Plasma-Material Interface: Giga Challenge For Fusion Energy...All energy from D-T fusion reactions passes through first wall. Flux of (particles + heat + 14 MeV neutrons) ~10 MW/m.

Managed by UT-Battellefor the Department of Energy NGC 2009, Hamilton, Canada, August 2009

Plasma-Material Interface:Giga Challenge For Fusion Energy

Predrag

Krstic

Physics Division, CFADCOak Ridge national LabOak Ridge TN, USA

Supported by DOE OFES & OBES, SciDAC, INCITE

NGC 2009, Hamilton, Canada, August 20092

Close collaborators:

Fred Meyer (ORNL)

Steve Stuart (Clemson U.)

Experiment:

Carlos Reinhold (ORNL)

Theory:

Eric Hollmann

(UCSD)

beam plasma

Our thanks to John Hogan (FED, ORNL)


What is fusion?

•Process in the core of our Sun and stars:H atoms fuse into He

•Release of tremendous energy:2m(H)>m(He) (T=15 mil K)

conversion of a fraction of the mass into energy upon E=mc2

d-t

fusion (more efficient) T=150 mil KAlpha-particles and neutrons carry most of the energy

Fusion on earth

Fusion in stars


All energy from D-T fusion reactions passes through first wallFlux of (particles + heat + 14 MeV

neutrons) ~10 MW/m2

Vac.

Supercon–ducting magnet

Shield Blanket

Turbine generator

Plasma

a

Plasma heating(rf, microwave, . . .)

Schematic magnetic fusion reactor ITER

4

DEMO

EU, Japan, Russia, China, Korea, US, India partnership, 30 year

agreement

ITER ranked 1st for US Investment of facilities in next 20-years (2003)Strongly supported by all US scientific and educational entities

Biggest international project of present times

Princeton/ ORNL partnership manage project office for US ITER activities


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

valid extrapolation needed!

ITER (> 2020) uses multi-matl wallsPulses ~ hundreds of sec~Be Main chamber wall(700m2 )Low Z + oxygen getter~W Baffle/Dome (100 m2)Funnels exhaust to divertor chamberLow erosion, long lifetime~ C Divertor Target (50 m2) (Graphite)

Minimize high-Z impurities(which lead to large radiative losses)

PMI strategy is evolving thru ITER towards DEMO reactor

5

Divertor: Magnetic filed lines end –

biggest flux of particles & energy


What does flux of 1025 particles/m2s mean?

at a box of surface of 3 nm lateral dim?a few thousands atoms (carbon)

The flux is 0.01 particle/nm2ns1)

1 particle at the interface surface of the cell each 100 ps.

But for deuterium with impact energy lessthen 100 eV: Penetration is less than 2 nm,typical sputtering process takes up to 50 psEach impacts independent, uncorrelated!

In effect interaction of an impact particle with nanosize

macromoleculePossibly functionalized!News is that each particle will change the surface for the subsequent Impact!


PMI: multi-scale, multi-dimensional, multi-mechanism problemSeek quasi-equilibrium: (burning plasma ⇔ wall)

Advances in theory &computing enable bottomup approach in contrast to topdown

7


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 costly.

8


PMI is key fusion research area and is getting a strong momentum

2007: DOE Greenwald Panel gap analysis for fusion• 4 of 5 key knowledge gaps which must be bridged to achieve

fusion power involve “taming the plasma-materials interface.”

2009: DOE Fusion Strategic Workshops recommendations• Decode and advance the science and technology of plasma-

surface interactions. • Develop improved power handling through engineering

innovation. • Establishment of new PMI facilities and programs

9


Beam-surface exp’t: precision control of projectiles & targets . . .

