1. Dynamic evolution of mixed materials bombarded with multiple ions beams and impurities
Tatyana Sizyuk
Ahmed Hassanein
School of Nuclear Engineering
Center for Materials Under eXtreme Environment
Purdue University
PFC community meeting at UCLA
August 4-6, 2010, Los Angeles CA

2. Outline ITMC-DYN Computer Simulation Package
Collision-induced reactions + Diffusion, Segregation, Chemical erosion, Surface recombination, and Desorption
Benchmarking for ELM averaged regimes
Benchmarking for TEXTOR discharges
PWI and material issues for ITER performance
Future code extension:
(1) Chemical reaction rates + detailed processes of chemical erosion
(2) 3D target structure for nonideal surface erosion
Summary

3. ITMC-DYN Integrated Models
A. Hassanein, “Surface effects on sputtered atoms and their angular and energy dependence”, Fusion Technology 8 (1985) 1735.
A. Hassanein and D.L. Smith, “Elastic and inelastic surface effects on ion penetration and the resulting sputtering and backscattering”, Nuclear Instruments and Methods in Physics Research B13 (1986) 225.
T. Sizyuk and A. Hassanein, "Dynamic analysis and evolution of mixed materials bombarded with multiple ions beams", J. Nucl. Mater., available online 1 July 2010
T. Sizyuk and A. Hassanein “Dynamic analysis of mixed ion beams/materials effects on the performance of ITER-like devices“, to be published J. Nucl. Mat. (2010)

4. ITMC-DYN Integrated Modeling
Collision models are integrated with detail models of time-dependent processes such as atom diffusion and segregation. Therefore, we consider actual fluences and exact irradiation times of laboratory experiments.
Time step is determined from processes of diffusion, molecular recombination, and surface segregation.
Additional surface boundary conditions are used to specify the recombination of a diatomic hydrogen isotope molecules and their release from the surface in modeling of both laboratory and reactor conditions.
Surface segregation models are implemented and helped to explain recent experimental results.
Chemical erosion of carbon is calculated in each layer near the surface, depending on carbon concentration. Erosion yield is compared with that predicted by well-defined and benchmarked formula.

5. Simulation Results of ITMC-DYN
Influence of binding energy on surface build-up and recession
2.0 eV binding energy of Cs
0 eV binding energy of Cs
Effect of potential and fluence on surface growth/erosion of Cs (on Si surface) with 0 eV binding energy of Cs in modeling

6. Testing Dynamic Update of Materials Composition
Nano-scale multilayers formation – 90 eV Fe+ and 50 eV C+ -> Si
High-resolution TEM micrograph of Fe/C multilayers with 4 bilayer periods: 0.75 nm, 1.25 nm, 2.5 nm and 5.0 nm in order from the Si substrate, respectively.
ITMC-Dyn – 4 bilayer periods: ~1.1 nm, ~ 1.8 nm, ~ 2 nm, ~ 4.3 nm.
(each from 2 bilayers)

7. Testing Dynamic Update of Materials Composition
Nano-scale multilayers formation – 90 eV Fe+ and 50 eV C+ -> Si
Sung-Yong Chun, “TEM-examination and computer simulation of nano-scale multilayers by pulsed cathodic arc deposition”, Journal of Ceramic Processing Research. Vol. 4, No. 3, pp. 115~117 (2003)
ITMC-Dyn – 4 bilayer periods: ~1.1 nm, ~ 1.8 nm, ~ 2 nm, ~ 4.3 nm.
(each from 2 bilayers)

8. Benchmarking Multiple Beams on Targets
Beams of hydrogen ions containing carbon impurity ions were impinged on pure tungsten samples heated from 653 to 1050 K.
Ion beams consisted of 70% of 333 eV H+, 10% of 500 eV H+, 20% of 1000 eV H+ and from 0.1% to 1% of C ions with 1000 eV energy.
Carbon concentration on the surface of tungsten was measured and conditions for blisters formation were investigated in dependence on the total fluence, on % of carbon concentration in ion beams, and on samples temperatures.
1) Y. Ueda, T. Shimada, and M. Nishikawa, "Impacts of carbon impurities in hydrogen plasmas on tungsten blistering", Nucl. Fusion 44 (2004) 62
2) T. Shimada, T. Funabiki, R. Kawakami, Y. Ueda, M. Nishikawa, "Carbon behavior on tungsten surface after carbon and hydrogen mixed beam irradiation", J. Nucl. Mater., 329–333 (2004) 747–751

