lee et al . [ 3] demonst r at ed t he possi bi l i t y of ext r act i ng hi gh cur r ent densi t y ( over 200 A/ c m2 ) f r om CS3 Sb phot ocat hode i l l umi nat ed by a f r equency doubl ed Nd : gl ass l aser . The f i r st r f gun desi gn consi st ed of one cavi t y oper at i ng at 1 . 3 GHz and i ncor por at i ng a Si nce at l east 1984, Los Al amos Nat i onal Labor at or y ( LANL) i s i nvol ved i n R&D pr ogr ams about l aser dr i ven r f guns and r el at ed t opi cs l i ke l aser t r on. Tabl e 1 Li st of r f gun pr oj ect s Labor at or y Ther auoni c r f guns St anf or d - Duke [ 2, 10] I HEP Ber i ng [ 451 Mi t subi shi [ 461 I PT Khar kov [ 47] AET/ Var i an/ SSRL [ 431 KEK[ 48] Nor t h Car ol i na Cent r al Uni ver si t y ( NCCU) [ 49] St anf or d Uni ver si t y [ 50] Laser - dr i ven r f guns Los Al amos [ 12] Los Al amos ( HI BAF) [ 24] St anf or d - > Duke [ 6] CEA [ 17, 19] Br ookhaven [ 20, 21] CERN [ 511 Ar gonne [ 52] Wupper t al [ 53] Twent e [ 36] Duke [ 9] Boei ng [ 54] Rockwel l [ 55] Los Al amos ( AFEL) [ 151 I NFN Mi l an [ 37] LAL Or say [ 56] I NFN Fr ascat i [ 57] MI T [ 58] KEK[ 591 UCLA [ 601 Vander bi l t Uni ver si t y [ 9] LLNL/ SLAC/ LBL [ 611 Fi el d emi ssi on r f guns I HEP Bei j i ng [ 62] LAL Or say [ 63] Count r y St at us Sept ember 90 Begi nni ng of desi gn Begi nni ng of t est s Pur pose USA Chi na Japan USSR USA Japan USA t est ed t est ed t est ed t est ed under t est s const r uct i on const r uct i on 1983 6/ 86 2/ 85 2/ 88 FEL FEL 1988? 1989 ? ? 1989 1989 1991 USA desi gn 1990 ? st or age r i ng i nj ect or st or age r i ng i nj ect or FEL/ ot her mi cr owave gener at or FEL USA USA USA Fr ance USA t est ed/ st opped oper at i ng t est ed under t est s under t est s 1984? 9/ 88 1987? 3/ 86 1987? 1985 5/ 89 9/ 88 1989 1989 Swi t zer l and USA Ger many Net her l ands USA USA USA USA I t al y Fr ance I t al y USA Japan USA USA USA r f t est s const r uct i on const r uct i on const r uct i on const r uct i on const r uct i on const r uct i on desi gn desi gn desi gn desi gn desi gn desi gn ? ? st opped 1988? 1988 1987? ? 9/ 89 ? ? 1988? 1988? 9/ 89 ? 1990 ? ? ? 1987 1990 ? ? ? 6/ 91 ? ? ? ? 1991 ? ? ° ? - Chi na Fr ance pr el i mi nar y desi gn st opped 1988? 1988 ? 1/ 89 9 r f gun FEL FEL FEL newaccel er at or met hods/ FEL col l i der wakef i el d accel er at or br i ght i nj ect or FEL st or age r i ng i nj ect or FEL FEL FEL FEL r f gun FEL col l i der col l i der FEL FEL hi gh gr adi ent accel er at or FEL r f gun V ACCELERATOR TECHNOLOGY C. Tr avi er / Rf guns br i ght i nj ect or s f or FEL 288 C ¢ L O N .-1 M ,v dr <: > v r. O N O 00 N O OOO M C N r, O O o n r N N O M MO O z .,4 r-1 c ~ M L' w z 'e Q u. a o I I ~ô , M u. Y O N b n. w . , N w z ¢ x ~ ° h LU = T° I I ô ~o CD N 00 N ° I ô °° O U O O o, - . ~O V , ô' o o° 10 i ~ ^ N Qj N T OI - N o Jz . o "n ~ _ n V' ï b O `r °^ r NN 1 U z F M y U ro s a CD O Mr - O CD N OO N MN T - p~ 1 W v~ . . y c V M M MO o0 O O `o I I U z G M r O ¢ y ô a a - b r, ô O V M U z I I I I -a w x o~o V N X I C V N I V I I I ~ I N I I r I I I ti . O E N N P. O NO I cG ô X O 0 ri C N I v~ o` NO r- O O O O CD n co ~ N "P , ' , O N N ti O -7 b b T C~ ON Vl GO VI ` n rn N r O V C ro V4 O U . 0 i M G N w w W K w CL cC N on C I I I O ti c- I .-1 X 1) O ~ O O 0o ~ oo M C N N ti ô r O 77- , O O V N ~ O x M 3 W w M oa i O J ', O N X r O Ô C . ,. â. N c â0 F â â > 4 3 cN7 c~d aa a. o aq y v ti ~ C â 3 .D C NN l m " Z~ r OV 57 ~ U C on , Q ro U ô b  z O U T cxd C C ~ o cX d P. 5: 7 ro C ~ ? °: , . . a) I O~ :d X_ C c v U b U ~. N > zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONM ? U " 9) Û ti N b W C Ny o G v O M N x 2 N x > Ç ro ro "a Û êê~ bn C N r O i n V1 M O O NI 00 N , ro "0 ti cd U c~C N pp °. b cod9 cG U y 3 °, 0 al d 7 ,^ a a ono 0 cU C C L ~ .V ~ ~ b ro E `C 8 p il. ti A U ~ ~ \ ¢ o ~o Ô û Û . >. . . >., C id N a .. N C ~ .~ C y ~Q ~x x â~3~~ C Tr avi er / Rf guns .- br i ght i nj ect or s f or FEL v -Z - ~i ~. .0 0 ,: :, - In ° ° 0 "T " 4D r , Do - 0 8 LL, w z '1: 1 1 Q) cl 00 S I f 00 CA 6 C4 N "" 1 ' ~ 'r ` Q CA M06 0 ~! 