. . . enabled development & validation of MD approach

ORNL: F.W. Meyer,

Krstic

& Meyer, 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

• Control of impact energy & angle.• Incident flux up to 1019

m-2

s-1

• Clean, well-characterized surfaces, pb

~10-10

torr• Temperature control of target• Absolute yields of interaction products• Direct line of sight for diagnostics (TOF, etc.)

exp with D 2+

exp with D +

CD3+CD 4

MD with D 2(*)

MD with D

total C

MD with D 2(g)

hydrocarbon

Impact energy (eV/D)7 8 9 10 20 30 40

Sput

terin

g yi

eld

(/D)

10-3

10-2

10-1

exp with D 3+

11

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

F. Meyer, 2007

P. Krstic, 2008

1 µm

Sub-surface structure (W)

grain ejection

D, H retentionBlistering

Surface morphology

Nano-fuzz on W irradiated with He

Chemical sputtering of hydrocarbons


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 techniquesDiffusion; transport


Addressing PMI Knowledge Gaps

•

Erosion/redeposition – fundamental understanding of the multi- species plasma mixed materials.

•

Tritium retention and permeation.•

Periodic off-normal heat-flux and energetic particle bombardment.

•

PFC lifetime and heat transfer – unprecedented exposures to high heat flux and durations.

•

Neutron irradiated materials – effect of high neutron fluence on PFC and internal component properties.

•

Development of new materials and concepts.

ErosionAblationMelting (metals)

Re-depositionCo-deposition

Implantation


PMI has many fundamental processes & synergies

elastic reflection

implantation

re-emission & sputtering & chemistry

trapping/detrappingretention

Plasma Material

diffusion, permeation

Give rise to synergistic effects

Damage Effects:Vacancies, bubbles, blisters, dislocations, voids, neutrons?

Drivers:Multi -T, -n, -species, plasma irradiation,neutronssheath acceleration


Beam-surface experiments:Prepared beam & target

15

MD and MC with plasma

synergy

ITER,DEMO

Molecular Dynamics (MD) & Monte Carlo (MC)

simulation

PMIDesign &Qualification

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

QuickTime™ and a decompressor

are needed to see this picture.

Toroidal

confinement experiments

15

Potential modelsQuantum-classical MDIncreases in computational power

Full

integration of the three main branches of the PMI research is happening!

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


Understanding synergies of species Exposure of pyrolytic

graphite to 5

MeV

C+ beam 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 beams:•

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


Bombardment by ions, atoms, molecules

Chemical reactions inside the surface(Breaking C-C bond, H pacifies bonds)

Produced volatile particles diffuse, or more likely travel through voids

Desorbed into the gas phase

Some promptly re-deposited,Most released into the plasma

Released particles transported with plasma, changed, deposited to some other PFW

The most complex PSI: Chemical sputtering

D atom (20 eV) on supersaturated a-C:H

Our basic simulation cell:~ 2500 atoms)


-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


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, retentionSurface desorption

FASTps-nsnm

SLOWms-s

Terascale-petascalechallenge

mμ

Simulation is computationally intensive and multi-scale

Short-time productssputtering, reflection,implantation


• Evolution of a system of particles over time: Duration ~order ps-ns

• Equations of motion are solved for each particle at a series of small time steps

(energy conservation) 0.1 fs

What is molecular dynamics?

aF m=

What is quantum-classical molecular dynamics?• Key physics input in MD is the inter-atomic potential function

Wide range of techniques for: Potential energy: Modeled, predefined function → Classical MD

• Potential energy is calculated at each time step by solving Schrödinger equation for electrons with adiabatic instantaneous Hamiltonian →

Quantum-Classical MD

This is happening now: ORNL Cray XT5 : petaflops

(142,000 processors)

U−∇=F


Classical MD is only as good as the interatomic potential model used

,

( ) ( ) ( )rep ij ij ij attr iji 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


Evolution of Density Profile

depth (A)

num

ber d

ensi

ty (a

.u.)

0

50

100

150

H density

C density

0 10 20-10

impactsu

rfac

e

5 15 25-5

Fluence (1020 m-2)

0



depth (A)

num

ber d

ensi

ty (a

.u.)