9. Blisters formation and dependence on target temperature
Blisters formation depends on mobility of implanted gas and on concentration of gas atoms in the target
Accumulation of H in tungsten target in the experiments can be due to lattice imperfections, ion-induced damage (that is not applicable for low energies of H ions), and carbon deposition
In these experiments carbon was deposited in the form of graphite or in binding with tungsten (WC or W2C) on the surface of the samples

10. Modeling of Ions/Target Interactions (ITMC-DYN) Beam of H with C Impurities on W surface
Fluence 3x1024/m2
0.8 % of C
T. Shimada, T. Funabiki, R. Kawakami, Y. Ueda, M. Nishikawa, "Carbon behavior on tungsten surface after carbon and hydrogen mixed beam irradiation", J. Nucl. Mater., 329–333 (2004) 747–751
ITMC-Dyn modeling with diffusion coefficient of H in dependence on carbon concentration in W target and for temperature of target 653 K

11. Modeling of Ions/Target Interactions (ITMC-DYN) Beam of H with C Impurities on W surface
Fluence 3x1024/m2
0.8 % of C
ITMC-Dyn modeling with diffusion coefficient of H in dependence on carbon concentration in W target and for temperature of target 453 K
It was reported in the experiment that at 453 K a high densities of blisters formed

12. Modeling of Ions/Target Interactions (ITMC-DYN) Beam of H with C Impurities on W surface
Fluence 3x1023/m2
0.8 % of C
ITMC-Dyn modeling with diffusion coefficient of H in dependence on carbon concentration in W target and for temperature of target 653 K
It the experiment blisters appeared under this fluence and target temperature 653 K

13. Dependence of blisters formation on impurity concentrations
The lower content of carbon of 0.11% in comparison with 0.84% resulted in the following differences in samples:
Peak carbon concentration changed from 40% to more than 60%
its location is shifted nm from the surface in the case of 0.11% and 7-8 nm in the case of 0.84%
target erosion is almost 10 times higher under the ion fluence containing 0.84% C
The difference in the target erosion explains the difference in peak location. Carbon atoms, redistributed inside the W target by incoming hydrogen ions, are then removed by surface erosion and this process is accelerated in the case of 0.84% C concentration in beam.

14. Modeling of Ions/Target Interactions (ITMC-DYN) Beam of H with C Impurities on W surface
Conclusion: Low level of impurity contents in plasma can significantly affect erosion lifetime, largely increase hydrogen isotope retention, and enhance bubble/blister formation in candidate reactor materials in fusion environments.
Evidence from other experiments:
“It is assumed that tungsten carbide formed on the W surface under exposure to the carbon-seeded D plasmas serves as a barrier layer for diffusion and prevents the outward transport of deuterium, thus increasing the D retention in the bulk of tungsten.”
V. Kh. Alimov, et al., “Deuterium retention in tungsten exposed to low-energy, high-flux clean and carbon-seeded deuterium plasmas”, J. Nucl. Mater. 375 (2008) 192

15. Effects of segregation and etching on carbon concentration in target
Surface segregation moves carbon atoms from bulk to surface layer because of increasing chemical potential of carbon in the bulk of W/C compound.
Implementing surface segregation can shift peak of carbon concentration profile closer to the surface
However simulating this processes self-consistently with ions beam deposition, target atoms sputtering, and atom cascade redistribution  have the effect that surface segregation loses its effectiveness

16. Influence of Post Irradiation Processes – Surface Segregation, Diffusion and Etching
Post-ion-deposition surface segregation, and subsequent processes of etching of samples
Etching is done using 1.7 keV Ar ions in experiments for measurement of carbon concentration profile
This can explain the reason of the enhanced carbon accumulation on the surface

17. Test of Diffusion Modeling
Carbon deposition, knock-out by H and diffusion in W
Self-consistently

18. Influence of Post Irradiation Processes – Surface Segregation and Diffusion

19. Influence of Post Irradiation Processes – Etching by 1.7 keV Ar ions

20. Benchmarking TEXTOR discharges
Textor limiter was exposed at a radial distance R = 48.5 cm from plasma center and 2.5 cm behind the last close flux surface (LCFS) during 125 plasma discharges with a total duration of 559 plasma seconds
Edge plasma parameters during the experiment were measured with fast probe and He-beam diagnostics. The ion fluence on the limiter estimated using He-beam data had a value of 4.9 · 1019 ion/cm2 averaged over the area of castellation
Surface temperature of the limiter was varied from 200 C to 400 C which agrees well with values expected for upper vertical targets of ITER divertor
[A. Litnovsky, et al., J. Nucl. Mater. 367–370 (2007) 1481]

