Tuning Plasma by the Nanoscale Lithium-Dopped Surfacesdoeplasma.eecs.umich.edu/files/Web_Krstic_Predrag_2012_06_01v0… · H alone Ar+ alone Hopf & von Keudell, 2003 H and Ar+ Ion

Managed by UT-Battelle for the Department of Energy LTP, June 01/2012

Predrag Krstić

Joint Institute of Computational Sciences, U. of Tennessee Ex: Physics Division, Oak Ridge National Laboratory

Support from: US DOE (OFES), ORNL LDRD program NCCS(DOE) & NICS(NSF) Computing

Tuning Plasma by the Nanoscale Lithium-Dopped

Surfaces

OFES Review, ORNL, March 02, 2010 Managed by UT-Battelle for the Department of Energy LTP, June 01/2012 2

Close collaborators:

Our thanks to John Hogan (FED, ORNL)

Jae Park (ORISE)

Jonny Dadras (UTK)

Chris (MTSU)

Eric Yang (Purdue)

Students Steve Stuart Clemson U.

Carlos Reinhold PD, ORNL

Theory:

Paul Kent ORNL

Alain Allouche CNRS, Fr

TBDFT modeling K. Morokuma,

Kyoto U.

J. Jakowski, NICS

H. Witek, Taiwan

PWDFT

Mat.

CMD

Stephan Irle Nagoja U, Japan

CMD

Experiment:

Fred Meyer (PD, ORNL)

Eric Hollmann (UCSD)

beam

plasma

JP Allain (Purdue)

Chase Taylor (Purdue)

beam

beam

Jeff Harris (FED, ORNL)

Rick Goulding (Fed, ORNL)

plasma

plasma

Chris Ehemann (MSTD)

OFES Review, ORNL, March 02, 2010 Managed by UT-Battelle for the Department of Energy LTP, June 01/2012

1. 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

4

• Flux of (particles + heat + 14 MeV neutrons) ~10 MW/m2

A FUSION REACTOR IMPLIES MANY INTERFACES BETWEEN THE PLASMA AND MATERIALS

Particles and surfaces

Unlike nuclear fission where energy is volume-distributed

Key role of PMI in fusion research well recognized in US and internationally

Why lithium? Carbon?

Why is PMI important?


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

Need bottom-up approach arising from the fundamental atomistic and nano science

5


How to build an effective

science for PMI?

2.Why bottom-up science?

or


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 10 ns. But for deuterium with impact energy less then 100 eV: Penetration is less than 2 nm, typical sputtering process takes up to 50 ps Is each impact independent, uncorrelated?

Each particle will functionalize the material, change the surface for the subsequent impact! Processes essentially discrete Atomistic approach!!!

8


3. 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.


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

Sputt

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

un

ts o

f R

-CD

x m

oie

ties

, x

=1

,2,3


4. 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


Eje

cti

on

en

erg

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

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).


5. 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…


Goal: Study synergy in the evolution of surface irradiated by plasma

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

He suppresses H retention in W

• He penetrates deeper than H

• Strong dependence on energy

• He bubbles: barrier to H diffusion?

He: 0.1% He: 0%

T=653 K


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).


6. What is outreach of the PMI science

at the plasma facilities?

Difficult to study PMI in thoroidal facilities !

Importance of the dedicated PMI facilities

Pisces-B, Magnum, …) = Bridge!!!

or

Do we need dedicated PMI plasma facility?

What do we want to do with it?


Beam-surface experiments: Prepared beam & target

MD and MC

with plasma

synergy

ITER,

DEMO

Molecular Dynamics (MD)

& Monte Carlo (MC)

simulation

PMI

Design &

Qualification

High-flux linear PMI experiment: Well-diagnosed plasma & target

QuickTime™ and a decompressor

are needed to see this picture.

Toroidal confinement experiments

Potential models

Quantum-classical MD

Increases in

computational power

Integration of theory & experiment basis for PMI research

Material

Science

Predictive science!!!

http://www.ipp.mpg.de/ippcms/de/images/pic/images_bereiche/w7x/158x110/grafik_stellarator_1.gif


What have we learned from carbonfacing plasma?

