FEBIAD ion source development at ISOLDE: efficiency improvement for all the elements

Transcription

1 FEBIAD ion source development at ISOLDE: eiciency improvement or all the elements L.Penescu, R.Catherall, J.Lettry, T.Stora CERN, AB-ATB-IF (ISOLDE) ISOLDE Workshop, November 2008

2 Ionization eiciency improvement at ISOLDE FEBIAD Ion source Ionization Eiciency He Ne Ar Kr Xe Rn* Standard MK7 [1] st proto (#380) nd proto (#317, #385) Multiplying actor #380 Nb (HRS) Record yields or Krypton: Yields 72 Kr 73 Kr Measured (at/µc) 1.1e4 1.2e6 Database (at/µc) 2.0e3 7.4e4 #385 UC x (HRS) Discovery o 229 Rn. [1] U.C. Bergmann et al, NIM B204 (2003) 204

3 Eiciency o the ISOL process Plasma Extracted ion beam ~na or Surace ~μa or FEBIAD ~ma or ECR Leaks Release loss Decay loss Leaks Condensation Decay loss Leaks Neutrals Condensation Sidebands Decay loss Multiply charged Nuclear reaction - cross section Diusion Eusion Ionization Beam quality Gas pumping

4 FEBIAD ion sources 1+ Ionization Eiciency Ionization potential: ANY element Volatility: elements with ΔH a < ~6eV Elements produced at ISOLDE Dependent on: Mass Ionization potential Volatility

5 The arc discharge plasma Neutral eusion Thermionic emission Transer line Electron impact ation Charge exchange Surace ation Recombination Chemical reactions Cathode Anode 16mm 22mm

6 Theoretical models or FEBIAD ation η calc Where: Best approximation: empirical equation [2] = 4D D l S 0 out l S out l = average path length or a particle in the plasma; D 0 = constant (cm 2 /s); S out = emission area o the source; T i = ion temperature; T e = electron temperature; I p = ation potential; l e = number o electrons in the valence shell o the atom with a given I p ; M i = mass o the species. π M I i p le exp 8kT i < kte > π M I p i le exp 8kT i < kte > Model parameters (FEBIAD): Fitted results: FEBIAD (Kirchner) and EBGP (Nitschke) sources Deducing: kt e = ev; T i = 2273 K; 4 l D 0 /A 0 = cm/s Not useul or: Implications Comprehension o IS internal parameters: - response time; - emittance; - collective behavior (=>selectivity) [2] G.D.Alton, NIM A382 (1996) 207

7 FEBIAD eect o operational parameters Temperature Magnetic ield Anodes potentials Operating pressure Density Energy Coninement Density Extraction ELECTRONS IONS Kr eiciency (%) 40% 35% 30% 25% 20% 15% 10% 5% Mag=0A Mag=1A Mag=2A Mag=3A Mag=4A 0% Anode potential (V)

8 FEBIAD - a parameter-oriented model Model rame No plasma coninement; Mainly direct electron beam ation; No ion heating. ε ~ σ M n e Inerred rom experimental study; Introduced in [3]. Te ev T T i n e n gas anode T cathode cathode Equivalent with: Buer gas not required or ation; Emittance deined mainly by the action geometry; Fast ation. ε = elec V source W Aexp kt E elec ln E l e E E elec M S i out T T = cathode temperature = gas & ion temperature; A, W = cathode parameters (Richardson constant & work unction); E elec = ing electrons energy; E, l e, M i = element parameters (ation energy; available electrons; mass) V source = source volume; S out = emission area o the source. [3] L.Penescu et al, NIM B 266, 4415 (2008)

9 FEBIAD prototypes ε ~ σ M n e Improvement: Extraction actor, + + V V active total 1 st Prototype (#380 Nb): optimization o acting ield Only MK7 ( cold plasma ) => Only noble gases 2 nd Prototype (#385 UC x ): reduction o wall losses MK7 and MK5 ( cold and hot plasma) => ALL elements

10 Expected element dependence ε ~ σ M n Ion source response time: e Pumping eect t Ar 1900 C Valid or isotopes ation action t Ar 1900 C =0.25 pumping sticking decay t t N VOLATILE STABLE = t 1 t t ~ σ N n e v rel Ion source response time improved Ion source response time < target + transer line resp. time No isotope lietime limitation rom the ion source t

11 Applicability Improvement valid or: ALL types o transer line (hot, cooled, quartz); ALL target materials; Atomic ation AND molecular sidebands. Eect on the MK7 eiciencies (and RIB yields): Element He Ne Ar Kr Xe Multiplying actor Eect on the MK5 eiciencies (and RIB yields): Element He Ne Ar Kr Xe Multiplying actor Expected to apply or ALL elements!

12 Thank you or your attention!