Multi-scale modelling of plasma-material interactions

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1 Multi-scale modelling of plasma-material interactions T. Ahlgren, C. Björkas, K. Heinola, K. O. E. Henriksson, N. Juslin, Ane Lasa, A. Meinander, K. E. Salonen, K. Vörtler of Physics and Helsinki Institute of Physics University of Helsinki, Finland

2 Aim No time (no point) to go through all the results of the last year(s) It's published We are a Molecular Dynamics group, but... Today we present our (successful) strategy to study the PWI: the multi-scale approach Different codes Link them Main results 2

but... Today we present our (successful) strategy to study the

3 A long trip... where are we? collaboration Euratom & Tekes Future Real plasmas Fusion in FI road map JET, ITER, DEMO... Plasma facing materials Fusion physics & diagnostics Remote handling & fusion engineering Edge Modelling Figs: efda.org and rworldpub.org

4 The rich materials science of plasma-wall interactions Crater Sputtered atom Adatom Amorphization Vacancy Just for a single ion all of the below may be result Interstitial? 1+1 = 2 3D extended defects For multiple ions (prolonged irradiation) many more things can happen, for instance: Interstitial-like dislocation loop Vacancy-like dislocation loop Implanted ion Spontaneous roughening or ripple formation [Norris et al, Nature communications 2, 276 (2011) T. K. Chini, F. Okuyama, M. Tanemura, and K. Phys. Rev. B 67, (2003)] Precipitate/nanocluster, bubble, void or blister formation [Bubbles e.g: K. O. E. Henriksson, K. J. Keinonen, D, Physica Scripta T108, 95 (2004); Nanocrystals e.g. 75S. Dhara, Crit. Rev. Solid State Mater. Sci. 32, 1 [2007)] 4

ion Spontaneous roughening or ripple formation [Norris et al, Nature communications 2, 276 (2011) T. K. Chini, F. Okuyama, M. Tanemura, and K. Phys. Rev.

5 Consequences of plasma-wall interactions for fusion How are all these relevant for fusion? Implantation => T retention => VERY BAD Sputtering => erosion => BAD Sputter heavy impurities into edge plasma => cooling => GOOD Sputter heavy impurities into main plasma => cooling => BAD Sputtered molecules can migrate => redeposition => BAD Damage the material => worse heat conduction => BAD Damage the material => material becomes brittle, may crack=> BAD Produce gas bubbles => blisters => flaking => dust => BAD So it is very problematic from many points of view, and improved understanding is needed to understand and avoid harmful effects! 5

main plasma => cooling => BAD Sputtered molecules can migrate => redeposition => BAD Damage the material => worse heat conduction => BAD Damage the material =>

6 What happens physically in the materials? Multiscale picture m Relevant region for DEMO Swelling Most relevant region for ITERChanges of macroscopic mechanical properties Length mm Dislocation mobility and reactions μm nm Sputtering; Bubble formation; Point defect mobility and recombination Primary damage production (cascades) ps ns μs ms s hours years Time 6

ITERChanges of macroscopic mechanical properties Length mm Dislocation mobility and

7 What is needed to model all this? m Rate equations μm nm Discrete dislocation dynamics BCA Length mm DFT Classical Molecular dynamics ps ns Kinetic Monte Carlo μs ms s hours years Time 7

8 ITER first wall materials The ITER design involved 3 elements: Be, C, W After operation commences, also mixtures of these materials will form plasma-wall interaction studies must include the Be-C-W system with all its mixtures Plus H and He arriving from the plasma [Image from G. Federici, PSIF 2005 at ORNL, www-cfadc.phy.ornl.gov/psif/federici.pdf] 8

must include the Be-C-W system with all its mixtures Plus H and He arriving from the

9 A small trip... Projects 1- H and He in W 1.1- Differences between H and He (DFT-MD-KMC) 1.2- W fuzz formation under He irradiation (MD-KMC) 2- Physico-chemistry of plasma-wall interactions: 2.1- Swift chemical sputtering of C and Be 2.2- Role of bond conjugation in BeC 9

10 1.1- Physics of H and He effects in W Difference of H and He bubble formation Both H and He bombard the W divertor in a fusion reactor Difference: depth of blisters vastly different. H: at micrometer depths He: close to projected range (<100 Å) Wh y? Options: We considered many possibilities: Damage: no, (also non-damaging irr. produces bubbles!!) Diffusivity: no, about the same Thermal gradients: no Different kinds of W samples in experiments: no Trapping behavior? The simplest: self-trap: Becomes immobile, seed for further bubble growth Our study: classical MD simulations and quantum-mechanical DFT calculations to examine the energy of two H or He atoms (dimers) at different distances Fusion Seminar

!) Diffusivity: no, about the same Thermal gradients: no Different kinds of W samples in experiments: no Trapping behavior?

11 1.1- Physics of H and He effects in W H vs. He self-trapping: energetics results MD energetics of H-H or He-He pair: (and confirmed by DFT) H-H He-He strong binding for He-He Almost no binding for H-H self-trapping & bubble formation The strain in W makes a noble gas bind! (~anti-chemistry effect!) [K. O. E. Henriksson, K. A. Krasheninnikov, and J. Keinonen, Appl. Phys. Lett. 87, (2005)] 11

He-He strong binding for He-He Almost no binding for H-H self-trapping & bubble formation The

12 1.1- Physics of H and He effects in W He bubbles: depths and evolution (animation) He bubble formation: mobile atoms red, immobile He in clusters orange, large clusters green, turning blue [K. O. E. Henriksson, K. A. Krasheninnikov, and J. Keinonen, Appl. Phys. Lett. 87, (2005); K. O. E. Henriksson, K. A. Krasheninnikov, and J. Keinonen, Fusion Science & Technology 50, 43 (2006)] He bubble depths by KMC with with self-trapping ~ experiments T(K) Our KMC Expt. Reference Å 62 Å Nicholson and Walls Å Å Fusion Seminar 2013 Chernikov and Zakharov

Keinonen, Appl. Phys. Lett. 87, 163113 (2005); K. O. E. Henriksson, K. A. Krasheninnikov, and J.

