1 EFFECTS OF 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

2 Outline of the presentation 1. Introduction, 2. Sources of carbon redeposition, 3. Simulation of dynamic mixing, 4. Experimental results, 5. Discussions, 6. Conclusions. 1

3 Introduction 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

4 Introduction 1 LIST OF PROCESSES Sketch of divertor

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

7 Lithuanian energy institute Materials Research and Testing Laboratory The goal: to form dense and hard W coatings The method: plasma activated deposition of W Plasma activated deposition Magnetron sputter deposition Samples Magnetrons 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 Sources of C redeposition The flux of ejected i atoms: - w i c i, 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 1. Wall collision back- scattering 2. Working gas collision scattering 3. Ballistic relocations 4. Redeposition scheme wiciwici

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

11 Model Kazimierz Dolny,Poland, June 2006 VI International Conference ION 2006, Kazimierz Dolny, Poland, June 2006 After introduction notationsand It is seen that rate equations can be rewritten as Three possible cases: (1)V a > V s – readsorption prevails (film growth rigime) (2)V s > V a – sputtering prevails (surface erosion regime) (3)V a = V s - readsorption and sputtering rates are equal (dynamic balance regime)

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

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

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 eV Ar + ion irradiation during C redeposition: - - SIMS carbon distribution profiles; - SEM and AFM surface topography analysis.

15 Experimental technique Experimental parameters : Source power – 200 W, Ar gas pressure – 10 Pa, Ar gas flow rate – 1.1 cm 3  min –1, Substrate temperature – 300 K The scheme of experimental device

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

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

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

19 SEM surface views of W film after irradiation during redeposition DNQ DNQ Adsorption prevails (V a >>V s ) Adsorption prevails (V a >V s ) Adsorption prevails (V a  V s ) Sputtering prevails (V s >V a ) 2,5  m 1  m 5  m 100  m

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

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

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

23 AFM surface topography sputtering prevails redeposition (V a >V s ) 28 µm 15 µm 5.1 µm 1.5 µm

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

25 Boundary region

26 XRD patterns of W film on the graphite substrate W2CW2C VaVsVaVs Diffraction angle, 2 

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

28 Mechanical erosion by pin-on disc technique As-deposited W film W film after C redeposition under irradiation

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 C adatoms 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 Conclusions Kazimierz Dolny,Poland, June 2006 VI International Conference ION 2006, Kazimierz Dolny, Poland, June 2006 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

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 Calculated (grey lines) and experimental depth profiles of carbon for target temperatures from 653 K to 1050 K. Beam fluence is 3×10 24 m -2. Y. Ueda, Y. Tanabe, etc., J. Nucl. Mater, 2004, W by 1.0 ke V of 0.1 % C + and H 3 + beam, flux - 3  m -2 ∙s -1, fluence – m -2, T= K

34 The model application to the published experimental results Calculated and experimental depth profiles of Ti in natural U P=10E-2 Pa Irradiation time -5 min Ion energy – 2.7 keV Flux – 1.3×10 20 m -2 s -1 β U = 0.83, β Ti = 0.89 V.I. Safonov, I. G. Marchenko, etc., surf. Coat. Technol., 2003, V by 2.7 keV Ti +, flux - 3  m -2 ∙s -1, time – 5 min, RT