Chemical Erosion and Critical Issues for ITER

Chemical Erosion and Critical Issues for ITER J. Roth Max-Planck-Institut für Plasmaphysik, Garching Chemical Erosion Studies Erosion yields: Dependence on temperature, energy and flux Emitted hydrocarbons and radicals Erosion Process and Modeling Individual atomic steps Analytic description Sticking Coefficient of Emitted Species In ion beam studies In fusion devices Summary of unresolved Issues

Chemical Erosion: Dependence on temperature J. Roth et al. JNM 63 (1976) 222 EROSION YIELD (at/ion) 0.20 D + -> C 15 ev 20 ev 30 ev 0.15 50 ev 75 ev 100 ev 200 ev 300 ev 0.10 0.05 0.00 M. Balden et al. JNM 280 (2000) 39 400 600 800 1000 TEMPERATUR (K) SPUTTERING YIELD (at/ion) 10-1 10-2 10-3 10-4 Be C W D +, 300 K theory 10 100 1000 10000 ENERGY (ev) Chemical erosion depending on temperature and energy

Chemical Erosion: Dependence on energy 10-1 D + ->C Flux = 10 20 /m 2 s Erosion Yield (at/ion) Threshold for physical sputtering 30 ev Chemical sputtering at room temperature has threshold around 1 ev No threshold for chemical erosion at high temperature 10-2 10-3 10-4 room temp. T max Y phys Salonen MolDyn IPP, ion beam U. of Toronto, ion beam Fantz, Paulin, low-t plasma 1 10 100 1000 Ion Energy (ev)

Chemical Erosion: Dependence on ion flux Collaboration in the framework of EU Task Force und ITPA Flussabhängigkeit J. Roth et al. NF 44 (2004) L21 J. Roth, R. Preuss, and EFDA Task Force, Nuclear Fusion Letter, (Oct. 2004) Chemical erosion Yield (at/ion) 10-1 10-2 10-3 Ion Beams (6), 15-50 ev ASDEX Upgrade (29), T e =8-13 ev 10 19 10 20 10 21 10 22 10 23 Ion Flux (m -2 s -1 ) T=300K Chemical Erosion Yield (at/ion) 10-1 10-2 10-3 Ion Beams IPP(6) PSI1 (17,19) JT-60U outer div (25) ToreS 1999 (22) ToreS 2002 (24) TEXTOR (27) PISCES (14) JET 2001 (20) 10 19 10 20 10 21 10 22 10 23 10 24 Ion Flux (m -2 s -1 ) T=T max Flux dependence resulting from normalized data Chemical erosion is adequately described as function of temperature, energy and flux

Chemical Erosion: Eroded species Emission of a variety of hydrocarbons Composition changes with ion energy Missing information: Mechanism of emission of higher hydrocarbons Origin of energy dependence (diffusion?) Contribution of radicals Ions of Reaction Products [a.u.] 1.0 0.5 0.0 C 3 C C 2 3 C 2 4 C 3 8 15 20 E. Vietzke, 25 A.A. 30 aasz (Academic 35 Press 401996) 45 Ion Mass mol. erosion C 3 1.00 1 C C 2.76 x 1.5 C C 3 x.5 1.5 C C 4 x. 08.3 C higher.12.7 C Σ 2.50 5.0 C C 3 3 C 3 5 C 3 6 C 4 10 E. Vietzke et al. JNM 145-147 (1987) 443

Chemical Erosion and Critical Issues for ITER Chemical Erosion Studies Erosion yields: Dependence on temperature, energy and flux Emitted hydrocarbons and radicals Erosion Process and Modeling Individual atomic steps Analytic description Sticking Coefficient of Emitted Species In ion beam studies In fusion devices Summary of Unresolved Issues

Chemical Erosion: Status of atomistic understanding ydration and erosion circle: thermal, REELS, TDS, isotope exchange, A. orn et al., Chem. Phys. Lett. 231, 193 (1994) ydration at room temperature of all possible absorption sites Steady state of abstraction and hydration Thermal release of C 3 radicals from activated sites σ = 1.1 Å 2 C 3 sp 3 σ = 0.05 Å 2 σ = 1.1 Å 2 2 C 3 σ = 1.1 Å 2 E act =1.7 ev sp 2 C 3

Chemical Erosion: Status of atomistic understanding ydration and erosion circle: thermal, REELS, TDS, isotope exchange, A. orn et al., Chem. Phys. Lett. 231, 193 (1994) ydration at room temperature of all possible absorption sites Steady state of abstraction and hydration Thermal release of C 3 radicals from activated sites At higher temperature thermal release of hydrogen Good description of temperature dependence E act =1.8 ev σ = 1.1 Å 2 sp 2

Chemical Erosion: Status of atomistic understanding Application to energetic ions: energetic +, C-isotope marker, J. Roth et al., Appl.Phys.Lett. 51 (1987) 964 Thermal release of hydrocarbon at end of ion range Yield increase due to damage production Steady state of methane production and breakup during out-diffusion CD 4 signal (a.u.) P. Franzen, thesis IPP9/92 (1993) A.A. aasz et al, JNM 220-222 (1995) 815 S. Chiu et al., JNM 218 (1995) 319 6 kev D+, 550 C on off σ = 1.1 Å 2 σ = 1.1 Å 2 Y surf C 3 C 3 sp 3 σ = 0.05 Å 2 σ = 1.1 Å 2 2 C 3 E act =1.8 ev E act =1.7 ev Time (sec) sp 2 C 3

