bottom-up integrated theoretical-experimental science for...
TRANSCRIPT
Managed by UT-Battelle for the Department of Energy IPAM, April 2012
Predrag Krstić
JICS, University of Tennessee
& Physics Division,
ORNL &
DBA TheoretiK
Support from: US DOE (OFES and OBES) and ORNL LDRD program
Bottom-up Integrated Theoretical-Experimental
Science for the Plasma-Material Interface
OFES Review, ORNL, March 02, 2010 Managed by UT-Battelle for the Department of Energy IPAM, April 2012 2
Close collaborators:
Fred Meyer (PD, ORNL)
Steve Stuart Clemson U.
Experiment:
Carlos Reinhold PD, ORNL
Theory:
Eric Hollmann (UCSD)
beam plasma
Our thanks to John Hogan (FED, ORNL)
jpg jpg
Paul Kent ORNL
JP Allain (Purdue)
Alain Allouche CNRS, Fr
Jae (ORISE)
Jonny (UTK)
Chris (MTSU)
Eric (Purdue)
Students
TBDFT modeling
K. Morokuma, Kyoto U.
J. Jakowski, NICS
H. Witek, Taiwan
Chase Taylor Purdue)
PWDFT Mat. CMD
beam and plasma
, Jeff Harris, Rick Goulding, FED, ORNL PhD PhD
Stephan Irle, Nagoya U
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Why PLASMA-MATERIAL INTERFACE
is such an important problem?
OFES Review, ORNL, March 02, 2010 Managed by UT-Battelle for the Department of Energy IPAM, April 2012
All energy from D-T fusion reactions passes through first wall
Vac.
Supercon–ducting magnet
Shield Blanket
Turbine generator
Plasma
a
Plasma heating
(rf, microwave, . . .)
Schematic magnetic fusion reactor ITER, DEMO
4
• Flux of (particles + heat + 14 MeV neutrons) ~10 MW/m2
Unlike nuclear fission where energy is volume-distributed
• Plasma-material interactions limit performance in present non-DT experiments
IR image of wall,
Tmax ~ 600°C TORE SUPRA
(France)
OFES Review, ORNL, March 02, 2010 Managed by UT-Battelle for the Department of Energy IPAM, April 2012
Guiding principle:
If Edison had a needle to find in a haystack, he would proceed at
once with the diligence of the bee to examine straw after straw
until he found the object of his search… I was a sorry witness of
such doings, knowing that a little theory and calculation would
have saved him 90% of his labor.
–Nikola Tesla, New York Times, October 19, 1931
The traditional trial-and-error approach to PMI for future fusion devices by successively refitting the walls of toroidal plasma devices with different materials and component designs is becoming prohibitively slow and costly
5
How to build an effective
science of PMI?
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Why bottom-up science?
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Probing the PMI requires integration of many experimental and theoretical
techniques spanning orders of magnitude in time, length, and energy scales
e.g., Rutherford backscattering,
elastic recoil detection
e.g., low energy ion scattering,
x-ray photoelectron spectroscopy
e.g., secondary neutral mass spectrometry
e.g., quartz crystal microbalance
Monte-Carlo techniques
Diffusion; transport
Courtesy of J.P. Allain
OFES Review, ORNL, March 02, 2010 Managed by UT-Battelle for the Department of Energy IPAM, April 2012
What does flux of 1025 particles/m2s mean (ITER) for a typical atomistic (MD) simulation?
At a box of surface of 3 nm lateral dim? a few thousands atoms (carbon) The flux is 0.01 particle/nm2ns 1) 1 particle at the interface surface of the cell each 100 ps. But for deuterium with impact energy less then 100 eV: Penetration is less than 2 nm, typical sputtering process takes up to 50 ps Each impact independent, uncorrelated!
In effect interaction of an impact particle with nanosize macromolecule functionalizes it! News is that each particle will change the surface for the subsequent impact!
8
OFES Review, ORNL, March 02, 2010 Managed by UT-Battelle for the Department of Energy IPAM, April 2012
PMI simulations: Erosion, sputtering, retention, redeposition,
reflection, displacement… originated from atomic processes
• Atomistic approaches therefore natural starting poi
–Quantum computbtational chemistry :
• Individual chemical reactions (and dynamics) for a few tens of atoms;
electronic excitations.