1 2 wl " , 2 0 1 0 ~) 289 o: N N C5 ' t 2 h 0 Am u2 u 0 0 + o p o N ri O z v 0 0 v 2 O o q 0 n ,-, , a} S. ,-r .1 , a 0 5. 5 0 z 6 CD 10 w u 00 0 C, 400 6 t- 0 0 n z" Z ~ 0 - 0J 0 10 il 11 "t I :L Z a ' 0 . o * 1y 0 C - -r - I 0 â C- ) u s! > > ", -Z ;, - g ~~ ~ > ~~ u > Or 7:2 -0 - " ~ -,, ' 53 ~~ ~2'E x " 0 m 42 :' 3 ~ ~ -; ~ - v , .2 m *6 , -, 2 0 z E) = 0 , - ts 0 ?, a, - y t - 10 Ci - u *0 0, ~9 It ~ 2 2 :5 ~ A i E . "0 . ~ E 0 z '. I lu M 0 ' 1 m r.2 LZ ~5 ~-~ 2 S2 Q . E E ' t ; ~ - a' 97 . 0 ' E ~ , lu . m0 0=~ E -C~ U ,d :3 E ~ ~ 04 z -,, Z! N g w u " e = M . .6 0 V 4) CL CLI 0. - - r 0 E 2 11 w V110, 9 C G V. ACCELERATOR TECHNOLOGY 290 C. Tr act er / Rf guns- br i ght i nj ect or s f or FEL a do appr oxi mat i on . Thi s gun pr oduced a 1 MeV beam of 70 ps pul ses wi t h a peak cur r ent of 200 A and a nor mal i zed enut t ance of 40m mmmr ad [ I 1] . A second cavi t y i ndependent l y power ed and phased was t hen added t o t he or i gi nal one. Meanwhi l e, t he adj unct i on of a pul se compr essor t o t he l aser al l owed t o gener at e 16 ps opt i cal pul ses . Thi s new gun t est ed i n 1987 and 1988, pr ovi ded a 2. 7 MeV beam. The maxi mumext r act ed char ge was 13 . 2 nC cor r espondi ng t o a peak cur r ent of 600 A. No enut t ance measur ement s wer e possi bl e due t o t he dr i f t space bet ween t he second cavi t y and t he " pepper - pot " devi ce t hat l ed t o excessi ve space- char ge emi t t ance gr owt h [ 12] . Dur i ng t hi s per i od, t he phot ocat hode l i f et i me was si gni f i cant l y i mpr oved by r epl aci ng Cs 3 Sb wi t h Cs K 2 Sb . QUADRUPOLE T -i `- VERTI CAL STEERI NG HORI ZONTAL STEERI NG Fi g. 3 . St anf or d/ Duk e r f gun l ayout ( f r om r ef . [ 44] ) . Cs 3 Sb phot ocat hode i l l umi nat ed by a f r equency doubl ed Nd : YAGl aser [ 4] . The cavi t y shape was desi gned t o mi ni mi ze t he nonl i near component s of t he r adi al el ect r i c f i el d, assumi ng Thi s l i f et i me r emai ned ver y dependent t hough on t he oper at i ng condi t i ons . Tabl e 2 summar i zes t he per f or mances of t he t wo exper i ment s named f or conveni ence phase I and phase 11 . Based on t he gai ned exper i ence, a new phot oi nl ect or was bui l t t o ser ve as an el ect r on sour ce f or t he HI BAF i nf r ar ed FEL [ 13] . Thi s new gun whi ch r epl aced t he pr evi ous convent i onal i nj ect or pr ovi des a 350 A, 16 ps and 351T mmmr ad beam. I t consi st s of si x cel l s wher e t he f i r st cel l cont ai ns t he Cs K2 Sb phot ocat hode ( f i g . 4) . Thi s phot oi nl ect or has been desi gned f or max i mum f l exi bi l i t y i n or der t o do par amet r i zat i on st udi es f or LOS ALAMOS NATI ONAL LABORATORY ACCELERATOR TECHNOLOGY DI VI SI ON GROI . P AT- 1 1300 Mi ; PHOTOI NJ ECTOR LI NAC Fi g . 4. Los Al amos HI BAF r f gun ( f r om r ef . [ 14] ) . C. Tr avi er / Rf guns : br i ght i nj ect or s f or FEL 291 Fi g. 5 . CEA phot oi nj ect or ( f r omr ef . [ 19] ) . si mul at i on codes ver i f i cat i on . ar e gi ven i n t abl e 2 [ 14] . Nomi nal per f or mances Car l st en pr oposed a ver y power f ul way t o r educe t he emi t t ance of a phot oi nj ect or [ 161 . Af t er i dent i f yi ng t he Because of t he possi bl e i ndust r i al use of FEL, i t i s i nt er est i ng t o devel op a ver y compact devi ce. Thi s wor k has been under t aken by LANL [ 15] . A compact FEL named AFEL i s now under desi gn . The phot oi nj ect or wi l l consi st of t en cel l s power ed by one r f sour ce. Al l f l exi bi l i t y wi l l be r emoved and t he par amet er s wi l l be chosen f or an opt i mum desi gn accor di ng t o si mul at i ons . Thi s i nj ect or shoul d be abl e t o pr oduce a beam br i ght ness of t he or der of 10 12 A/ ( m2 r ad 2 ) ( t abl e 2) . f our di f f er ent emi t t ance gr owt h mechani sms : ( a) l i near space char ge, ( b) nonl i near space char ge, ( c) nonl i near t i me- i ndependent r f ef f ect , ( d) l i near t i me- dependent r f ef f ect , he shows t hat f or a pul se pr oduced by a l aser , i t i s possi bl e t o r emove most of t he cor r el at ed emi t t ance ( var i at i ons i n t he t r ansver se phase space cor r el at ed wi t h l ongi t udi nal posi t i on) by a pr oper choi ce of posi t i on and st r engt h of magnet i c l enses . Such a t heor y i s not appl i cabl e f or convent i onal i nj ect or s si nce bunchi ng ~F TUNER Fi g . 