Outline of the presentation

1. EFFECTSOF CARBON REDEPOSITION ON TUNGSTEN UNDER HIGH-FLUX, LOW ENERGY Ar ION IRRADITAION AT ELEVATED TEMPERATURE Lithuanian Energy Institute, Lithuania Vytautas Magnus University, Lithuania Poitiers University, France Prof. habil. dr. L. Pranevičius 2006-11-15

2. 1 Outline of the presentation • Introduction, • Sources of carbon redeposition, • Simulation of dynamic mixing, • Experimental results, • Discussions, • Conclusions.

3. Issue: MATERIAL TRANSPORT AND EROSION /DEPOSITION FOR FUSION PROGRAMME The rate of erosion of the divertor targets and building up of deposited films may ultimately limit the choice of divertor materials and the operational space for ITER Introduction

4. LIST OF PROCESSES Sketch of divertor 1 Introduction

5. Introduction The present work is an attempt to explain: • the mixing mechanism of C contaminant on W substrate under high-flux, low-energy ion irradiation; • the experimentally observable anomalous deep C transport into W under prolonged irradiation at elevated temperature. The aim: • to deepen the understanding about the behavior of C contaminant on W .

6. T=300 K 200 eV H+ 20 eV H+ 20 eV H+ WC Helsinki University, 2005 MD simulations for WC target

7. Lithuanian energy instituteMaterials Research and Testing Laboratory The goal: to form dense and hard W coatings The method: plasma activated deposition of W Samples Magnetrons Plasma activated deposition Magnetron sputter deposition Kick-Off Meeting ASSOCIATION EURATOM 15 November, 2006, Kaunas, Lithuanian energy institute

8. Collaboration in Lithuania • E-beam deposition of hard coatings • Kaunas University of technology • SIMS carbon profiling • Vilnius university Kick-Off Meeting ASSOCIATION EURATOM 15 November, 2006, Kaunas, Lithuanian energy institute

9. 3 . Ballistic relocations 2 . Working gas collision scattering 1. Wall collision back-scattering wici The flux of ejected i atoms: - wici, where The flux of redeposited i atoms - where is the probability for i atom to be back-scattered, and is i atom probability to stick to j atom 4. Redeposition scheme Sources of C redeposition

10. where Model The system of rate equations on the surface and for the K monolayer including sputtering and readsorption processes

11. After introduction notations and It is seen that rate equations can be rewritten as • Three possible cases: • Va > Vs –readsorption prevails (film growth rigime) • Vs >Va – sputtering prevails (surface erosion regime) • Va =Vs- readsorption and sputtering rates are equal (dynamic balance regime) Model VI International Conference ION 2006 , Kazimierz Dolny,Poland, 26-29 June 2006

12. Surface erosion prevails (Va <Vs) The steady state solutions The characteristic thickness of an altered layer Conclusion: the steady state mixed layer is formed under simultaneous redeposition and sputtering (Va <Vs) Calculated distribution profiles Model

13. The system of rate equations on the surface and for the K monolayer including sputtering, redeposition and diffusion processes for K=1 for K1 The role of diffusion becomes important if Model

14. Experimental procedures The first stage :2 µm-thick W film deposition: - XRD characterization; - SEM and AFM surface view analysis. The second stage:erosion by 300 eVAr+ ion irradiation during C redeposition: - - SIMS carbon distribution profiles; - SEM and AFM surface topography analysis.