–Quantum (DFTB) and Classical Molecular dynamics (CMD): • Processes with hundreds and thousands of atoms;
classical dynamics on a predefined potential surface; as good as the potential is good
• DFTB parameterized QMD, 1000x slower than CMD, 1000x faster than
DFT based QMD
(a good fit to current supercomputers, we already parameterized
DFTB for all fusion materials)
–Binary collision approximation : • Good for physical sputtering cascades, elastic processes (TRIM),
displacement damage
–Kinetic Monte Carlo (KMC): • For long time-scale processes
• Diffusion of impurities in surfaces and bulk; depends on the
probability rates for individual processes determined by atomistic
approaches above
• Since irradiation creates dynamical surface (a modified interface), cumulative bom-
bardment is the key for modeling experiment (learned from previous PMI work)
… Material science: Continuum approaches
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Classical MD is only as good as the interatomic potential model used
,
( ) ( ) ( )rep ij ij ij attr ij
i j
E V r b r V r
Most advanced: hydro-carbon potential developed for chemistry
• Brenner, 1990 , 2002 : REBO, short range, 0.2nm
• more sophisticated AIREBO (Stuart, 2000, 2004, 1.1 nm)
• > 400 semi-empirical parameters, “bond order”, chemistry
EX: MD calc. of reflection coeff.
• Significant sensitivity to changes in
potential model for some processes
• Experimental validation essential to
establish credible MD simulation.
• Interatomic potentials for W and Be
are less mature than for carbon and
require more experimental validation.
Reinhold & Krstic, 2008
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Creation of the surface; “damaging”, annealing, hydrogenation
Short-time scale MD simulation
H collisional cascade; chemical processes
Probability rates:
diffusion, reactions
Long-time scale transport equation
(sources and sinks) for various particles
Monte Carlo simulation
Development of damage,
Diffusion of damage
Diffusion of hydrogen,
sputtering products
Total erosion, sputtering yield, retention
Surface desorption
FAST
ps-ns
nm
SLOW
ms-s
Terascale-
petascale
challenge
m
Simulation is computationally intensive and multi-scale
Short-time products
sputtering, reflection,
implantation
OFES Review, ORNL, March 02, 2010 Managed by UT-Battelle for the Department of Energy IPAM, April 2012
Why is PMI so difficult problem?
Interfacial physics, “when the two
worlds meet” : traditionally the most
challenging areas of science
Dynamical surface communicates
between two worlds: Plasma and Material
OFES Review, ORNL, March 02, 2010 Managed by UT-Battelle for the Department of Energy IPAM, April 2012 13
PMI has many fundamental processes & synergies
elastic reflection
implantation
re-emission &
sputtering &
chemistry
trapping/detrapping
retention
Plasma Material
diffusion, permeation
Give rise to synergistic effects
Damage Effects: Vacancies, bubbles, blisters, dislocations, voids, neutrons?
Drivers: Multi -T, -n, -species, plasma irradiation, neutrons sheath acceleration
Erosion
Ablation
Melting (metals)
Re-deposition
Co-deposition
When an ion or neutral arrives at a surface it undergoes a series of elastic and inelastic collisions
with the atoms of the solid.
OFES Review, ORNL, March 02, 2010 Managed by UT-Battelle for the Department of Energy IPAM, April 2012
-Type (H, H2, Ar, N, He)
- Impinging projectile energy
(1-100 eV), angle
- Internal state of the projectile
- Isotopic effect (D, T)
- Flux density (1021-1025 m-2s-1)
• Surface microstructure
- Crystalline, amorphous a-C,
polycrystalline); Doping (Si, B)
- Hydrogenation level (H/C ratio)
- Hybridization level (sp/sp2/sp3
ratio)
- Surface morphology; preparation
- Surface temperature (300-1500K)
• Predefined classical potentials (Brenner,
REBO, AIREBO (Stuart)) Limit to < 100 eV (D-D,D-C); <30 eV (C-C)
Function of :
Possible mechanism: Swift bond
breaking
OFES Review, ORNL, March 02, 2010 Managed by UT-Battelle for the Department of Energy IPAM, April 2012 15
PMI strategy is evolving thru ITER towards DEMO reactor
15
Divertor chamber
DEMO (> 2030?): • Steady-state, power flux ~ 10 MW/m2
• Hot walls (>600 C )
• Refractory metals
• Neutron irradiation 14.1 MeV (> 100 dpa)
Parameter range inaccessible in present devices
F valid extrapolation needed!