6 . BNL r f gun cavi t y ( f r omr ef . [ 21] ) . V ACCELERATOR TECHNOLOGY zyxwv 292 C Tr avi er / Rf guns: br i ght i nj ect or s f or FEL i nt r oduces mi xi ng bet ween l ongi t udi nal and t r ansver se phase- space pl anes . AFEL desi gn i s based on t hi s t heor y . ~ M LASER- DRI VEN GUNS EXPERI MEN- AL RESU_TS - ASER- DRI VEN GUNS DESI GN PARAhETEPS 10 4 " HERMI ONI C GJ NS EX~EPi r ' ENTAL PARAMEI ERS 3 3. CEA Br uyèr es- l e- Chât el The Commi ssar i at à l ' Ener gi e At omi que ( CEA) i n Br uy6r es- l e- Chât el ( Fr ance) has bui l t a l aser - dr i ven r f gun as an i nj ect or f or a hi gh- power i nf r ar ed FEL [ 17] . I n or der t o i mpr ove t he beam char act er i st i cs, a f r equency of 433 MHz was chosen f or t he l i nac and a subhar moni c f r equency of 144 MHz f or t he gun. At f i r st , one cavi t y i s used pr ovi di ng a 1 MeV beam ( f i g . 5) . Lat er , a second cavi t y wi l l be added . Cs 3 Sb and CsK, Sb phot ocat hodes ar e used . Ot her t ypes ar e al so t est ed . The goal i s t o pr oduce a 50- 100 ps, 10- 20 nC beam pul se . Desi gn par amet er s [ 18] ar e summar i zed i n t abl e 2 . Thi s gun i s now under t est i ng . An accel er at i ng f i el d of 25 MV/ m on t he cat hode has been obt ai ned . 20 nC, 100 ps pul ses have been accel er at ed t o 1 MeV. An emi t t ance of 901T mmmr ad was measur ed f or a 10 nC pul se [ 191 . 3 . 4 . Br ookhaven Nat i onal Labor at or y I n or der t o st udy sever al i deas r el at ed t o f ut ur e par t i cl e accel er at i on met hods, Br ookhaven Nat i onal Labor at or y ( BNL) i s bui l di ng an accel er at or t est f aci l i t y ( ATF) [ 20] whi ch wi l l al so be used f or FEL physi cs . The i nj ect or f or t he accel er at or i s a l aser - dr i ven r f gun oper at i ng at 2856 MHz. I t consi st s of a one and a hal f cel l cavi t y i ncor por at i ng a phot ocat hode i n t he end wal l of t he f i r st cel l . The cavi t y shape was desi gned so t hat r f f i el ds cause mi ni mal nonl i near di st or t i on of t he phase spaces ( f i g . 6) . Thi s was obt ai ned by usi ng a Four i er decomposi t i on of t he r f f i el ds and by set t i ng t he r adi al f i el d t o be l i near [ 21] . The gun i s desi gned t o oper at e wi t h a 100 MV/ m el ect r i c f i el d at cat hode. The l aser wi l l pr ovi de a 4 t o 6 ps pul se . 1 nC i s expect ed f r om t he yt t r i um phot ocat hode . The mai n desi gn par amet er s ar e shown i n t abl e 2. The gun has al r eady been t est ed wi t h a 20 ns pul se f r om an exci mer l aser . A 90 MV/ m accel er at i ng f i el d was obt ai ned at t he cat hode . A peak cur r ent of 0. 6 A was accel er at ed t o 3 . 6 MeV [ 20] . The pi cosecond Nd : YAG l aser i s now avai l abl e and exper i ment s wi l l pr oceed soon . Meanwhi l e, ext ensi ve cat hodes ar e done [ 22] . st udi es of met al l i c phot o- 10 100 1000 10000 CURRENT ( A) Fi g. 7 Rf gun br i ght ness vs peak cur r ent . t he br i ght ness as a f unct i on of t he peak cur r ent and t he bunch l engt h r espect i vel y, f or t he di f f er ent r f gun pr oj ect s. These f i gur es ar e onl y i ndi cat i ve and shoul d be consi der ed wi t h some car e . Fi g . 7 al so shows t he br i ght ness cor r espondi ng t o di f f er ent wavel engt h FEL. Fr om t hese f i gur es a f ew gener al comment s can be made . Bot h t her mi oni c and l aser dr i ven r f guns have demonst r at ed t hei r capabi l i t y t o pr oduce ver y br i ght beams ( r oughl y one or der of magni t ude hi gher t han convent i onal i nj ect or s) . Ther mi oni c r f guns pr ovi de smal l er peak cur r ent s t han phot oi nl ect or s . The best l aser - dr i ven r f gun shoul d be suf f i ci ent t o r each wavel engt hs mt he XUV domai n pr ovi ded t hat sui t abl e wi ggl er s can be bui l t f or such shor t wavel engt hs . X- r ay domai n i s out of r each f or t he cur r ent st at e of t he ar t . LASER- DRI VEN GUNS EXPERI MENTAL O 10' ' HER- 1ONI C GUNS 4 RESU- I S LASER- D- 2I VEN GUNS DESI GN PARAMETERS EXPERI MENTAL PARA- E - ERS E 10 12 Q cn cn w z 4. St at e of t he ar t ro A f i gur e of mer i t of t he beam qual i t y can be t he peak br i ght ness def i ned her e as t wo t i mes t he peak cur r ent di vi ded by t he squar e of t he nor mal i zed emi t t ance . I t i s expr essed i n A/ ( m2 r ad 2 ) . Fi gs . 