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Max-Planck-Institut für Plasmaphysik, EURATOM
Association
Multiscale modeling of hydrogen isotope
transport in porous graphite
Manoj Warrier
Thesis advisor Ralf Schneider
PhD work within IMPRS since January,
2002 Max-Planck Institut für Plasmaphysik Stellara
tor Theory Division, Edge Modeling Group
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Max-Planck-Institut für Plasmaphysik, EURATOM
Association 2. Outline
Plasma Wall Interaction and motivation
Multi-scale approach and results
Summary and conclusions
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Max-Planck-Institut für Plasmaphysik, EURATOM
Association
3. Plasma Wall Interaction in Fusion
Challenge Extremely high power loads
Requirement Pure plasma core
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Max-Planck-Institut für Plasmaphysik, EURATOM
Association
4. Graphite as a PFM
Good thermal conductivity, high sublimation
energy, low atomic number
V. Rohde (IPP, Garching)
But chemical sputtering, hydrogen isotope
inventory
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Max-Planck-Institut für Plasmaphysik, EURATOM
Association
5. Porous Structure of Graphite
Granule sizes microns Void sizes 0.1
microns Crystallite sizes 50-100 Å Micro-void
sizes 5-10 Å
Multi-scale problem in space (1 cm to 1 Å)
and time (ps to s)
H transport in complex, 3D, porous graphite
structure
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Max-Planck-Institut für Plasmaphysik, EURATOM
Association
6. Multi-scale approach
Macroscales KMC and Monte Carlo Diffusion (MCD)
Mesoscales Kinetic Monte Carlo (KMC)
Microscales Molecular Dynamics (MD)
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Max-Planck-Institut für Plasmaphysik, EURATOM
Association
7. Molecular dynamics at microscales
Hydrogen in crystal graphite (960 atoms)
Brenner potential, Nordlund long range
interaction
HCParcas Developed by Kai Nordlund
Berendsen thermostat (150K - 900K for 100 ps)
Reactive Empirical Bond Order (REBO) potential
allows simulation of hydrocarbon reactions
Periodic boundary conditions
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Max-Planck-Institut für Plasmaphysik, EURATOM
Association
8. MD simulation at 150K and 900K
150K
900K
Large jumps at high temperatures gt 450K No
diffusion across graphene layers
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Max-Planck-Institut für Plasmaphysik, EURATOM
Association
9. MD simulation results
Two diffusion channels
Non-Arrhenius temperature dependence for hydrogen
isotope diffusion in crystal graphite
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Max-Planck-Institut für Plasmaphysik, EURATOM
Association
10. Kinetic Monte Carlo - basic idea
Poisson process (assigns real time to the
jumps) Jumps are independent (no memory)
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Max-Planck-Institut für Plasmaphysik, EURATOM
Association
11. Mesoscales - Comparison with experiments
standard graphites
highly saturated graphite
Large variation in observed diffusion coefficients
Strong dependence on void sizes and not void
fraction Saturated H ?0105s-1 and step sizes
1Å (QM?)
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Max-Planck-Institut für Plasmaphysik, EURATOM
Association
12. Effect of voids
B 20 voids
C 20 voids
A 10 voids
Larger voids
Higher diffusion
Longer jumps
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Max-Planck-Institut für Plasmaphysik, EURATOM
Association
13. KMC and MCD at macroscales
Trapping - detrapping (2.7 eV) Desorption (1.9
eV) Surface diffusion (0.9 eV)
KMC with Jump lengths depend on the process
Monte Carlo Diffusion (MCD) used to simulate TGD
?
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Max-Planck-Institut für Plasmaphysik, EURATOM
Association
14. Results at macroscales
variation of 3D structure
surface diffusion 0.9 eV
adsorption- desorption 1.9 eV
Different processes dominate at different
temperatures Diffusion in voids dominates
Diffusion coefficients without knowledge of
structure are meaningless
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Max-Planck-Institut für Plasmaphysik, EURATOM
Association
15. Further results at macroscales
Interpretation of diffusion?
Subdiffusion
Superdiffusion
H atom desorption begins above 1200 K
Closed pores efficiently supress hydrogen
diffusion
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Max-Planck-Institut für Plasmaphysik, EURATOM
Association
16. Defect agglomeration
Graphite surface during hydrogen bombardment
(STM analysis from T. Angot et
al., Univ. of Provence, Marseille)
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Max-Planck-Institut für Plasmaphysik, EURATOM
Association
17. Defect agglomeration
Defect agglomeration on graphite surface
reproduced by simulation
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Max-Planck-Institut für Plasmaphysik, EURATOM
Association
17. Defect agglomeration
Defect agglomeration on graphite surface
reproduced by simulation
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Max-Planck-Institut für Plasmaphysik, EURATOM
Association
Summary
Multi-scale model developed
M. Warrier, R. Schneider, E. Salonen, K.
Nordlund, Physica Scripta, T108 (2004) 85. M.
Warrier, R. Schneider, X Bonnin, Computer Physics
Communications, 160, 1 (2004) 46 R. Schneider,
et. al., Computer Physics Communications 164
(2004) 9.
Model reproduces experimental results
H atom
desorption, diffusion coefficients, defect
agglomeration
M. Warrier, R. Schneider, E. Salonen, K.
Nordlund, J. Nucl. Mater (In press). M. Warrier,
R. Schneider, E. Salonen, K. Nordlund, Contrib.
Plasma Phys., 44, 1-3 (2004) 307
Model suited for predictions
diffusion
coefficients, isotope exchange, chemical
sputtering
Diffusion coefficients without knowledge of
structure are meaningless