1. Molecular Dynamics modelling of mixed layer formation K. Nordlund, C. Björkas, N. Juslin, K. Vörtler Accelerator Laboratory, University of Helsinki P. Erhart Lawrence Livermore National Laboratory, Livermore, USA K. Henriksson Kungliga Tekniska Högskolan, Stockholm, Sweden

2. Carolina Björkas and Kai Nordlund 2007 2 Outline Motivation for MD simulation of mixed materials What is needed to model mixed materials by molecular dynamics? Current status of BeWCH potential development Simulations of mixed material formation and erosion WC formation from melt WC structural formation and erosion by D ions Outlook/workplan WC cobombardment BeC BeW

3. Carolina Björkas and Kai Nordlund 2007 3 Motivation Central phenomena for mixed materials in ITER are erosion, sputtering and redeposition All of these originate in atomic level processes To attempt to understand these theoretically it is natural to use atomic-level methods In this context 4 main levels of methods can be considered relevant: Binary collision approximation (BCA): -Ideal for purely ballistic effects, e.g. linear cascade sputtering Quantum chemistry (in practice mainly DFT methods): -Individual chemical reactions for a few tens of atoms Molecular dynamics (MD): -Long time scale processes for thousands of atoms Kinetic Monte Carlo (KMC) -Diffusion of impurities in surfaces and bulk

4. Carolina Björkas and Kai Nordlund 2007 4 What is needed for MD We do all of these levels of simulations, but in the remainder of this talk I will concentrate on MD MD simulations always require the existence of an interatomic potential for the system to be modelled A potential must describe the relevant interactions at least qualitatively correctly For fusion reactor issues the potentials must in addition be fully reactive in the sense that they should be able to describe all coordination states and chemical bond breaking => molecular mechanics models are out Until recently, reactive MD potentials existed only for elemental and binary systems Clearly not enough for the ITER BeWCH materials mix…

5. Carolina Björkas and Kai Nordlund 2007 5 Examples of past MD plasma-wall work Swift chemical sputtering mechanism which explains the low-temperature chemical erosion of C in divertors Now much additional work by LLNL, ORNL, FOM on this; good agreement on main points H and He interactions with W surfaces: big difference between H and He clustering explained [Salonen, EPL 52 (2000) 50; PRB 63 (2000) 195415 ] [Henriksson et al, Nucl. Instr. Meth. B 244 (2005) 377]

6. Carolina Björkas and Kai Nordlund 2007 6 Status of availability of divertor- relevant potentials: overview HHeCWBe H He C W Be 2002, done by others 2006, CarWMod [Juslin et al, J. Appl. Phys. 98, 123520 (2005)] BeTunCMod aim 2008

7. Carolina Björkas and Kai Nordlund 2007 7 What is needed for a new potential We use a 3-stage fitting procedure, which has been proven to work well in a wide range of complex systems e.g. Si, C, SiC, WCH, Pt, Zn, ZnO, GaAs, GaN, Fe,... Our fitting stages are, for each interaction pair: 1. Obtain data to be fit: obtained from experiments and DFT calculations, relevant to problems of interest 2. Fit a Tersoff-Brenner-like potential into the data 3. Test important non-fitted properties, iterate back to stage 2 as long as necessary… Expected effort needed for success/interaction pair: 2-12 person-months depending on complexity

8. Carolina Björkas and Kai Nordlund 2007 8 Status of availability of divertor- relevant potentials: status of today HHeCWBe H He C W Be 2002 2006, CarWMod 1. DFT 2. Pot. fit Almost all DFT data for fitting calculated, some more for Be-H and Be-He coming from Alain Allouche Be-Be has good fit, Be-C fit still to be improved

9. Carolina Björkas and Kai Nordlund 2007 9 WC formation from melt One way to form WC mixed layers is by cooling from a melt, quenching Not necessarily same structure as that formed during irradiation But good as ground state comparison point for irradiated layers -E.g. in Si same comparison shows similar densities between irradiated and quenched cells We obtained reasonable WC amorphous layers W x C 1-x, x = 0.5 – 0.9 considering e.g. density compared to crystals and melt

10. Carolina Björkas and Kai Nordlund 2007 10 D irradiation of WC D irradiation of initially crystalline showed a clear amorphization effect in the cells What is surprising is that the amorphization correlates with D implantation depth and not deposited energy Not only ballistic collisions, but also chemistry needed 20 eV 50 eV 100 eV 200 eV Original 10 eV 1000 eV 2000 eV Structure of WC after 2000 D impacts

11. Carolina Björkas and Kai Nordlund 2007 11 D irradiation of WC We observed (as expected) preferential sputtering of C increasing W surface concentration Results for change of W surface concentration compared to experiment: Experiment at 300 eV D on WC: 63 ± 10 at.% /(10 18 ions/cm 2 ) Simulation at 300 eV D on WC: crystalline: 57 ± 7 at.% /(10 18 ions/cm 2 ) amorphous: 110 ± 10 at.% /(10 18 ions/cm 2 ) Excellent agreement considering there are no adjusted parameters! [Träskelin et al, Phys. Rev. B 75 (2007) 174113]

12. Carolina Björkas and Kai Nordlund 2007 12 Outlook Potential construction: Complete Be-C-H system potential, test it Make Be-W system potential, test total Be-W-C-H potential Make He-Be potential WC mixed layer erosion/formation: Response of WC to mixed D + 10% He/Ne/Ar/C/W bombardment -Especially C/W interesting: deposition vs. erosion -Details still to be determined

13. Carolina Björkas and Kai Nordlund 2007 13 Outlook Formation and erosion of BeC and BeW mixed layers: Construction of quenched a-BeC and a-BeW layers and simulation of their erosion by D -Relatively fast Simulation of formation of mixed layers by Be/C/W deposition on Be or C or W -Very slow simulations -Comparable to results of Doerner/Linsmeier/Krug presented earlier during this meeting -Time scale a challenge, but high T might work Details still to be determined -Will be fixed based on discussion with EFDA, you, and technical simulation limitations

14. Carolina Björkas and Kai Nordlund 2007 14 Conclusions It is now possible to simulate on an atomic level mixed layer formation and erosion in the ternary WCH system Plus any noble gas (He/Ne/Ar/…) with a pair potential Results until now show no obvious major shortcomings => results likely to be qualitatively reliable Potentials now under development for the quaternary BeWCH system We are hopeful this will do at least as well as the WCH one But this is one of the first fully reactive potentials ever made for a quaternary system for any application area. Complexity is high and I am sure there will be shortcomings somewhere. -But some potential is still better than no potential…

15. Carolina Björkas and Kai Nordlund 2007 15 Thank you for your attention!