7 and 8 show 10 1 10 BUNCH 100 LENGTH ( PS) Fi g . 8 . Rf gun br i ght ness vs bunch l engt h 1000 C Tr act er / Rf guns br i ght i nj ect or s f or FEL What ar e t he pr esent l i mi t at i ons i n r f gun t echnol ogy? As ment i oned ear l i er , t her mi oni c r f guns suf f er f r om t he backbombar dment pr obl em. The ext r a heat due t o t he backbombar ded el ect r ons i ncr eases t he peak cur r ent dur i ng t he r f macr opul se whi ch i s not good f or FEL oper at i on. I t al so l i mi t s t he dut y cycl e . Addi ng a t r ansver se magnet i c f i el d i mpr oves t he si t uat i on si nce t he most ener get i c el ect r ons come back on t he cavi t y wal l besi de t he cat hode . Usi ng t hi s t echni que, St anf or d coul d obt ai n a 10 ps macr opul se r epeat ed at 15 Hz wi t hout cur r ent i ncr ease dur i ng t he macr opul se [ 10) . I f mor e cur r ent and hi gher dut y cycl es ar e needed, t he backbombar dment pr obl em wi l l st i l l be a l i mi t at i on . Convent i onal t her t moni c cat hodes have cur r ent densi t i es of a f ew t ens of A/ c m2 . Recent l y, however , mor e t han 100 A/ c m2 has been r epor t ed [ 23] . These val ues ar e never t hel ess much smal l er t han t hat r epor t ed f or phot ocat hodes. Si nce i n a t her mi oni c r f gun, ever y r f bucket i s f i l l ed, t he beam l oadi ng i s ver y i mpor t ant . Ref l ect ed el ect r ons and secondar y el ect r ons al so par t i ci pat e t o beam l oadi ng . At equi val ent peak cur r ent , t her mi oni c r f guns r equi r e mor e r f power t han l aser - dr i ven r f guns . Thi s f act coul d be a l i mi t at i on f or hi gh cur r ent s . Phot oi nj ect or s r el y on phot ocat hodes and l aser t echnol ogy . The opt i mum phot ocat hode ( good quant um ef f i ci ency, l ong l i f et i me, good abi l i t y t o wi t hst and hi gh f i el ds) has not yet been f ound . The most successf ul phot ocat hodes mai nl y st udi ed at Los Al amos ( Cs 3 Sb, Cs K2 Sb) have a ver y good quant um ef f i ci ency ( 2 t o 8%) . Thei r l i f et i me depends on t he oper at i ng condi t i ons . For t hei r t ypi cal oper at i ng condi t i ons ( 200 t i mes 5 nC mi cr opul ses r epeat ed at 1 Hz, 2 x 10 - 9 Tor r pr essur e) , Los Al amos HI BAF phot ocat hodes have a 15 h l i f et i me [ 24) , t he pr oduct i on t i me f or one cat hode bei ng 3 h. I n or der t o i mpr ove t hi s l i f et i me, Shef f i el d suggest ed t o use a gl ow di schar ge t o cl ean t he cavi t y pr i or t o i t s oper at i on [ 251 . The mai n al t er nat i ve t o t hese mul t i - al kal i ne phot ocat hodes ar e t he t her mi oni c cat hodes ( di spenser , LaB6 ) and t he met al l i c cat hodes . They bot h have a l ower quant umef f i ci ency and ar e mor e r obust . Onl y LaB6 has been ser i ousl y t est ed i n an r f gun [ 6) . Di spenser cat hodes ar e now st udi ed i n a do gun at Or say [ 261. Met al l i c cat hodes have been ext ensi vel y st udi ed at BNL bot h i n pi cosecond and f emt osecond r egi mes [ 27, 28] i n a do conf i gur at i on . Quant um ef f i ci ency as hi gh as 7. 