• 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


• We know from in-situ experiments labs, and more than 7 different tokamak machines (TFTR , CDX-U, FTU, DIII-D, TJ-II, EAST , and NSTX ) that work with graphite with thin lithium coatings have a "significant" effect on plasma behavior and more specifically on hydrogen recycling. Controlled experiments demonstrated reduced recycling, improved energy confinement time tE, and a reduction of edge instabilities known as edge localized modes (ELMs) Notice the ration of the diemsnions of the plasma and Li layer!!!

• 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 wall conditioning improves confinement! Why?

~ 1’s m

~ 1-10’s nm

D+


Lithium dynamics: Problem to study theoretically because Li polarizing features when interacting with other elements

Electronegativity is chemical property of an element defining its tendency to attract electrons: Li has it exceptionally low in comparison to H , C, O, Mo, W.

Consequence: Bonding between Li and other atoms covalent and polar; Long-range nonbonding: Coulomb :1/R Lennard-Jones :1/R6, 1/R12

Electronegativity and size of atoms related!


How to treat the dynamics in quaternary system Li-C-O-H

•Li-C , Li-H, Li-O are of very different electronegativities and therefore these could be considered as ionic solids/liquids

•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, including multipole interactions

•Classical MD is not a good answer, cannot self-consistently calculate charges (EEM+Tersoff-Brenner covalent models too expensive) Quantum-Classical MD based on Self-Consistent-Charge Density-Functional Tight-Binding (SCC-DFTB) method (developed by Bremen Center for Computational Mat. Science, Germany) a possible answer for qualitative phenomenology is our choice


Chemistry and sputtering/reflection/retention dynamics in lithiated&oxidized 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-O-H, validated by experiments.

2) Experiments from Purdue and NSTX (PPPL) indicate higher retention and lower erosion rate with D whenever Li present in C, however XPS diagnostics show dominating D-O-C chemistry. Why – is the question now?

Simulation Goals:

C.N. Taylor, B. Heim, J.P. Allain, Journal of Applied Physics 109, 053306 (2011)

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

Simulation of deuterium impact to lithiated and oxidized carbon surface (quantum-classical approach, DFTB)

•Cell of a few hunreds atoms of lithiated and oxidated amorphous carbon •(~20% of Li, and/or ~20% of O), at 300K How? •By random seed of Li and O in amorphous carbon and energy minimization, followed by thermalization •bombarded by 5 eV D atoms, up to 500fs for the full evolution •Perpendicularly to the shell interface

•5004 random trajectories (embarrassingly parallel runs at Jaguar, Kraken); Time step 1 fs; several 10,000 CPU hours per run

LTP, June 01/2012


26 LTP, June 01, 2012

Only C+H

C-Li-O

Slabs studied: Periodicity in x-y


From experiments: There was correlation between hydrogen irradiation and the behavior change of the O(1s) and C(1s) peaks ONLY IN THE PRESENCE OF LITHIUM. The Li(1s) peak was always invariant????

What do experiments teach us? II

600 500 400 300 200 100 0

290 283 57 52535 530

Li 1sC 1s

Virgin Graphite

Lithiated Graphite

Inte

nsity (

a.u

.)

Binding Energy (eV)

Post D Bombardment

O 1s

O 1s C 1s Li 1s

x0.2

Norm

alize

d

Inte

nsity (

a.u

.)


Co

un

ts

100

101

102

C

Li

D

Partial charge (e)

-1.0 -0.5 0.0 0.5 1.0No

rmal

ized

cu

mu

lati

ve

dis

trib

uti

on

0

20

40

60

80

(a) Distribution of average charges of D, Li and C, with impact of D on a-C:Li surface (~23% Li concentration, Li/(Li+C)). D has a slight preference for interacting with Li rather than with C.

D

(a)

(b)

Does Li bond more D’s?

Krstic et al, FE&D, 2011


29

Large percentage of impact D (40%) prefer closeness of Li to settle down

Not much attention to O (already bonded to Li)

But there is only 20% Li in C, < 5% of O

The first try of the model had a nice story. The electropositive nature of Li gave us a clear picture on its influence on hydrogen retention when we compared "with Li" and "without Li" cases in the graphite matrix. The model remained somewhat unconnected to the XPS data that clearly showed it was the O(1s) that was really active and correlated to hydrogen irradiation. BUT ONLY IN THE PRESENCE OF LITHIUM. What was missing...???

