Deuterium retention mechanisms in beryllium

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Deuterium retention mechanisms in beryllium
M. Reinelt, Ch. Linsmeier Max-Planck-Institut
für Plasmaphysik EURATOM Association, Garching
b. München, Germany
PSI-18 28 May 2008
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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
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Motivation
Be 700 m2
D implantation into beryllium Deuterium
retention / thermal recycling of ITER main
wall Be Fast reaction with O2 / H2O Previous
experiments BeO contaminated surfaces
Investigation of System Be (BeO) / D Clean
Be / D NO DATA !
D,T
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Motivation
Be 700 m2
Anderl et al. 1999
D,T
Variation by 1 ORDER OF MAGNITUDE !
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Concept

• Possible reasons
• Undefined BeO coverage
• Undefined crystallinity
• Unclear retention mechanisms
• (Needed for quantification)

• Issues to be solved
• Retention in pure Be ?
• Crystallinity
• Influence of BeO ?
• Retention mechanisms ?

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Experimental concept
in situ... (10-11 mbar)
TPD Temperature Programmed Desorption

• Sequential release of D
• Limited by combination of
• bulk surface
• processes
• Energy barriers for ...
• Diffusion
• Detrapping
• Recombination

Ar sputter cleaning annealing XPS /
LEIS Control of surface
Thermal release
NRA
Retention mechanisms
1 keV D implantation
Hydrogen retention
Single crystalline Be
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TPD Spectrum
Sequential release Energetically different rate
limiting steps

• 1 keV D Implantation (300 K)
• 21017 D cm-2

m/q 4 (D2)
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TPD Increasing fluence
TPD Spectra recorded in random order ! ? Fluence
dependent behaviour
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TPD Increasing fluence
Trapping in ion induced defects
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TPD Increasing fluence
Structural modifications
Trapping in ion induced defects
Local saturation of available binding sites
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TPD Increasing fluence
Structural modifications
Sample saturation
Threshold
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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?

(cut off)
SDTrim.SP Calculation
TPD Experiments
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Structural modifications / Surface desorption

1st order release 1 DTrapped ? DMobile 2nd
order release 2 D ? D2 (Surface
desorption)
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Structural modifications / Surface desorption
Peak shape Desorption peak is 1st
order Surface area (AFM) Release of 60 x T
(saturation coverage T 0.5) AFM max.
1.2 T ? Surface recom-bination is not the
rate-limiting step

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TPD Influence of BeO-coverage
No change of EA of release from binding
states No recombination limit ?
Trapping in the bulk Formation of
BeO-D at the surface
BeOD (surface)
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TPD Elevated implantation temperatures
530 K
300K
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TPD Elevated implantation temperatures
Different retention mechanism !
Change of the binding states in the
supersaturated areas
Ion-induced trap sites unaffected
300K
530 K
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TPD Elevated implantation temperatures
Different retention mechanism !
Change of the binding states in the
supersaturated areas BeD2 formation
(Decomposition 570 K)
BeOD (surface)
Ion-induced trap sites unaffected
BeD2
300K
530 K
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Implanted / Co-deposited
1 keV Ion implanted (this work) D/Be plasma
co-deposited (de Temmerman)
300 K
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Implanted / Co-deposited
300 K
1 keV Ion implanted (this work) D/Be plasma
co-deposited (de Temmerman)
Supersaturated material
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Implanted / Co-deposited
300 K
1 keV Ion implanted (this work) D/Be plasma
co-deposited (de Temmerman)
Ion-induced traps in the bulk
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Implanted / Co-deposited
Formation of BeD2 see also poster P3-05 by R.
Doerner
300 K
600 K 530 K
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Qualitative interpretation of data
Ion-induced traps in the bulk lattice
Structural modifications
BeD2
Constant retention
Be ( BeO) 1 and 1.5 keV
clean Be (1 keV)
Anderl et al. 1999
300 400 500 600 700 800
900 1000 Specimen exposure
temperature K
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Identification of retention mechanisms Quantifi
cation TMAP7 / Rate equations
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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 ET1 1.88 eV ET2 2.05
eV
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Schematic energy diagram
TPD Spectrum ? Activation energies
Atomic D E0 0
0
Position / State
E (D-Atom)
Temperature K
Desorption rate a.u.
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Schematic energy diagram
Positions in the undisturbed bulk lattice
E (D-Atom)
Mobile state ?ED 0.29 eV
Atomic D E0 0
ES -0.10 eV
Temperature K
Desorption rate a.u.
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Schematic energy diagram
Surface processes
E (D-Atom)
Mobile state ?ED 0.29 eV
Atomic D E0 0
ES -0.10 eV
?EAd 0.87 eV
Temperature K
Molecular D2 EBE (1/2 D2) -2.278 eV
Surface
Desorption rate a.u.
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Schematic energy diagram
Activation energies obtained from modeling of TPD
spectra
E (D-Atom)
Mobile state ?ED 0.29 eV
Atomic D E0 0
ES -0.10 eV
?EAd 0.87 eV
Temperature K
Molecular D2 EBE (1/2 D2) -2.278 eV
Surface
Desorption rate a.u.
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Schematic energy diagramm
Activation energies obtained from modelling of
TPD spectra
E (D-Atom)
Mobile state ?ED 0.29 eV
Atomic D E0 0
ES -0.10 eV
?EAd 0.87 eV
?E 1.25 eV 1.33 eV
BeD2
Temperature K
Molecular D2 EBE (1/2 D2) -2.278 eV
?E 1.88 eV 2.05 eV
Surface
Desorption rate a.u.
Ion-induced defects
Supersaturated states
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Conclusion

• Deuterium retention in beryllium
• Binding states / retention mechanisms identified
  and quantified
• Hydrogen retention in ITER negligible
  contribution of "pure" Be wall
• (lt 7 g T by implantation)
• Thin BeO surface layers are not rate-limiting
  for thermal recycling
• Formation of BeD2 at elevated temperatures
• Currently DFT calculations
• ? Detailed understanding of D / Be

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Appendix
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Surface desorption ?
Literature data Lossev,Küppers 1993
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Substrate characterization SEM
Cleaning Cycles of 3 keV Ar / 1000 K ?
Recrystallization erosion
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Substrate characterization SEM
SEM
(1010)
(1120)
T ? 1000 K, several hours Recrystallization to
low indexed facets

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Substrate characterization AFM
AFM
500 nm

• Recrystallization
• Erosion
• D Induced structural modifications

• Cycles of
• Cleaning
• D Implantation
• Degassing 1000 K

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SEM bubble channel formation ?
This work
Anderl et al. 1992
Higher fluences ? Const. retention ?
Aggregation ! ? Bubbles, pores, OPEN
channels
Fluence 41017 D cm-2 ? CLOSED nanosized
structural modifications !
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Chemical surface composition
XPS
cleaned annealed

• BeO coverage lt 0.2 ML
• lt 1day (10-11 mbar)
• gt 1000 K

• Cleaning by Ar
• bombardment
• Annealing 1000 K