ITER (> 2020) uses multi-matl walls
Pulses ~ hundreds of sec ~Be Main chamber wall(700m2 )
Low Z + oxygen getter
~W Baffle/Dome (100 m2)
Funnels exhaust to divertor chamber
Low erosion, long lifetime
~ C Divertor Target (50 m2) (Graphite)
Minimize high-Z impurities
(which lead to large radiative losses)
OFES Review, ORNL, March 02, 2010 Managed by UT-Battelle for the Department of Energy IPAM, April 2012 16
Materials exposed to plasma are modified, resulting in a “dynamical” surface
Methane sputtering requires H loading of the surface
Plasma irradiation results in a different surface
Deuterium impact of carbon
Surface morphology
Chemical sputtering of hydrocarbons Amorphization depends on penetration depth rather
than of deposited energy
Depth (A)
-5 0 5 10
sp3 d
ensi
ty (
1/A
3)
0.000
0.005
0.010
0.015
0.0207.5 eV/D
5 eV/D
15 eV/D
20 eV/D
30 eV/D
surf
ace
impact
Krstic et al, New J. of Phys. 9, 219 (2007).
0 50 100 3500 3600
10
15
20
25
30
35
0 50 100 1900 2000
10
20
30
40
50
60
70
No
rma
lize
d M
ass 2
0 s
ign
al
Time (s)
saturated
signal
wall contribution to
saturated signal: ~ 20 %
Beam ONbackground level
Time (s)
saturated
signal
wall contribution to
saturated signal: ~ 10 %
Beam ON background level
10 eV / D
60 eV / D
depth (A)
num
ber
den
sity
(a.
u.)
0
50
100
150
H density
C density
0 10 20-10
impact
surf
ace
5 15 25-5
Fluence (1020
m-2
)
0
0.07
0.14
0.28
0.43
0.57
0.71
0.85
1.00
Reaching “steady state”
CD2
CD
CD3
Impact energy (eV/D)
5 10 15 20 25 30
Sp
utt
erin
g Y
ield
/D
2x10-3
4x10-3
6x10-3
8x10-3
1x10-2
sp3
sp2
sp
E (eV/D)
5 10 15 20 25 30
5
10
15
z<5
Co
unts
of
R-C
Dx m
oie
ties
, x
=1
,2,3
OFES Review, ORNL, March 02, 2010 Managed by UT-Battelle for the Department of Energy IPAM, April 2012 17
How to validate theory with experiments (and vv)?
OFES Review, ORNL, March 02, 2010 Managed by UT-Battelle for the Department of Energy IPAM, April 2012 18
Beam-surface exp’t: precision control of projectiles & targets . . .
. . . enabled development & validation of MD approach
Meyer et al, Physica Scripta T128, 50 (2007).
Remarkable agreement of theory & exp’t
when simulation mimics exp’t. No fitting
parameters! Key: simulation prepares surface by bombardment!
• Fluence (not flux) like that in experiment
• Type, internal state, energy, angle as in exp’t
What have we learned from the “next door”
beam-surface experiments?
Beam-surface experiments: Prepared beam & target
OFES Review, ORNL, March 02, 2010 Managed by UT-Battelle for the Department of Energy IPAM, April 2012
Chemical sputtering creates a dynamic, evolving surface • Competition between beam induced C=C bond breaking, H passivation, HC precursor formation, and H
loss, HC product emission
• At steady state dynamic equilibrium between H loading and H2 loss has been reached
0 50 100 3500 3600
10
15
20
25
30
35
0 50 100 1900 2000
10
20
30
40
50
60
70
No
rma
lize
d M
ass 2
0 s
ign
al
Time (s)
saturated
signal
wall contribution to
saturated signal: ~ 20 %
Beam ONbackground level
Time (s)
saturated
signal
wall contribution to
saturated signal: ~ 10 %
Beam ON background level
10 eV / D
60 eV / D
Time evolution of methane signal Steady state methane yield/D
Vergara et al., 2006
D3+ incident on ATJ graphite
Total carbon
CD3+CD4
No yield !!