21. Benchmarking TEXTOR discharges
At carbon concentration in beam of 3%, carbon layer covers tungsten surface and can prevent hydrogen diffusion to the bulk
Parameters for modeling were used based on fast probe measurements:
120 eV D ions, 300 eV C ions
Parameters for TEXTOR experiments:
Ion flux x1017 D/sm2/s; 1%, 2% and 3% of C in the beam; 600 K temperature in W

22. Blistering at similar to TEXTOR conditions
1x1017 D/cm2/s flux ‘clean’ and carbon-seeded D plasmas (ion fluence of about 2x1020 D/cm2);  200 eV/D
Impurity concentration in the near surface layer:
V. Kh. Alimov, et al., “Deuterium retention in tungsten exposed to low-energy, high-flux clean and carbon-seeded deuterium plasmas”, J. Nucl. Mater. 375 (2008) 192

23. Integrated Modeling issues for PWI
Erosion, re-deposition
Mixed ions beams with impurities, erosion of compounds, coupling with above surface plasma codes
D and T retention
Hydrogen diffusion, molecular recombination and desorption, chemical erosion, saturation in C and Be, agglomeration and blistering in W
He retention
He diffusion, bubbles and blisters formation, W surface modification
Surface morphology
Physical sputtering and impurities deposition in dependence on surface conditions
Mixing and alloying
Chemical reactions rates for compounds formation
V. Philipps, 19th PSI San Diego 2010

24. Code extension: reaction rates for compounds formation at first wall materials
Elementary reactions, equations for reaction fluxes and experimental reaction parameters (sample of Be and C compound):
formation reaction fluxes
2 Be + C → Be2C
Be2C → 2 Be + C
destruction reaction fluxes
Solving rate equations:
Be2C formation:
ΔE=1.8 eV (exp.), k0 = m4/s
Be2C dissociation:
ΔE=3.0 eV (from ΔHf), k0 = 1013 s-1
Ch. Linsmeier, 19th PSI San Diego 2010

25. Code extension: detailed processes of chemical erosion
Three processes determine the chemical erosion of carbon under low-energy hydrogen bombardment:
Reaction of thermalized ions within implanted surface proceeds via the hydrogenation of carbon atoms at edges of graphitic planes to CH3–C complexes
Thermal reaction is enhanced by radiation damage induced in material that provides open bonds for hydrogen attachment.
At low surface temperatures: kinetic hydrocarbon emission or surface effects, since hydrocarbon radicals are bound to surface with much less binding energy (≈1 eV) than are carbon atoms in their regular lattice environment (7.4 eV)
J. Roth, et al., Nucl. Fusion 44 (2004)

26. Code extension: 3D target structure for exploring nonideal surface erosion
For exploring of:
Surface roughness effects on impurities deposition
Sputtering and deposition in W fuzz
Shin Kajita, 19th PSI San Diego 2010
Y. Ueda, J. Nucl. Mater. 390–391 (2009)
This will be accomplished by significantly increasing mesh size and target structure with parallel implementation of code running at ANL Supercomputers

27. Summary & Conclusion
A new dynamic version of ITMC code is developed (ITMC-DYN)
This version is capable of dynamically simulating target composition changes and all physical processes as a function of irradiation time
New models and simulation of diffusion, segregation, chemical erosion, and etching processes are developed and implemented
Initial results of modeling were presented and compared quite well with recent laboratory experiments of mixed ion beams and the effect of blister formation in tungsten material
Future work ($$ ?) will include more detailed reaction processes and simulating more complicated experiments, real reactor conditions, and to propose/recommend laboratory experiments to enhance our understanding of mixed materials behavior and effects

28. Thank You

29. Examples of Simulation Results
Influence of binding energy on surface build-up and recession
W. Eckstein et al.,”Ion-induced alkali-silicon interfaces: Atomistic simulations of collisional effects”, Nucl. Instr.Meth. B119 (1996)
ITMC-DYN – Effect of potential and fluence on surface growth/erosion of Cs (on Si surface) with 0 eV binding energy of Cs in modeling

30. Fluence 3x1024/m2; target temperature – 653 K; C/H = 0.84%
Rates of mass losses
Fluence 3x1024/m2; target temperature – 653 K; C/H = 0.84%

31. PWI and Material Issues for ITER Performance
Erosion, re-deposition
T retention
He retention
Surface morphology
Cracking
Melting and melt stability
Mixing and alloying
V. Philipps, 19th PSI San Diego 2010

32. Code extension: detailed processes of chemical erosion
Total methane erosion can be calculated as sum of temperature dependent erosion YT (Mech model)
And kinetic part Y0 (Hopf model for temperature 300 K)
We will implement also in model:
Inventory of different hybridization states of carbon atoms (sp2, sp3 and sp3H)
Control of upper limit of H atoms bounded to C (1/3)
Control of maximum sp3 centers as function of temperature