Deuterium retention mechanisms in beryllium M. Reinelt, Ch. Linsmeier Max-Planck-Institut für Plasmaphysik EURATOM Association, Garching b. München, Germany PSI-1828 May 2008

Outline  Motivation  Results of thermal release experiments Variation of... * Irradiation fluence * Implantation temperature * BeO coverage  Modeling  Energy diagram of D / Be  Conclusion ITER cross section

D implantation into beryllium Deuterium retention / thermal recycling of ITER main wall Be: Fast reaction with O 2 / H 2 O Previous experiments: BeO contaminated surfaces Investigation of System Be (+BeO) / D Clean Be / D: NO DATA ! D implantation into beryllium Deuterium retention / thermal recycling of ITER main wall Be: Fast reaction with O 2 / H 2 O Previous experiments: BeO contaminated surfaces Investigation of System Be (+BeO) / D Clean Be / D: NO DATA ! Be: ~700 m 2 D,T Motivation

Be (+ BeO) 1 and 1.5 keV D implantation Motivation D,T Variation by 1 ORDER OF MAGNITUDE ! Variation by 1 ORDER OF MAGNITUDE ! Be: ~700 m 2 [Anderl et al. 1999]

Concept Issues to be solved: 1. Retention in pure Be ? 2. Crystallinity 3. Influence of BeO ? 4. Retention mechanisms ? Issues to be solved: 1. Retention in pure Be ? 2. Crystallinity 3. Influence of BeO ? 4. Retention mechanisms ? Possible reasons: 1. Undefined BeO coverage 2. Undefined crystallinity 3. Unclear retention mechanisms (Needed for quantification) Possible reasons: 1. Undefined BeO coverage 2. Undefined crystallinity 3. Unclear retention mechanisms (Needed for quantification)

Experimental concept Thermal release NRA TPD Temperature Programmed Desorption TPD Temperature Programmed Desorption Sequential release of D Limited by combination of bulk + surface processes Energy barriers for...  Diffusion  Detrapping  Recombination Sequential release of D Limited by combination of bulk + surface processes Energy barriers for...  Diffusion  Detrapping  Recombination Single crystalline Be Retention mechanisms 1 keV D implantation Hydrogen retention in situ... ( mbar) Ar sputter cleaning+ annealing XPS / LEIS: Control of surface Ar sputter cleaning+ annealing XPS / LEIS: Control of surface

TPD: Spectrum Sequential release: Energetically different rate limiting steps  1 keV D + Implantation (300 K)  2·10 17 D cm -2 m/q= 4 (D 2 )

TPD: Increasing fluence TPD Spectra recorded in random order !  Fluence dependent behaviour TPD Spectra recorded in random order !  Fluence dependent behaviour

TPD: Increasing fluence Trapping in ion induced defects

TPD: Increasing fluence Structural modifications Local saturation of available binding sites Local saturation of available binding sites Trapping in ion induced defects

TPD: Increasing fluence Structural modifications Sample saturation Threshold

Retention: Simulation by SDTrim.SP Super saturation zone D accumulation in a depth of 40 nm Bulk saturation concentration: 26 at% D (D/Be = 0.35) * Supersaturation * Structural modifications: Surface process? D accumulation in a depth of 40 nm Bulk saturation concentration: 26 at% D (D/Be = 0.35) * Supersaturation * Structural modifications: Surface process? (cut off) SDTrim.SP Calculation TPD Experiments

Structural modifications / Surface desorption 1 st order release 1 D Trapped  D Mobile 2 nd order release 2 D  D 2 (Surface desorption)

Structural modifications / Surface desorption * Peak shape Desorption peak is 1st order * Surface area (AFM) Release of 60 x Θ (saturation coverage Θ ~ 0.5) AFM: max. 1.2 Θ  Surface recom- bination is not the rate-limiting step * Peak shape Desorption peak is 1st order * Surface area (AFM) Release of 60 x Θ (saturation coverage Θ ~ 0.5) AFM: max. 1.2 Θ  Surface recom- bination is not the rate-limiting step

TPD: Influence of BeO-coverage No change of E A of release from binding states No recombination limit ↔ Trapping in the bulk Formation of BeO-D at the surface No change of E A of release from binding states No recombination limit ↔ Trapping in the bulk Formation of BeO-D at the surface BeO:D (surface)

TPD: Elevated implantation temperatures 300K 530 K

TPD: Elevated implantation temperatures 300K 530 K Different retention mechanism ! Change of the binding states in the supersaturated areas Different retention mechanism ! Change of the binding states in the supersaturated areas Ion-induced trap sites unaffected

TPD: Elevated implantation temperatures 300K 530 K Different retention mechanism ! Change of the binding states in the supersaturated areas BeD 2 formation (Decomposition ~ 570 K) Different retention mechanism ! Change of the binding states in the supersaturated areas BeD 2 formation (Decomposition ~ 570 K) Ion-induced trap sites unaffected BeD 2 BeO:D (surface)

Implanted / Co-deposited 1 keV Ion implanted (this work) D/Be plasma co-deposited (de Temmerman) 300 K