What is model teaching us (I)?

LTP, June 01/2012


Comes very interesting and theoretically anticipated result: C. Taylor shows that the chemistry is incident particle energy independent... as expected. But, again a result came by observing the O(1s); in presence of Li Not even the C(1s) showed much.

Is it the impact energy? QM model used 5 eV and experiment (cannot afford higher energy…)

No problem with impact energy!!!

What do experiments teach us? III


Quantum-mechanical, PWDFT “static”

calculations finger -point in the same direction,

Qualitatively benchmark the DFTB findings:

•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 (conf. E)

LTP, June 01/2012

Configuration

A B C D E

Bo

nd

ing

of

H (

eV)

-5.0

-4.5

-4.0

-3.5

-3.0

-2.5

-2.0

-1.5

-1.0

-0.5

d=1.73A

1.88A

5.71A

3.53A

PWDFT

DFTB

Configurations

A B C D E

Ch

arg

es (

e)

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

Li

H

Allouche&Krstic, Carbon (2012)


Result: The quantitative amount of lithium deposition in the in-situ Purdue studies correlated DIRECTLY with results by Maingi et al on amounts of Li deposited in NSTX and subsequent changes on plasma behavior. 300-500 nm THIN FILM only OF LITHIUM MADE A SIGNIFICANT CHANGE IN PLASMA BEHAVIOR of spatial scales of meters!! NO ELMS. But this behavior was limited in time and for some reason saturated. Then comes another pioneering result by C. Taylor showing how this saturation is attained in lithiated graphite: Connected with O saturation

What do experiments teach us? IV


Here comes the experiment again (Chase): 1) At most 5% oxygen content on the surface of NON-

LITHIATED graphite... AS EXPECTED.

2) With lithium you get 10% of Oxygen

3) IMPORTANT: with LOW-ENERGY IRRADIATION one gets over 20% oxygen on the surface.

..... B/C LITHIUM BRINGS IT THERE WHEN LITHIATED GRAPHITE IS IRRADIATED.

What do experiments teach us? V


!"#

$"#

100 µm 100 µm

50 µm

0 µm

50 µm

0 µm

1.289 µm

0.644 µm

1 µm

1 µm

0.5 µm 0.5 µm

0 µm

29.115 nm

14.558 nm

0 nm

0 nm

0 µm

• Polished graphite samples have surface height variations of >1 µm (for a 100 x 100 µm domain) in addition to local height variations on smaller domains, as shown in the inset atomic force micrographs (AFM) in (b-c).

• The effective film “thickness” of Li is significantly less than the nominal dose due to the increased effective surface area.

• Li rapidly intercalates into graphite following deposition, causing a modest rise in surface oxygen content (10%).

• But in some samples, D ion bombardment dramatically increases the surface oxygen content to as much as 45%.

And more: Effective surface exposed to Li Large

Courtesy of C. Taylor


Multipeak structure is consequence of the discrete structure of the bonding centers

0% Li

0% O

20% Li

5% O

20% O

5% Li

20% Li

20% O

Cou

nts

0

10

20

30

40

50

60

0

10

20

30

40

50

60

Penetration depth (Angs)

-5 0 5 10 150

10

20

30

40

50

60

10

20

30

40

50

60

70

0

<d> = 3.9100

SD = 3.2528

<d> = 5.5704

SD = 3.2651

<d> = 5.3764

SD = 3.3006

<d> = 5.0761

SD = 3.6662

Depth (A)

-5 0 5 10sp

3 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

Classical (2500 atoms)

Quantal (250 atoms)

Interestingly: QM and classical give similar penetrations for a:C (deut.)

What is modeling teaching us?

II

Penetration depth?


Indicate that D has a preference for interacting with O and C-O structures rather than with Li or Li-C structures when there is enough O

(c)

How do Li and O compete?