15. The scheme of experimental device Experimental technique Experimental parameters: Source power – 200 W, Ar gas pressure – 10 Pa, Ar gas flow rate – 1.1 cm3min–1, Substrate temperature – 300 K

16. Ar plasma + + + + + + W film GRAPHITE Plasma parameters: Electron concentration – 81010 cm-3, Electron temperature – 3.1 eV, Sheath bias – 11 V, Ion flux – 5.51015 cm–2s–1. Experimental

17. SEM cross-sectional view 2 µm Diffraction angle, 2 Without bias voltage Bias voltage – 100 V Diffraction angle, 2 W film characterization

18. As-deposited SIMS carbon distribution profiles in W film Carbon distribution profiles in tungsten

19. 100 m 5 m 2,5 m Adsorption prevails (Va>>Vs) Adsorption prevails (Va>Vs) 1 m 1 m Adsorption prevails (VaVs) Sputtering prevails (Vs>Va) SEM surface views of W film after irradiation during redeposition DNQ-117-2-2 DNQ-116-1-1

20. 0,5 m 0,5 m 1 m 2 m SEM surface views of W film after irradiation during redeposition when sputtering prevails

21. Not-irradiated Va > Vs Vs >Va 29 µm 29 µm 5 µm W surface roughness after irradiation during redeposition After irradiation during carbon redeposition Ra=38.3 Ra=13.5 nm Roughness: Ra=2.9 nm VI International Conference ION 2006 , Kazimierz Dolny,Poland, 26-29 June 2006

22. 1 – 1 s 2 – 5 s 3 –10 s Coverage Number of monolayer Target Surface roughness 1. W=2, =1 2. W=1, =2 Number of monolayer Time W surface roughness (mechanism)

23. 28 µm 15 µm 1.5 µm 5.1 µm AFM surface topography sputtering prevails redeposition(Va>Vs)

24. 28 µm 1.5 µm AFM surface topography to the C transport into the W film mechanism VI International Conference ION 2006 , Kazimierz Dolny,Poland, 26-29 June 2006

25. Boundary region

26. VaVs Diffraction angle, 2 Diffraction angle, 2 XRD patterns of W film on the graphite substrate W2C

27. Diffraction angle, 2 XRD patterns of W film on the graphite substrate

28. 2 0 -2 -4 2 0 -2 -4 0 40 80 120 0 40 80 120 0 40 80 120 0 40 80 120 As-deposited W film W film after C redeposition under irradiation Mechanical erosion by pin-on disc technique

29. Discussions • The main deduced results: • the dynamic mixing results in the formation of an layer (modeling); • the efficient C transport from the surface into W film takes place during the weight decrease regime when W surface is only partially covered by C atoms (experiment); • the C transport efficiency sharply decreases when continuous amorphous C film is formed on the W surface (experiment).

30. Discussions • The deduced results may be explained if to assume: • during high-flux, low-energy ion irradiation the surface chemical potential of W increases and difference of potentials between activated surface and grain boundaries acts as the driving force for Cadatoms transporting them into the bulk of W film; • as continuous amorphous C layer is formed on the W surface the transport of C adatoms from the surface is blocked;

31. The redeposition and surface relocation effects forms: (i) steady state mixed layer on the surface in the regime of surface erosion, (ii) formation of continuous film in the regime when redeposition prevails, and (iii) mixed layer with thickness increasing in time as where Conclusions VI International Conference ION 2006 , Kazimierz Dolny,Poland, 26-29 June 2006

32. Conclusions - The surface roughness increases when sputtering yield of surface contaminants is low in comparison with matrix material; - The efficient carbon transport from the surface into the W film was observed in the regime when sputtering prevails redeposition.

33. The model application to the published experimental results Y. Ueda, Y. Tanabe, etc., J. Nucl. Mater, 2004, W by 1.0 ke V of 0.1 % C+ and H3+ beam, flux - 31020 m-2∙s-1, fluence – 1022 -1024 m-2, T=653 -1050 K Calculated (grey lines) and experimental depth profiles of carbon for target temperatures from 653 K to 1050 K. Beam fluence is 3×1024 m-2.

34. The model application to the published experimental results V.I. Safonov, I. G. Marchenko, etc., surf. Coat. Technol., 2003, V by 2.7 keV Ti+, flux - 31020 m-2∙s-1, time – 5 min, RT Calculated and experimental depth profiles of Ti in natural U P=10E-2 Pa Irradiation time -5 min Ion energy – 2.7 keV Flux – 1.3×1020 m-2s-1 βU = 0.83, βTi = 0.89