Steady state
depth (A)
num
ber
den
sity
(a.
u.)
0
50
100
150
H density
C density
0 10 20-10
impact
surf
ace
5 15 25-5
Fluence (1020
m-2
)
0
0.07
0.14
0.28
0.43
0.57
0.71
0.85
1.00
(a) No impacts (b)1000 impacts
Fluence = 1.4 x1020 D m−2
(c)3000 impacts
Evolution of the density profile
MD
Stuart et al, J. Phys. Conf. Ser. 194, 012059 (2009)
OFES Review, ORNL, March 02, 2010 Managed by UT-Battelle for the Department of Energy IPAM, April 2012
Eje
cti
on
energ
y (
eV
)0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
Translational
Rovibrational
Rotational
Vibrational
Hydrocarbon ejection
D Impact energy (eV)
0 5 10 15 20 25 300.0
0.5
1.0
1.5
2.0
2.5CD3 ejection
(a)
(b)
New physics from theory-experiment cannot reach
Ejection energy Ejection temperature
Functions of hydrocarbon mass and impacting D energy
Approximate thermalization indicated
energy (eV)
0 2 4 6 8 10 12
Co
un
ts
0
100
200
300
400
500
600
700rotational distribution of ejecta
Fit based on Maxwell of 0.34 eV
H2 with D impact (10 eV)D2 with D impact (10 eV)
Krstic et al, J. Appl. Phys. 104, 103308 (2008).
OFES Review, ORNL, March 02, 2010 Managed by UT-Battelle for the Department of Energy IPAM, April 2012
3) Angular distributions dn/dΩ
vx (107 m/s)
-0.10 -0.05 0.00 0.05 0.10
vy (
10
7 m
/s)
-0.10
-0.05
0.00
0.05
0.10total hydrocarbon
E=30 eV/D
Angular moments L
sin law or isotropic
CM velocities follow cos-law
4) Isotope effects increase with the mass of the projectile
•Bond-breaking threshold are nearly mass independent and are dominated by associative-dissociative reactions
Krstic et al, J. Appl. Phys. 104, 103308 (2008).
Reinhold et al,J. Nucl. Mat. 401, 1 (2010).
OFES Review, ORNL, March 02, 2010 Managed by UT-Battelle for the Department of Energy IPAM, April 2012
Where MD Fails?
Do we need better potentials? Do we need direct line of sight experiments?
Open problem: simulations overestimates the yield of large molecules ->Erosion of carbon oversized at higher energies
We have developed a new hydrocarbon potential which includes ZBL corrections for higher impact energies and additional hydrocarbon params
Krstic et al, Nuc. Instr. Meth. B 267, 704 (2009).
OFES Review, ORNL, March 02, 2010 Managed by UT-Battelle for the Department of Energy IPAM, April 2012
Carb
on y
ield
(C
/D)
Temperature (K)
15 eV D impact
at deuterated a-C
200 400 600 800 1000
0.01
0.1
Legend
MD C yield
Exp Balden
MD C yield with red D
Example : Where MD fails?
Dependence of Carbon erosion on the surface temperature
Time scale of thermal D-release longer than ps-ns scale
Dadras et al, 2010
OFES Review, ORNL, March 02, 2010 Managed by UT-Battelle for the Department of Energy IPAM, April 2012
•Controlled experiments on many thoroidal devices demonstrated reduced recycling, improved energy confinement time E, and a reduction of edge instabilities known as edge localized modes (ELMs)
• Initially the experimentalists conjecture was that there was some "functionality" that governed the behavior of the Li-C-O-H system observed indirectly by analyzing the O(1s) and C(1s) peaks. For "some reason" the Li(1s) peaks didn't show much information.
Lithium dynamics is difficult to study theoretically because Li polarizing features when interacting with other elements
Lithium wall conditioning improves confinement! Why?