0

50

100

150

H density

C density

0 10 20-10

impactsu

rfac

e

5 15 25-5

Fluence (1020 m-2)

0

0.07

Presenter

Presentation Notes

Needs a trimmed version for just t=0.



depth (A)

num

ber d

ensi

ty (a

.u.)

0

50

100

150

H density

C density

0 10 20-10

impactsu

rfac

e

5 15 25-5

Fluence (1020 m-2)

0

0.07

0.14

Presenter

Presentation Notes




depth (A)

num

ber d

ensi

ty (a

.u.)

0

50

100

150

H density

C density

0 10 20-10

impactsu

rfac

e

5 15 25-5

Fluence (1020 m-2)

0

0.07

0.14

0.28

Presenter

Presentation Notes




depth (A)

num

ber d

ensi

ty (a

.u.)

0

50

100

150

H density

C density

0 10 20-10

impactsu

rfac

e

5 15 25-5

Fluence (1020 m-2)

0

0.07

0.14

0.28

0.43

Presenter

Presentation Notes




depth (A)

num

ber d

ensi

ty (a

.u.)

0

50

100

150

H density

C density

0 10 20-10

impactsu

rfac

e

5 15 25-5

Fluence (1020 m-2)

0

0.07

0.14

0.28

0.43

0.57

Presenter

Presentation Notes




depth (A)

num

ber d

ensi

ty (a

.u.)

0

50

100

150

H density

C density

0 10 20-10

impactsu

rfac

e

5 15 25-5

Fluence (1020 m-2)

0

0.07

0.14

0.28

0.43

0.57

0.71

0.85

Presenter

Presentation Notes




depth (A)

num

ber d

ensi

ty (a

.u.)

0

50

100

150

H density

C density

0 10 20-10

impactsu

rfac

e

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

Presenter

Presentation Notes




•

Surface interface supersaturated with D

•

Lower C density at surface

•

D/C ~1 at surface

•

“Steady state”

achieved by fluence

of ~

1020

m–2

•

Surface depth ~20 Å

depth (A)

num

ber d

ensi

ty (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

Presenter

Presentation Notes




RESEARCHHIGHLIGHTS


1) Hybridization depth profile, Mechanism of sputtering

Depth (A)-5 0 5 10

sp3 d

ensi

ty (1

/A3 )

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

sp3sp2

sp

E (eV/D)5 10 15 20 25 30

0

2

4

6

8

10

z<15

Parti

al c

ount

5

10

15

z<5

a)

b)

C-CD3 : sp3C-CD2 : sp2C-CD : spchange

C2Dn, n>2CD2CD

C3Dn

C2D

C2D2CD3

E (eV/D)5 10 15 20 25 30

Sput

terin

g Y

ield

/D

2x10-3

4x10-3

6x10-3

8x10-3

1x10-2 CD4

Terminal groups bond-braking(TGBB)-subset of swift BB (Salonen)

Presenter

Presentation Notes



Amplitude (Angtroms)0 1 2 3 4 5 6 7 8

Yie

ld

10-3

10-2

C

CD3+CD4C2D2

E=10 eV/D

R (a.u.)0 2 4 6 8 10

Pote

ntia

l ene

rgy

(eV

)

-5

-4

-3

-2

-1

0

1

2

0

40

H2+ (1sσg) +H(1s)

H2(X1Σg+)

mixed diss. continuum

R (a.u.)0 2 4 6 8 10

Pote

ntia

l ene

rgy

(eV

)

-5

-4

-3

-2

-1

0

1

2

R (a.u.)0 2 4 6 8 10

Pote

ntia

l ene

rgy

(eV

)

-5

-4

-3

-2

-1

0

1

2

0

40

H2+ (1sσg) +H(1s)

H2(X1Σg+)

mixed diss. continuum

E (eV)101 102C

harg

e tra

nsfe

r cro

ss se

ctio

n (c

m2 )

10-17

10-16

10-15 vf=4H2

+(v=0) + H -> H2(vf) + H+

vf=3

vf=5

vf=0

(a)

(b)

Krstic, 2002

2) Sputtering with (ro)vibrationally excited projectiles

At low impact energies strong influence, most likely due to dissociation balance (4.5 eV)

Neutralization of D2 ionsmost likely with deposited D

++ +→+ DDsDgsD )6~()1()( 22 ν

Europhys. Lett. 77, 33002 (2007).


