4 The Electron Cyclotron Resonance Ion Source (ECRIS) HF ion sources

Transcription

1 4 Th Elctron Cyclotron Rsonanc Ion Sourc (ECRIS) HF ion sourcs 4.1 Th ECR: Microwav ion sourcs (f > 1 GHz) hav bn dvlopd for th production of multipl chargd ions. Th first sourcs which usd th ECR-hating togthr with a magntic-plasma-confinmnt wr built at th nd of th 1960's in th group of R. Gllr. H took advantag from his xprinc with "mirror machins" for fusion-plasma-studis. For this purpos a "Min-B-configuration" was chosn. Th bttr th magntic confinmnt, th highr charg stats of th ions can b producd. On can distinguish btwn ECR-sourcs with f < 2.45 GHz for intns bams of low chargd ions and f > 2.45 GHz for highly chargd ions. Th principl stup of an ECR is shown on th right hand sid. Th magntic plasma confinmnt in axial dirction is providd by solnoids, th radial on by a multi-pol configuration of prmannt magnts. A microwav is guidd into th chambr and nsurs th plasma-production. Th plasma chambr is insulatd and is biasd with high voltag for xtraction. WS2015/16 4.1

2 If a volum with sufficint high magntic fild, is irradiatd with microwavs, it is possibl to ignit a plasma by lctron rsonanc at a givn gas prssur lvl. With a spcial arrangmnt of axial and radial magntic fild componnts (min B configuration) th maximum plasma confinmnt is rachd for th production of hot plasma lctrons by microwav hating. Th plasma chambr is a microwav cavity. For th microwav hating th condition applis: f c m B B 2 m BT For B = 1 T this lads to f = 28 GHz 10 (4.1) multipol fild B solnoidal fild ECR zon B ECR =/m or f = 10 GHz rquirs B = 0.36 T. Microwav magntic fild distribution Th mirror-rlation B max /B min is in gnral 2-4 and B max is of th ordr of two tims th lctron cyclotron (EC)-rsonanc fild valu. For frquncis abov 20 GHz magntic fild abov 1.5 T ar ndd, which can b rachd with supr-conducting magnts. Th multi-pol gomtry dtrmins th shap of th loss lins and loss-aras. Th highr th numbr of pols, th biggr ar th total loss-aras on both nds of th confinmnt. Th loss-aras ar th rsult of a compnsation of th axial fild by th fild of th prmannt magnts forming th multi-pol at th positions of opposit polarity. WS2015/16 4.2

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4 Modrn ion sourcs ar using an iron yok to rduc th powr consumption. Furthr mor, an iron yok also rducs th influnc of stray fild on th plasma and th xtractd ion bam. Th first ion sourc, which was th basis of all ECR sourcs, was Gllr's SuprMAFIOS (1975). Th powr consumption of 3MW was not accptabl for applications. Hnc, Gllr and coworkrs dvlopd th MiniMAFIOS with 100 kw powr consumption and 10 Ghz microwav frquncy. WS2015/16 4.4

5 In 1987, Gllr and Sortais build th first ECR consisting out of prmannt magnts. Th so calld NoMAFIOS only nds th powr for th microwav hating. Th ions insid th plasma ar not hatd By th microwav, but rmain thrmal. Hnc, th intraction of th "slow" plasma ions with th magntic fild is wk. Th confinmnt of th plasma ions can thrby only b don by th lctrostatic spac-charg fild of th magntically confind plasma lctrons. Th ECR plasma can b clarly distinguishd from othr plasma, by th significant diffrnc btwn th ion and lctron tmpratur. Whil th plasma-lctron tmpratur can b up to som hundrd kv, th tmpratur of th plasma-ions is of th ordr of V, i.. th man kintic nrgy is quit low. Th magntic momnt of th plasma-ions is thrby quit low and thr intraction with th xtrnal magntic fild is small. Hnc, a magntic confinmnt of th slow plasma-ions is not possibl. WS2015/16 4.5

6 Charactristic of ECR plasma: Small potntial dip of th cntral ara with rspct to th hatd zon. This is th rsult of th two-componnt plasma with diffrnt lctron tmpraturs. Th lctron distribution adjusts in a way that j, cold j, hot, coldvcold, hot bcaus v hot v cold, cold, hot v hot hot zon, ECR j, hot j, cold cold zon WS2015/16 4.6

7 Th dpndnc of th lctron dnsity n and tmpratur T on th microwav hating powr shows intrsting bhaviours. Th lctron dnsity saturats and vn dcrass abov a crtain powr lvl. Th tmpratur also saturats. This is du to th dcras of th confinmnt tim and th incras of th plasma volum with highr hating powr. Primarily, th stochastic-lctron-hating bcoms infficint at too high hating powr lvls. Th plasma dnsity is limitd by f cutoff f p f wav. WS2015/16 4.7

8 f p 2 f 3 p n m Hz n m f cm ==> (4.2) 81 Th lctron confinmnt tim is T n 3 / 2 const (Diffusion by collisions Spitzr formula) 2 T and thrwith For powr balanc follows 1/ 2 P V rf p n kt n k n P V rf p T 1/ 4 (4.3) V p is th plasma volum, which lads, guidd by th fild lins, to th xtraction ara. Th xtraction ara incrass fastr than th HF powr and th lctron tmpratur. Th ion currnt with charg stat q follows I q n q q V q p n V p B 2 V p 2 HF (4.4) Hr q is th ion confinmnt tim in th plasma. Typical valus for th lctron dnsity ar n ~ cm -3. For highly chargd ions q must b q 0. 01s. WS2015/16 4.8