25 x 10 - 4 has been measur ed f or samar i um phot ocat hodes . A cur r ent of 21 k A/ c m2 has been ext r act ed f r om a 7 mm2 yt t r i um phot ocat hode . Thi s r esul t cor r esponds t o t he " hi ghest cur r ent densi t y r epor t ed t o dat e f or a macr oscopi c phot oemi t t er " . These phot ocat hodes have now t o be t est ed i n an r f gun conf i gur at i on . Sever al ot her phot ocat hode st udi es ar e cur r ent l y un- 293 der way i n many di f f er ent l abor at or i es . A good over vi ew can be f ound i n r ef . [ 29) . An i mpor t ant i ssue f or any t ype of phot ocat hode i s t he dar k cur r ent ( cur r ent due t o f i el d emi ssi on) . Thi s cur r ent depends on t he phot ocat hode mat er i al and t he f i el d l evel i n t he cavi t y . At Los Al amos, f or t he oper at i ng condi t i ons ment i oned above, a cur r ent of 100 f C per r f cycl e was measur ed [ 301 . I n case of cw oper at i on, t hi s dar k cur r ent coul d car r y a si gni f i cant power . The l aser used t o i l l umi nat e t he phot ocat hode shoul d pr ovi de ver y shor t pul ses, at t he wavel engt h cor r espondi ng t o t he phot ocat hode spect r al r esponse . Gi ven t he phot ocat hode quant um yi el d, t he l aser macr opul se shoul d car r y enough ener gy t o ext r act t he desi r ed char ge . The l aser shoul d be synchr oni zed t o t he r f , i . e . t he l aser mi cr opul se shoul d cor r espond t o a det er mi ned phase of t he r f si gnal . Thi s phase shoul d be r epeat ed f r om pul se t o pul se wi t h a t i mi ng j i t t er much l ess t han t he pul se l engt h . The l aser mi cr opul se shoul d be as uni f or m as possi bl e i n bot h t r ansver se and l ongi t udi nal di r ect i ons . Uni f or m t r ai ns of mi cr opul ses ar e r equi r ed at a r epet i t i on r at e t hat var i es f r om a f ew t ens of MHz f or FEL t o a f ew GHz f or l i near col l i der s' dr i vi ng beams . Over t he l ast f ew year s, l aser s sui t abl e f or r f gun oper at i on have been devel oped. A l ot mor e wor k i s r equi r ed t o i mpr ove some of t he per f or mances and br i ng t hem t o a hi gh l evel of r el i abi l i t y . The most cr i t i cal i ssues ar e t he uni f or m pul se shape, t he phase and ampl i t ude st abi l i t y and t he ver y hi gh mi cr opul se r epet i t i on r at e especi al l y when ver y shor t ener get i c pul ses ar e r equi r ed. The t ypi cal pr esent per f or mances ar e : mi cr opul se l engt h 10- 20 ps, mi cr opul se ener gy 10- 100 pJ, r epet i t i on r at e 20- 100 MHz, ampl i t ude j i t t er 1%, t i mi ng j i t t er 1- 2 ps . Tabl e 3 Compar i son of r f gun vs convent i onal i nj ect or s Convent i onal Rf gun i nj ect or t her mi oni c l aser - dr i ven Hi gh accel er at i ng f i el d Cur r ent densi t y Shor t pul se Pul se f or mat f l exi bi l i t y Cat hode l i f et i me Rf syst em si mpl i ci t y Compact ness Ti mmgj i t t er Ener gy f i t t er Devel opment st at e Cost + + + + + + + + + - + + + + + + V. ACCELERATORTECHNOLOGY C Tr aoi er / Rf guns br i ght i nj ect or s f or FEL 294 5. Rf guns vs convent i onal i nj ect or s Convent i onal i nj ect or s i ncl udi ng a do gun and one or sever al buncher s have al r eady pr oduced ver y br i ght beams ( e . g. , SLC [ 31] , Boei ng [ 32] ) . Ther ef or e sever al new FEL desi gns make use of a convent i onal i nj ect or ( e . g . , CLI O [ 33] , FELI X [ 34] , CDRL Ber kel ey [ 35] ) . Meanwhi l e sever al ot her r ecent desi gns r el y on r f guns ( e. g . , Duke [ 10] , Los Al amos [ 15] , Twent e Uni ver si t y [ 361, Mi l an [ 37] , et c . ) . Bot h t echnol ogi es have advant ages and dr awbacks t hat ar e summar i zed i n t abl e 3 . The t wo t echnol ogi es ar e i n a ver y di f f er ent st at e of devel opment . The st at ement s pr esent ed i n t abl e 3 ar e t hus subj ect t o evol ut i on i n t he next f ew year s . I nf r ar ed FEL user s' f aci l i t i es t o be bui l t now ar e ver y l i kel y t o use a convent i onal i nj ect or si nce i t s per f or mances ar e suf f i ci ent and i t s r el i abi l i t y much gr eat er . For oper at i on i n t he vi si bl e r egi me, r f guns ar e advi sabl e owi ng t o t he f act t hat t he consi der abl e ef f or t s made t o i mpr ove t hei r per f or mances ( st abi l i t y, r el i abi l i t y) shoul d succeed soon . For even shor t er wavel engt hs, t hey ar e pr obabl y t he onl y choi ce t o dat e . Pbot oi nj ect or s ar e al so ver y i nt er est i ng f or compact i nf r ar ed FELs t hat mi ght be devel oped f or commer ci al use . Compact ness al l ows t o r educe t he amount of concr et e necessar y f or shi el di ng t hus maki ng t he FEL cheaper . 