4) Mass/energy spectra of sputtered hydrocarbons(with excited D2 impact!!!)

E s (e

V)

10-2

10-1

100

101

10 20 30 40 50 60 7010-2

10-1

100

101

Ms (amu)10 20 30 40 50 60 70

E=7.5 eV/D E=10 eV/D

E=20 eV/D E= 30 eV/D

CDy C2Dy CDy C2Dy

CDy C2Dy C3Dy C4Dy C2Dy C3Dy C4Dy C5Dy

Ms (amu)10 20 30 40 50 60 70

E s (e

V)

10-2

10-1

100

30 eV/D10 eV/D

15 eV/D7.5 eV/D 20 eV/D

CDy C2Dy C3Dy C4Dy C5Dy

Es (eV)0.0 0.5 1.0 1.5 2.0 2.5 3.0

Cou

nts

0

50

100

150

200

150 e-Es(eV)/0.6)

E=20 eV/amu

Heavier species at higher energy impact Hot hydrocarbon ejecta (5-10000K)

Approximate thermalization along a collision cascade

Kinetic desorption

Ms (amu)10 20 30 40 50 60 70

0.0

0.4

0.8E=20 eV/D

Yie

ld/D

(%

)

0.0

0.1

0.2

0.3E=7.5 eV/D E=10 eV/D

20 30 40 50 60 70

E=30 eV/DCDy

C2Dy

C3Dy C4Dy

Hot

Time-of-flight experiment of Vetzke (2002) sees hot hydrocarbons (a few tens of eV)


5) Deuterium ejection characteristics upon D impact

Depth(A)


vx (107 m/s)

-0.10 -0.05 0.00 0.05 0.10

v y (1

07 m/s

)

-0.10

-0.05

0.00

0.05

0.10 total hydrocarbonE=30 eV/D

6) ANGULAR DISTRIBUTIONS dn/dΩ

Angular momenta

Lsin law or isotropic

Velocities –

cos

law


Ejec

tion

ener

gy (e

V)

0.00.51.01.52.02.53.03.5

TranslationalRovibrational

RotationalVibrational

Hydrocarbon ejection

D Impact energy (eV)0 5 10 15 20 25 300.0

0.51.01.52.02.5

CD3 ejection

(a)

(b)

7) Rovibrational

energy distributions: Hydrocarbons

Ejection energy Ejection temperature

Functions of hydrocarbon mass and impacting D energy


8) Approximate thermalization

indicated (rovibrational)

D2

and HC ejecta


Typical Monte Carlo trajectory approach (ex: a-C:H carbon)

carbon

CH3is created

diffusion

Conversion to CH4

diffusion

desorptionH impact

sp2

spxHH

sp3 HH

H

Macroscopic master equation for the long-time evolution

Boltzmann like collisional

relaxation operator(rates form MD)


PLASMA IRRADIATION=

Randomization of impact parameters


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”

nanostructureson W irradiated by He

high W temperatures (>1000K)

Baldwin,2008


E (eV)0 2 4 6 8 10

Cou

nts

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 -0.2 0.0 0.2-0.25

-0.20

-0.15

-0.10

-0.05

0.00

T=10,000KAll VZ>0 changed sign

E (eV)0 50 100 150 200

Cou

nts

0

100

200

300

400

500

600

~sqrt(E) exp(-E/kT)

T=200,000 KMaxwell-Boltzmann

T=200,000 KAll VZ>0 inverted sign

VX VY

VZ

-1.5 -1.0 -0.5 0.0 0.5 1.0 -1.5 -1.0 -0.5 0.0 0.5 1.0-1.0

-0.8

-0.6

-0.4

-0.2

0.0

Impact plasma distributions (D atom) : Examples

Plasma Irradiation!!!


