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
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
2
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)
3
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.
4
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
5
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)
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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 nanoscale 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)
7
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
8
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
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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
10
Modeling of Ions/Target Interactions (ITMC-DYN)
Beam of H with C Impurities on W surface
Fluence 3x1024/m2
0.8 % of C
It was reported in the experiment that at 453 K a high
densities of blisters formed
ITMC-Dyn modeling with diffusion coefficient
of H in dependence on carbon concentration in
W target and for temperature of target 453 K
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Modeling of Ions/Target Interactions (ITMC-DYN)
Beam of H with C Impurities on W surface
Fluence 3x1023/m2
0.8 % of C
It the experiment blisters appeared under this fluence
and target temperature 653 K
ITMC-Dyn modeling with diffusion coefficient
of H in dependence on carbon concentration in
W target and for temperature of target 653 K
12
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 15-20 nm from
the surface in the case of 0.11% and 78 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.
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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
14
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
15
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
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Test of Diffusion Modeling
Carbon deposition, knock-out by H and diffusion in W
Self-consistently
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Influence of Post Irradiation Processes –
Surface Segregation and Diffusion
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Influence of Post Irradiation Processes –
Etching by 1.7 keV Ar ions
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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]
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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 - 2.5x1017 D/sm2/s; 1%, 2% and 3% of C in the beam; 600 K temperature in W
21
Blistering at similar to TEXTOR conditions
1x1017 D/cm2/s flux ‘clean’ and carbonseeded 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
22
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
23
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
Solving rate equations:
1
  [Be] [C]k exp kTE
1
m2 s
2
1
1
with k1
 
m4
s
destruction reaction fluxes
2
  [Be C]k exp  kTE
1
m2 s
2
2
2
with k2
1s 
d [ Be2C ]
d [ Be ]
 1  2  ...
 21  22  ...
dt
dt
Be2C formation:
ΔE=1.8 eV (exp.), k0 = 10-29 m4/s
Be2C dissociation:
ΔE=3.0 eV (from ΔHf), k0 = 1013 s-1
Ch. Linsmeier, 19th PSI San Diego 2010
24
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)
25
Code extension: 3D target structure for exploring
nonideal surface erosion
For exploring of:
Sputtering and deposition in W fuzz
Shin Kajita, 19th PSI San Diego 2010
Surface roughness effects on
impurities deposition
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
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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
27
Thank You
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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) 477486
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
29
Rates of mass losses
Fluence 3x1024/m2; target temperature – 653 K; C/H = 0.84%
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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
31
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
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