6. Desi gn phi l osophy Fr om t he Ki m equat i ons [ 38] , i t can be shown t hat t he mi ni mum emi t t ance expr essed i n i T mmmr ad f or an r f gun beam i s equal t o e mm - 1 . 57 X 10- _ ~f axa6~2 wher e I i s t he peak cur r ent t o A, f t he r f f r equency i n MHz, a, t he r ms t r ansver se beam si ze i n mm, a b t he r ms bunch l engt h i n ps . Thi s equat i on i s val i d f or shor t pul ses and Gaussi an di st r i but i ons, Thi s mi ni mumemi t t ance i s obt ai ned f or an opt i mum accel er at i ng f i el d expr essed i n MV/ m: Eopt -- 7. 2 x 10 5 f 7Y a b Sa _ x + l . 50 6 Fi g . 9 shows t he opt i mum f i el d as a f unct i on of t he f r equency f or a cur r ent of 100 A, a bunch si ze ax of 3 mm and di f f er ent pul se l engt hs . Si nce t he above equat i ons ar e onl y val i d f or shor t pul ses, f r act i ons of t he r f per i od ar e shown t o i ndi cat e what ar e t he r easonabl e bunch l engt hs t o consi der . The pr act i cal l i mi t of t wo t i mes t he Ki l pat r i ck val ue i s al so r epr esent ed showi ng t hat i t i s not al ways possi bl e t o r each t he opt i mumf i el d. Fr om t hi s pi ct ur e i t appear s t hat l ow f r equenci es r equi r e a ver y hi gh f i el d. The cor r espondi ng mi ni mum I = 100 A - si gma . . = 3 mm 0 _J w w 0 10 100 1000 FREQUENCY ( MHz) Fi g . 9 . Opt i mumel ect r i c f i el d emi t t ances ar e shown i n f i g. 10 . When t he opt i mum f i el d i s hi gher t han t he pr act i cal l i mi t , t he emi t t ance i s cal cul at ed f or t hi s pr act i cal f i el d l i mi t . Fi gs . 9 and 10 pr ovi de a qui ck est i mat e of r f gun per f or mances . For hi gher cur r ent , t he r esul t s ar e easi l y scal ed si nce bot h E. t and f r zyxwvutsrqponml , ar e prkjihopor gfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA t i onal t o t he P squar e r oot of t he cur r ent . For t he r ms bunch l engt h of i nt er est ( l ess t han 10 ps) , f i g . 10 shows t hat t he best f r equency t o choose i s ar ound 1 GHz whi ch cor r esponds t o L- band. I f ver y shor t pul ses can be pr oduced f r om t he l aser , hi gher f r equenci es ar e possi bl e . I f onl y l ong pul ses ar e possi bl e or i f ver y hi gh char ges ar e needed, l ower f r equenci es ar e t o be chosen . I n t hi s case t he r f gun has t o be f ol l owed by a magnet i c compr essor . Thi s scheme i s pr obabl y t he best way t o obt ai n cur r ent s l ar ger t han 500 A. I n or der t o mi ni mi ze t he emi t t ance gr owt h occur r i ng I = 100 A - si gma x = 3 mm E E C 100 w U Z Q Eui 10 100 1000 FREQUENCY ( MHz) Fi g . 10 . Mi ni mumenut t ance . C. Tr aur er / Rf guns . br i ght i nj ect or s f or FEL l ow ener gy, dur i ng dr i f t s at it i s bet t er t o use sever al 295 Ref er enc es cel l s t hus pr oduc i ng a beam wi t h an ener gy l ar ger t han 10 MeV at t he end of t he gun. C. Tr avi er , The abov e equat i ons di d not i ncl ude t he us e of a magnet i c l ens t o r educe t he emi t t ance as suggest ed by Car l st en . Howev er , Ki m [ 39] s howed t hat t hi s met hod Pr oc . I CFA Wor k s hop on Shor t Cur r ent Cat hodes, Bendor , t o be publ i shed . [ 21 G. A. Wes t ens k ow and J. M. J . Madey, c an be appl i ed onl y i f ar i d 30 Laser and Par t i cl e Beams 2 ( 1984) 223. C. H. Lee et I n yq < 1, Pul se Hi gh Fr ance, 1990 ( Ed. Fr ont i èr es) . 7Ez r Vancouver , 2 al . , I EEE Par t i cl e Accel er at or Pr oc. 1985, I EEE Tr ans Canada, Conf Nucl . Sci . NS- 32 ( 5) ( 1985) 3045 [ 4] J . S. Fr aser et al . , i bi d . , p. 1791 . wher e yq i s t he el ect r on ener gy ( m uni t s of l ocat i on of gr adi ent t he l ens, I MCZ ) at t he i s t he peak cur r ent i n A, i n MV/ m and r [ 51 R. L . Shef f i el d, E. R. Gr ay and J. S . Fr aser , Nucl . I nst r and E t he t he beam r adi us i n mm. For Met h . A272 ( 1988) 222. . Cur t i n et A. , Pr oc. 11t h I nt . FEL Conf . , Napl es, FL, [ 6] M t ypi cal val ues of cur r ent , beam r adi us and el ect r i c f i el d, t he abov e condi t i ons s how t hat emi t t ance r educt i on due t o t he magnet i c l ens i s onl y possi bl e f or l ow f r equenci es . Sever al ot her USA, 1989, Nucl . I nst r and Met h . A296 ( 1990) 127 . [ 71 As k nown t o t he aut hor by Sept ember 1990 . [ 8] C. B. Mc Kee and J . MJ Madey, Pr oc . 