Output (reflected) distributions of D

Energy of re-emitted D (eV)0 5 10 15 20 25 30

Cou

nts

0

100

200

300

400

500

exp(-E/2.9eV)

exp(-E/0.6eV) sputtered

inelastically reflected

Tp=50,000 K

Boltzmann distributions

VX VY

VZ

-0.40.0

0.4 -0.4 0.0 0.4 0.60.00

0.10

0.20

0.30

Energy of re-emitted D (eV)0 5 10 15 20 25 30

Cou

nts

0

100

200

300

400

500

600

exp(-E/6eV)

exp(-E/0.8eV) sputtered

inelastically reflected

Boltzmann distributions

Tp=100,000 K

VZ VY

VZ

-0.8 -0.4 0.0 0.4 -1.0 -0.5 0.0 0.5 1.00.0

0.2

0.4

0.6


Ec.m. (eV)0 2 4 6 8 10

Cou

nts

0

20

40

60

80

100

exp(-E/1.9eV)c.m. energy

Tp=200,000 K

Erot (eV)0.0 0.5 1.0 1.5 2.0 2.5 3.0

Cou

nts

0

20

40

60

80

100

exp(-E/0.45eV)rotational energy

Tp = 200,000 K

Distributions of sputtered hydrocarbons

Vx VY

VZ

-0.15-0.10-0.050.000.050.100.15 -0.15 -0.10 -0.05 0.00 0.05 0.100.00

0.02

0.04

0.06

0.08

0.10

LX LY

LZ

-15-10 -5 0 5 10 15 20 -20 -10 0 10 20 30 40-30

-20-10

010

2030

Angular momentum distributions


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 ejectedD2 sputtered

plasma

beam

D2 sputtering and D re-emission with beam and plasma

With plasma irradiation:Reflection significantly higher, sputtering smaller!!!


Plasma irradiation –

1st

direct comp of atomistic calc with plasma experiment: designed by E. Hollmann

(UCSD) 2008

0

0 s

T TT T

α −=

−

Accommodation coefficient

Graphite tube

Measured (assumption Tv

=5000K, Tr=

Tk

=800K)MD simulated


•

Multi-species, multi-state plasma•

Multi-component, multi-directional distribution function

Incident plasma

Modelling of high flux plasma-wall interaction is complicated by surface response to plasma bombardment

•

History of irradiation, heating•

Surface deposition & damage

Evolving target

•

Particle reflection, implantation, sputtering

•

Synergetic surface chemistry

Products

Retained deuterium concentration in C, Be and W co-deposition conditions (J. Roth et al., PPCF 50 (2008)103001)

Reliability of extrapolation to DEMOdepends on validation of theory with thorough experimental characterization

Deu

teriu

m c

once

ntra

tion

(D/X

)

DEMO


-New targets: C crystalline, polycrystalline structures; CFC, doped C, Tungsten, Beryllium …target temperature variation-New plasma particles: N (N2 ), C, W, Be, inert gases

isotopic effects-Development of new methods:

New potentials (C,W,Be,H) !!!!!CNMS of ORNL ZBL corrections; Barrier corrections (done!)

Excited state MDCharge transfer (impinging ions), El. excitations of projectiles and target atomsExpansion of computation capabilities

Where do we see continuous challenges?

New materials, inclusion of quantum mechanical effects, long times, T…

EXPERIMENTAL VALIDATION OF HIGH IMPORTANCE!!!

Plasma-Material Interface: Giga Challenge For Fusion Energy...All energy from D-T fusion reactions passes through first wall. Flux of (particles + heat + 14 MeV neutrons) ~10 MW/m.

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