9 Th hating of th plasma lctrons with microwavs at EC-rsonanc bcoms optimal, if th lctricfild-vctor of th microwavs rotats prpndicular to th lins of th static magntic fild, i.. th lctromagntic fild has to b a circular polarizd. Hnc, during ach half-wav th lctron gains nrgy. An lctron has to cross th hating zons svral tims to b hatd to high nrgis. X-Ray spctroscopy shows that lctrons can b acclratd up to 1 MV. Th fild-strngth of th microwavs is only about 100 V/cm, i.. th lctrons hav thn crossd th hating zons many tims. Th dcoupling of th microwavs into th "multimod"-plasma-chambr can b don at svral positions, as shown abov. WS2015/16 4.9

10 Th production of mtal-ions is don by vaporation of th mtal insid an ovn or by sputtr targts, which ar movd clos to th plasma. Th xtraction of ions can b both continus or in th so-calld "aftrglow mod". In th "aftrglow mod" th HF is switchd of and thrby th confinmnt of th ions. Aftr an optimization of th magntic fild configuration it follows a high currnt of highly chargd ions in on puls. For that purpos, th confinmnt has to b considrably good. On can s that th dc currnt dcrass significantly for optimum aftrglow mod sttings. Th puls lngth varis btwn ms. WS2015/

11 Concrning xtrnal injction th ions ar transportd to th plasma via th groundd lctrod and dclratd to a fw V during th injction into th plasma, bcaus th plasma chambr is put on high voltag. Th ions ar only capturd if thy ar dclratd sufficintly du to collisions with plasma-ions. WS2015/

12 Typical spctrum: Mtal ion production by vaporation of lad with an ovn. 171 na Pb 2+ injctd 560 na In + injctd Spctrum for xtrnal injction of mtal-ions lik.g. In 1+ or Pb 1+. WS2015/

13 Dvlopmnt of th ECR tchnology in 30 yars. VENUS is an ECR sourc of th third gnration (high prformanc dvic) wr th magntic filds ar only producd by supr conduction coils. Th structur and th data of VENUS ar givn blow. In addition, th scaling-laws, valid for modrn ECR sourcs ar givn. WS2015/

14 Hr M is th mass of th ion-spcis, which is xtractd. Gnrally a gas is injctd to produc highly chargd ions of th dsird isotop. For th production of xotic nucli an xtrnal injction of singl chargd ions is ncssary. Th following magntic-fild rlations ar valid for high powr ECRs: B inj /B cr ~ 4, B xt /B cr ~ 2, B min /B cr ~ 0.8, B rad /B cr > 2, B xt /B rad < 0.9 (4.5) Hr B inj (B xt ) is th maximum fild strngth on th injction sid (xtraction sid), B rad th radial fild of th sxtupol at th plasma chambr wall and B min th minimum fild strngth btwn th magntic mirrors. WS2015/

15 intnsity (µa) Typical valus: Microwav powr of W: 16 O 6+ -and 40 Ar 10+ -ion currnts of mor than 2μA and up to 10μA for 197 Au Bi 25+ and 238 U ions can b producd Comparison of diffrnt ECR sourc typs: GHz SC-ECRIS (xtrapolatd) 28 GHz SERSE 18 GHz SERSE 18 GHz RT-ECRIS 14 GHz GSI-CAPRICE II X charg stat WS2015/

16 4.2 High frquncy (HF) drivn ion sourcs: Th us of rf-gnratd plasma dats back to th 1940s. Th rf-drivn volum sourcs can b opratd with any working gas, bcaus thr is no filamnt that can b damagd. An xampl is Oxygn, which dgnrats th tungstn filamnts of th volum sourcs. Th basic principl is shown with a Thonmann-sourc-typ. Typical working prssurs ar mbar. Th rf-powr to b applid varis from a fw hundrd Watt up to svral kw. Th applid radio-frquncy is of th ordr of MHz. Thr ar two ways for th rf-coupling: Capacitiv btwn two lctrods Typically a stup lik a cylindrical capacitor Inductiv via an induction coil. Th coil can b put outsid th vacuum chambr if th chambr is mad out of glass or quartz. Or insid th chambr as spiral shapd antnna. Th advantags of th sourc: Almost no war parts, bttr discharg stability, indpndnt of th cathod charactristics. Hnc, thy can b usd as ion thrustr in spaccrafts. WS2015/

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18 xtrnal antnna intrnal antnna WS2015/

19 Plasma gnration via rf-incoupling: x m v dv dt v 0 E E Ep sin( t) p sin( t ) E p cos cos t m RF Without collisions, th lctrons do not tak nrgy out of th rf-fild! Th phas of th lctron movmnt can b changd by collisions. Thrwith lctrons can tak nrgy from th rf-fild. Nw lctrons ar gnratd by ionization procsss. If th production rat is biggr than th loss rat, a stabl rf-discharg follows. WS2015/

20 Th circuit, which provids th coupling of th rf is opratd in rsonanc mod. Pulsgnrator Frqunzgnrator Vorvrstärkr HF-Vrstärkr HF- Anpassungsntzwrk Z il 1 R V 1 ic 1 1 il 2 1 R L A chang in th plasma paramtrs lads to a chang in R L. WS2015/

21 By inductiv coupling, an azimuthal AC-lctric fild is producd, which Gnrats oscillating lctrons insid th gas and thrwith transfrs nough nrgy for th gnration of a plasma. With inductivly HF-drivn multicuspvolum-sourcs, currnt dnsitis of about 1A/cm² can b rachd. Th achivabl currnt dnsitis Dpnd on th ion mass, spcially for vry light ions. WS2015/

22 Typical spctra of a multicusp ion sourc: WS2015/