Implanted / Co-deposited Supersaturated material Supersaturated material 1 keV Ion implanted (this work) D/Be plasma co-deposited (de Temmerman)

300 K Implanted / Co-deposited Ion-induced traps in the bulk 1 keV Ion implanted (this work) D/Be plasma co-deposited (de Temmerman)

300 K 600 K 530 K Implanted / Co-deposited Formation of BeD 2 see also poster P3-05 by R. Doerner Formation of BeD 2 see also poster P3-05 by R. Doerner

Qualitative interpretation of data [Anderl et al. 1999] clean Be (1 keV) Be (+ BeO) 1 and 1.5 keV Structural modifications BeD 2 Ion-induced traps in the bulk lattice ~ Constant retention Specimen exposure temperature [K]

Identification of retention mechanisms Quantification: TMAP7 / Rate equations

TMAP7: D transport bulk / surface Input parameters Trap concentration profile by SDTrim.SP Saturated trap sites (TPD) Temperature ramp (TPD) Literature: Diffusion barrier 0.29 eV Dissolution energy 0.1 eV Free parameters Detrapping energies E T1 = 1.88 eV E T2 = 2.05 eV Detrapping energies E T1 = 1.88 eV E T2 = 2.05 eV

Schematic energy diagram E (D-Atom) TPD – Spectrum  Activation energies Atomic D E 0 ≡ 0 Position / State 0 Temperature [K] Desorption rate [a.u.]

Schematic energy diagram [E S = eV] Atomic D E 0 ≡ 0 E (D-Atom) Positions in the undisturbed bulk lattice Mobile state [ΔE D = 0.29 eV] Temperature [K] Desorption rate [a.u.]

Schematic energy diagram [E S = eV] Atomic D E 0 ≡ 0 Surface E (D-Atom) Surface processes Mobile state [ΔE D = 0.29 eV] [ΔE Ad = 0.87 eV] Molecular D 2 [E BE (1/2 D 2 ) = eV] Temperature [K] Desorption rate [a.u.]

Schematic energy diagram [E S = eV] Atomic D E 0 ≡ 0 [ΔE Ad = 0.87 eV] Molecular D 2 [E BE (1/2 D 2 ) = eV] Surface Mobile state [ΔE D = 0.29 eV] E (D-Atom) Activation energies obtained from modeling of TPD spectra Temperature [K] Desorption rate [a.u.]

Schematic energy diagramm [E S = eV] Atomic D E 0 ≡ 0 [ΔE Ad = 0.87 eV] Molecular D 2 [E BE (1/2 D 2 ) = eV] Surface Mobile state [ΔE D = 0.29 eV] ΔE = 1.25 eV 1.33 eV E (D-Atom) ΔE = 1.88 eV 2.05 eV Ion-induced defects Supersaturated states Activation energies obtained from modelling of TPD spectra BeD 2 Temperature [K] Desorption rate [a.u.]

Conclusion Deuterium retention in beryllium  Binding states / retention mechanisms identified and quantified  Hydrogen retention in ITER: negligible contribution of "pure" Be wall (< 7 g T by implantation)  Thin BeO surface layers are not rate-limiting for thermal recycling  Formation of BeD 2 at elevated temperatures  Currently: DFT calculations  Detailed understanding of D / Be Deuterium retention in beryllium  Binding states / retention mechanisms identified and quantified  Hydrogen retention in ITER: negligible contribution of "pure" Be wall (< 7 g T by implantation)  Thin BeO surface layers are not rate-limiting for thermal recycling  Formation of BeD 2 at elevated temperatures  Currently: DFT calculations  Detailed understanding of D / Be

Appendix

Literature data [Lossev,Küppers 1993] Surface desorption ?

Substrate characterization: SEM Cleaning: Cycles of 3 keV Ar + / 1000 K  Recrystallization + erosion Cleaning: Cycles of 3 keV Ar + / 1000 K  Recrystallization + erosion

Substrate characterization: SEM (1010) (1120) T  1000 K, several hours: Recrystallization to low indexed facets T  1000 K, several hours: Recrystallization to low indexed facets SEM

Substrate characterization: AFM 500 nm Recrystallization Erosion D Induced structural modifications Recrystallization Erosion D Induced structural modifications AFM Cycles of Cleaning D Implantation Degassing 1000 K Cycles of Cleaning D Implantation Degassing 1000 K

SEM: bubble & channel formation ? This work: Anderl et al. [1992]: Higher fluences  Const. retention  Aggregation !  Bubbles, pores, OPEN channels Higher fluences  Const. retention  Aggregation !  Bubbles, pores, OPEN channels Fluence ≤ 4·10 17 D cm -2  CLOSED nanosized structural modifications ! Fluence ≤ 4·10 17 D cm -2  CLOSED nanosized structural modifications !

Chemical surface composition BeO coverage < 0.2 ML < 1day ( mbar) > 1000 K BeO coverage < 0.2 ML < 1day ( mbar) > 1000 K Cleaning by Ar + bombardment Annealing 1000 K Cleaning by Ar + bombardment Annealing 1000 K XPS cleaned & annealed