bottom-up integrated theoretical-experimental science for...

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

OFES Review, ORNL, March 02, 2010 Managed by UT-Battelle for the Department of Energy IPAM, April 2012

Why PLASMA-MATERIAL INTERFACE

is such an important problem?


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)


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?


Why bottom-up science?


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


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


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


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


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


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


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.


-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


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)


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



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


How to validate theory with experiments (and vv)?


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


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



Beam ONbackground level

Time (s)

saturated

signal



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)


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).


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).


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).


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


•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):


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

http://www.bccms.uni-bremen.de/


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


29 PPPL, 2nd ISLAFD, April 28, 2011

Only C+H

C-Li-O

Slabs studied: Periodicity in x-y

IPAM, April 2012


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)


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)


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…


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


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


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).


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

bottom-up integrated theoretical-experimental science for...

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