MODEL

-0.3 0.0 0.3 -0.3 0.0 0.3

0

20

40

60

80

100

-0.3 0.0 0.3 -0.3 0.0 0.3 -0.3 0.0 0.3

d)

c) i)

j)

Inte

gra

ted

dis

trib

ution

a)

Matrix

composition:

Matrix

composition:

Matrix

composition:

B EMatrix

composition: A C D

b)

Partial Charge (e)

No

rmalized

Counts

(a.u

.)

e)

f)

h)

g)

A B C D E0

50

100

Matrix Composition

Ne

are

st

Ne

igh

bo

r O

ccu

rre

nce

(%

)

C

Li

O

H

D

52%

16%

16%

16%

E

80%

20%

C

60%

20%

20%

B

80%

20%

A

100%C

Li

O

H

9%

3%5%

9%

91%100%

67%

27%

9%

16%

71%

30%

70%

Courtesy of C. Taylor


(c)

What do the first neighbors to D say? Again: O preference!


0

10

20

30

40

50

60

70

80

Binding energy (eV)

-10 -5 0 5-10 -5 00

10

20

30

40

50

60

70

<Eb> = -4.2029

SD = 0.9706

0% Li & O

<Eb> = -4.2519

SD = 0.99597

20% O & 5% Li

<Eb> = -4.0141

SD = 1.1195

20% Li & O

<Eb> = -3.7450

SD = 2.1216

20% Li & 5% O (b)

(d)

Distribution of binding energies of D in various mixtures consistent with O dominance

0

20

40

60

80

100

120

140

0

20

40

60

80

100

120

Final kinetic energy of bound D (eV)

0.0 0.2 0.4 0.6 0.8 1.00

20

40

60

80

100

120

Co

un

ts

0

20

40

60

80

100

120

0% Li

0% O

20% Li

5% O

20% O

5% Li

20% Li

20% O

Most of the bound D’s in the ground Vib state – width ~0.2 eV) Widths are signatures of various bonding sites


6x1015

6x1016

0

5

10

15

20

25

30

35

40

D+ Fluence (cm

-2)

Post-O

irradiation

Post-Li

deposition

O 1

s S

urf

ace C

oncentr

ation (

%)

Virgin

Graphite

D+ bombarded O-graphite

D+ bombarded Li-graphite

D+ bombarded virgin graphite

Experiment confirms: Li brings enough oxygen into the surface

D+ bombardment of virgin or oxidized graphite surfaces suppresses O content in time Lithiated graphite increases O-content upon D+ irradiation to 20-40%

0 1 2 3 4 50

10

20

30

40

50

60 C 1s %

F 1s %

Li 1s %

O 1s %

N 1s %

Irradiation Time (hr)

Surf

ace

Conce

ntr

atio

n (

%)

C. Taylor et al, 2012


CONCLUSIONS

Li is an excellent oxygen getter: Lithiation of C brings A LOT OF Oxygen inside C and this the main role of Li. Oxygen and Oxygen-Carbon bond D strongly:

suppressing erosion & increasing D retention. Li mainly ineffective in this process.

10

20

70

80

90

A B C D E0.0

0.5

1.0

1.5

Pro

ba

bili

ty p

er

D

Retention

Reflection

a)

b)

Sp

utt

eri

ng

Yie

ld (

%)

Matrix Composition

Carbon

Total

A B C D E0

50

100

Matrix Composition

Ne

are

st N

eig

hbo

r O

ccu

rre

nce

(%

)

C

Li

O

H

D

52%

16%

16%

16%

E

80%

20%

C

60%

20%

20%

B

80%

20%

A

100%C

Li

O

H

9%

3%5%

9%

91%100%

67%

27%

9%

16%

71%

30%

70%

Simulation:

P. Krstic et al, Nature SR, 2012


It is not Lithium that suppresses erosion of C, and increases retention of H OXYGEN plays the key role in the binding of hydrogen. Lithium is the oxygen getter If there is a SIGNIFICANT amount of oxygen on surface with lithium present in the graphite matrix, OXYGEN becomes the main player; NOT LITHIUM!!! ... consistent with the XPS data!!

What is model teaching us?

A beautiful music of full harmony of theory and experiments

Documents

Tuning Plasma by the Nanoscale Lithium-Dopped Surfacesdoeplasma.eecs.umich.edu/files/Web_Krstic_Predrag_2012_06_01v0… · H alone Ar+ alone Hopf & von Keudell, 2003 H and Ar+ Ion