13 1.2- He in W Near-surface blistering of W by He Cumulative MD simulation of 100 ev He W Surface growth by loop punching & blistering surface W He? loop punching Fusion Seminar

14 1.2- He in W What happened after impact 2580? Fusion Seminar

1.2- W fuzz formation W under high He fluence Under very high fluence (105 He impacts, by MD), this leads to the formation of a swiss-cheese like structure Also:

15 1.2- W fuzz formation W under high He fluence Under very high fluence (105 He impacts, by MD), this leads to the formation of a swiss-cheese like structure Also: bubble coalescence, effect of C and temperature, surface growth, Implemented all of this in a new KMC code. [ S. K. Tähtinen and K. Nordlund (2013) submitted] 15

Also: bubble coalescence, effect of C and temperature, surface growth,

16 1.2- W fuzz formation W fuzz formation Fuzz growth with the same (time) dependence as in experiments! After discarding other effects: the W fuzz formation can be explained by a balance of loop punching surface growth and bubble rupture leading to a surface roughening that scales as (time) Fig: Jyrki Hokkanen, CSC [ S. K. Tähtinen and K. Nordlund (2013) submitted] 16

After discarding other effects: the W fuzz formation can be explained by a balance of loop

17 1.2- W fuzz formation W fuzz growth animation 17

18 2.1- Physico-chemistry of plasma-wall interactions The swift chemical sputtering mechanism for carbon In we showed that sputtering of C can occur when incoming ~ ev H The H ion hits the middle of a C-C bond. This raises the energy enough to break the chemical bond Process is energetically unfavorable (endothermal) [Salonen et al, Europhys. Lett. 52 (2000) 504; Phys. Rev. B 63 (2001) ; Krasheninnikov et al, Comput. Mater. Sci. 25 (2004) 427] 18

This raises the energy enough to break the chemical bond Process is energetically unfavorable (endothermal) [Salonen

19 2.1- Physico-chemistry of plasma-wall interactions The carbon sputtering mechanism A model system of a single H atom colliding with a C dimer gives insight to the basic mechanism Momentum transfer (in y) dependent: H does not penetrate (reflected) H penetrates slowly => large τ => large py => bond breaking occurs H penetrates rapidly => small τ => small py => no bond breaking The sputtering is in decent agreement with experiments! [Krasheninnikov et al, Contrib. Plasma Phys. 42 (2002) 451; Comput. Mater. Sci 25, 427 (2002)] [Salonen, Physica Scripta T111 (2004) 133; Krasheninnikov et al, Comput. Mater. Sci. 25 (2004) 427; Nordlund et al, Pure and Appl. Chem. 78 (2005) 1203] Fusion Seminar

τ => small py => no bond breaking The sputtering is in decent agreement with experiments! [Krasheninnikov et al, Contrib. Plasma Phys. 42 (2002) 451; Comput. Mater.

20 2.1- Physico-chemistry of plasma-wall interactions: D irradiation of Be Be, a metal, is eroded as BeD molecules at low energies Chemical sputtering! This fraction decreases with ion energy The same trend is seen in the experiments (by R. Doerner, San Diego) Physics,ofUH University Helsinki 20

$This fraction decreases with ion energy The same trend is seen in the$

21 2.1- Physico-chemistry of plasma-wall interactions A comprehensive view We compared systematically the SCS mechanism for all the dimers Be-Be, C-C, W-W, Be-C, Be-W and C-W [K. Nordlund et al, Nucl. Instr. Meth. Phys. Res. B, 269 (2011) 1257] W: high mass, high bond-strength: lateral energy transfer not enough to cause bond breaking in practice (in theory always possible) Conclusion: Physics,ofUH University Helsinki Be, C, Si, Be-C, Be-W and C-W can sputter by SCS W not good news for once! 21

22 2.2- Chemistry of plasma-wall interactions Role of bond conjugation Bond energy We also found that the conjugation of chemical bonds is very important for the sputtering of Be-C at low H energies! 3-body energy Bond conjugation [A. Meinander et al, NIMB (2013) accepted]. Physics,ofUH University Helsinki 22

23 Conclusions Plasma-wall interactions in fusion reactors involve a complex and rich interplay of physics and chemistry, at very different scales Multi-scaling allows a more complete description of the PWI DFT MD KMC RE Pure W behaves according to physics, but everything else (C, Be, D) show also chemical effects (simulated in MD) Swift chemical sputtering Molecular sputtering Physics,ofUH University Helsinki 23

Chemical Sputtering. von Kohlenstoff durch Wasserstoff. W. Jacob

Chemical Sputtering. von Kohlenstoff durch Wasserstoff. W. Jacob Chemical Sputtering von Kohlenstoff durch Wasserstoff W. Jacob Centre for Interdisciplinary Plasma Science Max-Planck-Institut für Plasmaphysik, 85748 Garching Content: Definitions: Chemical erosion, physical