11t h I nt . FEL conf . , Napl es, met hods t o r educe t he emi t t ance ar e bei ng i nvest i gat ed such as l aser pul se s hapi ng [ 40] t i me- dependent rf f ocusi ng [ 41, 42] . Comput er t he pr act i cal i t y of st r at ed . such met hods i s t o be demon- Wi t h t hese schemes, t he r esi dual USA, 1989, A296 USA, 1989, Nucl . I nst r . and Met h . A296 ( 1990) 110. emi t t ance i s Washi ngt on, DC, USA, 1987, I EEE 87 CH2387- 9, p. 1705 . R. L . Shef f i el d et al . , Pr oc . Li near Accel er at or Conf . , Wi l l i amsbur g, VA, USA, 1988, CEBAF- Repor t - 89- 001 ( 1989) P. 520 . [ 131 W . D. Cor nel i us et al . , Pr oc. 11t h I nt . FEL Conf . , Napl es, 7. Concl usi on FL, USA, and 1988, Bet ween 1983 it was pr ov en t hat t her - [ 14] el ect r on beams. St ar t i ng i n 1989, R. L. Shef f i el d, pr i vat e communi cat i on . [ 15] Ef f or t s ar e made t o pus h t he t echnol ogy t o t he l i mi t whi l e t r yi ng t o i mpr ov e i t s r el i abi l i t y . The f i r st r out i ne t hen be possi bl e t o t est t he di f f er - cat hodes i n Long hour s of 1989, Nuc l . I nst r and Met h . A296 ( 1990) 282 . B. E. Car l st en and R. L . Shef f i el d, Pr oc . Li near Accel er at or Conf . , Wi l l i amsbur g, VA, USA, 1988, CEBAF- Repor t - 89001 ( 1989) p. 365 . 1990, sever al l abor at or i es wi l l be r eady t o ex per i ment t hei r r f guns ( CEA, BNL, CERN, DUKE, et c . ) . I t wi l l J . C. Gol dst ei n et al . , Pr oc . 11t h I nt . FEL Conf . , Napl es, FL, USA, [ 16] oper at i on of an r f gun i s st ar t i ng at SSRL [ 43] . St ar t i ng i n t he f al l of al . , t hese Pr oceedi ngs ( 12t h I nt . FEL ( 1991) 224, r f guns began t o be wi del y st udi ed and us ed i n di f f er ent l abor at or i es . 1989, Nucl . I nst r . and Met h . A296 ( 1990) 251 . D. W. Fel dman et Conf . , Pans, Fr ance, 1990) Nucl . I nst r . and Met h . A304 mi oni c and l aser - dr i ven r f guns coul d pr oduc e br i ght of and Met h, [ 11] J . S. Fr aser et al . , Pr oc. I EEE Par t i cl e Accel er at or Conf [ 12] t ypes I nst r . S. V. Benson et al . , Pr oc . 11t h I nt . FEL Conf . , Napl es, FL, mai nl y due t o t he t her mal emi t t ance of t he c at hode. ent Nuc l . S. V Benson, pr i vat e communi cat i on . si mul a- t i ons s how si gni f i cant r educt i on of t he beam emi t t ance but FL, ( 1990) 716 . and an accel er at or env i r onment . oper at i on exper i ence wi l l gi ve bi r t h t o new i deas t o br i ng t he r f gun t echnol ogy t o a mat ur e [ 17] R. Dei - Cas, Nucl [ 18] S. Jol y, pr i vat e communi cat i on . I nst r . and Met h . A285 ( 1989) 320. [ 19] R. Dei - Cas et al . , Pr oc . I CFA Wor k s hop on Shor t Pul se Hi gh Cur r ent Cat hodes, Bendor , Fr ance, 1990 ( Ed. Fr ont i èr es) t o be publ i shed. [ 20] st at e . K. Bat chel or , Pr oc . buquer que, NM, USA, [ 21] Li near Accel er at or Conf . , Al - 1990, t o be publ i shed. K. T . Mc Donal d, Desi gn of t he Laser - Dr i ven RF El ect r on Gun f or t he BNL Accel er at or Test Faci l i t y, DOE/ ER/ 3072- 43 ( Pr i ncet on Uni ver si t y, 1988) . Ac k nowl edgement s [ 221 J. Fi sher , Pr oc . I CFA Wor k s hop on Shor t Pul se Hi gh Cur r ent Cat hodes, Bendor , Fr ance, 1990 ( Ed. Fr ont i èr es) I woul d l i ke t o t hank ver y muc h J. Gao, B. Car l st en f or hel pf ul Chen, A. Enomo, A. Mi chal ke, di scussi ons . S. V. H. Li u and Bens on, S. C. G . J . Er nst , W . Gai , S. Jol y, K. J . Ki m, R. L . Shef f i el d, Y. Tur and S. Zhong ar e ac k nowl edged f or pr ovi di ng me wi t h a par amet er l i st f or t hei r pr oj ect s. Fi nal l y, I am i ndebt ed t o J. Le Duf f t he oppor t uni t y t o wor k on t hi s subj ect . t o be publ i shed . [ 231 L. Fal ce, i bi d . , t o be publ i shed. [ 24] P. G. O' Shea et a] . , Pr oc . Li near Accel er at or Conf . Al buquer que, NM, USA, [ 26] H. Ber ger et , M . Boussoukaya, J. Le Duf f , who gav e me 1990, t o be publ i shed [ 251 R. L. Shef f i el d, i bi d, R. Chehab, B. Lebl ond and Pr oc . I CFA Wor k s hop on Shor t Pul se Hi gh Cur r ent Cat hodes, Bendor , Fr ance, 1990 ( Ed. Fr ont i èr es) t o be publ i shed . V. ACCELERATOR TECHNOLOGY 29 6 [ 27] C Tr avi er / Rf guns T Sr i ni vasan- Rae et al . , submi t t ed t o J Appl . Phys . br i ght i nj ect or s f or FEL [ 47] [ 28] T. Tsang et al , submi t t ed t o Phys . Rev . B [ 29] Pr oc I CFA Wor k s hop Cat hodes, Bender , on Fr ance, Shor t Cur r ent Fr ont i èr es) [ 31] M . B . James, Ph. D Di sser t at i on, SLAC- 319, UC- 28 ( A) Chi cago, et al . , I L, USA, Pr oc . Par t i cl e Accel er at or 1989 I EEE Cat al og no Conf . , ( 1989) [ 34] P. W Van Amer sf oor t et al . , Pr oc. Eur op . Par t i cl e Accel er at or Conf Ni ce, Fr ance, 1990 ( Ed. Fr ont i èr es) p 547 . Chemi cal Dynami cs [ 36] Resear ch Labor at or y . Concept ual [ 53] [ 37] R. Boni f aci o et al , Nucl I nst r . and Met h A289 ( 1990) 1 [ 38] K. J Ki m, Nucl [ 39] K, J . Ki m, ESGNot e- 106, LBL- 29538 . ( 1990) Pr oc. Eur op . Par t i cl e Accel er at or al . , Pr oc . Eur op Par t i cl e Accel er at or War r en et al . , I L, USA, Pr oc . Par t i cl e Accel er at or 1989, I EEE Cat al og no H. B . Knowl es, pr i vat e communi cat i on, [ 56] C Tr avi er et a] . , Pr oc quer que, NM, Li near Accel er at or Conf . , Al bu- USA, 1990, t o be publ i shed . L. Ser af i ni et al . , Pr oc. Eur op. Par t i cl e Accel er at or Conf . , Ni ce, Fr ance, 1990 ( Ed. Fr ont i èr es) p. 143 [ 58] S. C Chen, pr i vat e communi cat i on . ATF- 90 J ( 1990) . [ 59] S. Takeda et al . , Par t . Accel . 30 ( 1990) 153. [ 42] J Gao, [ 60] C [ 43] M . Bor l and et al . , Pr oc. Li near Accel er at or Conf . , Al bu- LAL/ SERA 90- 188/ RFG ( 1990) . Conf . , 89CH2669- 0 ( 1989) p. 420. [ 57] CAP 54- al . , [ 55] [ 40] J C. Gal l ar do, pr i vat e communi cat i on. BNL 43862, Conf . , Al - zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQ Ni ce, Fr ance, 1990 ( Ed. Fr ont i èr es) . H. Chal oupka et [ 54] J . L. I nst r and Met h . A275 ( 1989) 201 . Pal mer , El ect r on unpubl i shed. Nor t h- al . , Pr oc. Li near Accel er at or et P. Schoessow et Chi cago, 1989, Nucl . I nst r . and Met h . A296 ( 1990) 304 . Over vi ew. A Novel I nf r ar ed, Conf , Rome, I t al y, 1988 ( Wor l d Sci ent i f i c) p . 1312 LBL- pub- 5266 ( 1990) . [ 41] J . C. Gal l ar do and R. B NCCU Pr oj ect Y. C. Huang, pr i vat e communi cat i on . Conf G. J . Er nst et al . , Pr oc . 11t h I nt . FEL Conf . , Napl es, FL, USA, 14t h Meet i ng on Li near Accel er a- 1989, i n Japanese. buquer que . NM, USA, 1990, t o be publ i shed . [ 52] J . M Or t ega et al . , Nue] . I nst r . and Met h . A285 ( 1989) 97 . Desi gn Summar y, C R. Jones, [ 51] Y. Bac onmer 89CH2669- 0 [ 33] [ 35] Eur op. Par t i cl e Accel er at or Car ol i na Cent r al Uni ver si t y ( 1990) . [ 50] Yer emi an Pr oc. Beam Sour ce f or t he Far ( 1987) A. al . , T Ur ano et al , Pr oc t or s, Nar a, Japan, [ 49] B. Car l st en, pr i vat e communi cat i on . [ 32] [ 48] t o be publ i shed . [ 30] Dov bny a et Conf . , Ni ce, Fr ance, 1990 Pul se Hi gh 1990 ( Ed . A. N Pel l egr i m, buquer que, Pr oc. NM. USA, Li near Accel er at or Conf ., Al - 1990, t o be publ i shed . quer que, NM, USA, 1990, t o be publ i shed [ 61] [ 44] S V. Benson et al . , Nucl . I nst r . and Met h . A272 ( 1988) 22 [ 62] J. Xi e, H. Li u and R. Zhang, Mi cr owave el ect r on gun wi t h [ 45] H. Li u, Nucl [ 46] S I nst r . and Met h . A294 ( 1990) 365 Ni shi har a and M. Ki mur a, Pr oc 14t h Meet i ng on Li near Accel er at or s, Nar a, Japan, 1989, i n Japanese . S Chat t opadhyay et al . , LBL- 25699, ESGNot e- 55 ( 1988) . f i el d emi ssi on cat hode, submi t t ed t o Act a El ect r oni ca Smi ca ( 1989) i n Chi nese [ 631 G Bi envenu, LAL/ SERA 89- 76 ( 1989) i n Fr ench.