Consequence: Bonding between Li and other atoms covalent and polar; Long-range nonbonding: Coulomb :1/R Lenard-Jones :1/R6, 1/R12
OFES Review, ORNL, March 02, 2010 Managed by UT-Battelle for the Department of Energy U. Of Provence, May 23, 2011
Chemistry and sputtering/reflection dynamics in lithiated carbon material, bombarded by slow deuterium atoms is studied. The objectives of this research are two-fold:
1) To develop the realistic methods for computational
simulation of the Li-C-H-O, validated by experiments.
2) To explain the specifics of the chemistry of deuterium bonding in lithiated carbon. Experiments from Purdue indicate that bonded C-Li-O? sites are preferable for H bonding:
Goals:
OFES Review, ORNL, March 02, 2010 Managed by UT-Battelle for the Department of Energy
Quaternary system Li-C-O-H
•Li-C , Li-H and Li-O are of very different electronegativities and therefore these could be considered as ionic solids. •The implication is that, as a result of partial charge transfer from one element to the other, the dominant long-distance binding force between particles is the Coulombic attraction between opposite charges.
tot bound noboundE E E nobound coul vdWE E E
i j
coul
i j i ij
q qE
r
12 6[( ) 2( ) ]
ij ij
vdW ij
i j i ij ij
E Dr r
Total energy in LiXY mixture
Currently our biggest efforts in th!! (6 members of the team on it):
OFES Review, ORNL, March 02, 2010 Managed by UT-Battelle for the Department of Energy
Charging changes at each simulation step:
Quantum-Classical Molecular dynamics a must
Employed the Self-Consistent-Charge Density-Functional Tight-Binding (SCC-DFTB) method (developed by Bremen Center for Computational Materials Science, Germany) It is an approximation to the DFT method in which only valence orbitals are considered, and difficult Integrals parameterized in advance. In comparison to other TB methods: Improved self-consistent interaction of electronic charges This enables computational efficiency about 1,000 time faster than ab initio quantum methods (and about 1,000 time slower than Classical Molecular Dynamics) Parameterization for Li-C-H-O provided by K. Morokuma group (Kyoto-ORNL-Emory) Inclusion of Mo and He in parameterization: H. Witek (Taiwan U.)
Electrons: Quantum mechanically at each step, resulting in charges and forces Nuclei: Classical motion
[1] M. Elstner, D. Porezag, G. Jungnickel, et al, Phys. Rev. B 58, 7260 (1998) [2] G. Zheng, M. Lundberg, J. Jakowski, at al, Int. J. Quantum Chem. 109, 1841 (2007).
U. Of Provence, May 23, 2011
OFES Review, ORNL, March 02, 2010 Managed by UT-Battelle for the Department of Energy
Simulation of deuterium impact to lithiated and oxidated carbon surface
•Cell of a few hunreds of atoms of lithiated and oxidated amorphous carbon (~30% of Li, < 10% of O), at 300K •By random seed of Li and O in amorphous carbon and energy minimization, followed by thermalization •bombarded by 5 eV D atoms and 2.5 eV H, •Perpendicularly to the shell interface •5004 random trajectories
5004 processors of Cray XT5 (Jaguar, Kraken), Time step 0.2-1fs, 24 hours: 200-400 fs. One run over 120,000 CPU hours (TeraGrid project)
IPAM, April 2012
The cell swelled during the structure optimization
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29 PPPL, 2nd ISLAFD, April 28, 2011
Only C+H
C-Li-O
Slabs studied: Periodicity in x-y
IPAM, April 2012
OFES Review, ORNL, March 02, 2010 Managed by UT-Battelle for the Department of Energy
Quantum-mechanical, PWDFT “static”
calculations of Alain Allouche of CNRS
(Marseille) finger -point in the same direction:
•graphene bilayer with Li and H on the surface
When a lithium atom is co-adsorbed on surface bonding energy of H grows up to values ranging from -2.2 to -2.5 eV, with decreasing the Li-H distance. (compared with -1.9 eV for pure graphite)
The bonding E enhancement is also observed when Li is sandwiched 1 layer below the surface layer, but disappears when Li is 2 layers below the surface.
•graphene 4-layers, Li inside the slab, H on surface
Another remarkable finding of Allouche: With inclusion of defects the energy shows similar shift like in XPS exper.
IPAM< April 2012
A. Allouche, Carbon (2012)
OFES Review, ORNL, March 02, 2010 Managed by UT-Battelle for the Department of Energy IPAM, April 2012
Here comes the experiment J.P. Allain, C. Taylor, Purdue U): 1) At most 5% oxygen content
on the surface of NON-LITHIATED graphite... AS EXPECTED.
2) With lithium it increases to 10% of Oxygen
3) With LOW-ENERGY IRRADIATION over 20-40% oxygen AT the surface.
Surface contains much more Oxygen than expected
Our quantum-classical molecular dynamics predictions showed that if there is comparable quantity of oxygen in the surface, O takes over and dominates the Retention chemistry
Submitted to Nature Communications (2012)
OFES Review, ORNL, March 02, 2010 Managed by UT-Battelle for the Department of Energy IPAM, April 2012 32
Is the PMI phenomenology with plasma
Irradiation different than with a beam
irradiation?
Can we derive plasma-irradiated PMI from the
beam-PMI?
Can we model plasma irradiation?
Answer is: Yes, No, Yes…
OFES Review, ORNL, March 02, 2010 Managed by UT-Battelle for the Department of Energy IPAM, April 2012 33
Synergies in tungsten: dramatic effects in plasma experiments
Blistering related to D, H retention
• Sparser for T > 600 K H implantation(2-20 nm)
grain ejection
H accumulation
@ grain boundaries
Dome-like blisters
> 1 µm
Ueda et al,2008
He suppresses H retention
• He penetrates deeper than H
• Strong dependence on energy
• He bubbles: barrier to H diffusion?
He: 0.1% He: 0%
T=653 K
“Fuzz” nanostructures
on W irradiated by He
high W temperatures (>1000K)
Baldwin,
2008
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Goal: Study synergy in the evolution of surface irradiated by plasma
Exposure of pyrolytic graphite to
5 MeV C+ simulates NEUTRON
DAMAGE:
• more nucleation enhanced erosion
• sites for H retention
• increased HC density?
• increased ejection probability?
B.I. Khripunov et al. 2009
H alone Ar+ alone
Hopf & von Keudell, 2003
H and Ar+ Ion flux = 3.5 * 1012 cm-2 s-1
H flux = 1.4 * 1015 cm-2 s-1
physical
Experiments with Ar+ and H:
• Sputtering = (chemical) + (physical) • Surface preparation by H impact for
chemical sputtering
• Impurity atoms in plasma are efficient
precursors for erosion
• PM processes very dependent on
inventory of H in the material
OFES Review, ORNL, March 02, 2010 Managed by UT-Battelle for the Department of Energy IPAM, April 2012 35
E (eV)
0 2 4 6 8 10
Co
un
ts
0
100
200
300
400
500
600
T=10,000K
sqrt(E) exp(-E/kT)
VXVY
VZ
-0.3 -0.2 -0.1 0.0 0.1 0.2 0.3 -0.20.0
0.2-0.25
-0.20
-0.15
-0.10
-0.05
0.00
T=10,000KAll VZ>0 changed sign
Simulations of the plasma irradiation (D atoms)
Impact energy of D (eV)
0 5 10 15 20 25 30
Yie
ld
0.0
0.2
0.4
0.6
0.8 D ejected
D2 sputtered
plasma
beam
With plasma irradiation:
Reflection significantly higher than
beam, sputtering suppressed !!!
Krstic et al, 2009
Maxwell
uniform
Impact distributions
HOW?
Krstic et al, AIP Conf. Proc. 1161, 75 (2009).
OFES Review, ORNL, March 02, 2010 Managed by UT-Battelle for the Department of Energy IPAM, April 2012
CONCLUSIONS:
• PMI extremely difficult interfacial problem (Material mixing create SURFACE entity; its scale depends on impact energy: for sub-100eV => nm-ns scales
• PMI science can be built from bottom-up recognizing its multiscale character and building from shortest time/spatial scales (fs/Angstrom) up
• Theory&modeling of PMI has to be validated by experiment (and v.v.)
• Irradiation create dynamical surface, changing interface, cumulative bombardment is the key for agreement with experiment
• Surface responds to synergy in plasma irradiation (angles, energies, particles), NOT following linear superposition principle; NEED plasma irradiation modeling and experiments; dedicated plasma devices a must
• Oxygen is doing D chemistry in lithiated carbon surfaces