Chemical Erosion: Status of atomistic understanding Application to energetic ions: energetic +, C-isotope marker, J. Roth et al., Appl.Phys.Lett. 51 (1987) 964 Thermal release of hydrocarbon at end of ion range Yield increase due to damage production Steady state of methane production and breakup during out-diffusion σ = 1.1 Å 2 Y surf C 3 C 3 sp 3 σ = 0.05 Å 2 σ = 1.1 Å 2 2 C 3 Ion induced release of weakly bound hydrocarbon radicals complexes σ = 1.1 Å 2 E act =1.8 ev E act =1.7 ev sp 2 C 3

Chemical Sputtering: Status of atomistic understanding E. Salonen et al., Phys. Rev. B63 (2001) 195415 ydrogenic atom attacks the region between two carbon atoms Core-core repulsion pushes the C atoms apart from each other Increase in potential energy is sufficient to result in bond rupture Resulting yields in good agreement with experiments

Chemical Erosion: Analytic description of the data 10-1 Flux = 10 20 /m 2 s 10-1 Flux = 2x10 22 /m 2 s Erosion Yield (at/ion) 10-2 10-3 10-4 Y tot at RT Y surf 1 10 100 1000 Ion Energy (ev) Y tot at T max Salonen MolDyn IPP, ion beam U. of Toronto, ion beam Fantz, Paulin, low-t plasma Erosion Yield (at/ion) 10-2 10-3 10-4 Y tot at RT Y surf Y tot at T max AUp 300K PISCES 600K 1 10 100 1000 Ion Energy (ev) Full description of chemical erosion as function of temperature, energy and ion flux Y tot = Y therm (1+Y dam ) + Y surf + Y phys

Chemical Erosion and Critical Issues for ITER Chemical Erosion Studies Erosion yields: Dependence on temperature, energy and flux Emitted hydrocarbons and radicals Erosion Process and Modeling Individual atomic steps Analytic description Development of low erosion graphites Sticking Coefficient of Emitted Species In ion beam studies In fusion devices Summary of unresolved Issues

Mechanism of chemical erosion: Methods of reduction Enhancement of hydrogen desorption by addition of metallic impurities C 3 sp 3 E act =1.8 ev sp 2 2 C 3 C 3 Chemical Erosion Yield (C-atoms/ion) 0.06 0.04 0.02 0.00 D +,50 ev pyrol. Graph. USB 15 Erel=1.8 ev Erel=1.2 ev Chemical Erosion Yield (C-atoms/ion) 0.15 0.10 0.05 400 600 800 1000 Temperature (K) D +, 200 ev, C. Garcia-Rosales et al. (1992) pyrol. Graph. USB 15 Erel=1.8 ev Erel=1.2 ev 0.00 400 600 800 1000 Temperature (K) Variation of E act from 1.2 to 1.8 ev

Methane Yield (C 4 / + ) Erosion of plasma-facing materials: Development of low-erosion materials 0.10 0.08 0.06 0.04 0.02 +, 1keV, A.A. aasz et al. (Univ.of Toronto) 14at% Si 8at% Ti 10at% W pyr.graphit CD 4 production yield (a.u.) Pure Graphite 0.06 Graphite 4at% Ti Doped a-c: 0.05 10-13at% Ti Model 0.04 E =1.8eV E =1.0ev 0.03 0.02 0.01 30eV D + 0.00 400 600 800 1000 Temperature (K) 0.00 300 600 900 Temperature (K) Development of fine grain graphites with high heat conductivity: UTIAS (Toronto), IPP efei (China), CEIT (Spain) and IPP Garching Doping can eliminate thermal chemical erosion

Carbon transport to remote locations in the JET vessel 1: impurity production at main chamber walls C +, + W. Jacob (2000) 5: pumping stable species 3: re-deposition of species with large sticking coeff. C 4, 2 2: re-erosion at divertor plate C 3 4: re-deposition of species with low sticking coeff. C 2, C 2 3

Chemical Erosion Eroded species and sticking coefficient J. Roth, C. opf, JNM 334 (2004) 97 80 Sticking Coeff. 60% Sticking Coeff. <1% Sticking Coeff. close to 0 Phys. Sputtering Thermal chem. Erosion 100% 88% 29% 0 12% 71% 0 factor 4 from weight loss Multiple species are emitted under chemical erosion conditions Sticking coefficient ranging from 0.6 to 10-3 More data needed on eroded species Chem.Enhanced Sputtering 0 Carbon Deposit (10 15 at/cm 2 ) Carbon Deposit (10 15 at/cm 2 ) 70 60 50 40 30 20 10 1 kev D+,750 K 0 0 1 2 3 4 5 80 70 60 50 40 30 20 10 Position (3mm) Boden Deckel Modell 12% 0.001 88% 0.6 40 ev D+, RT 0 0 1 2 3 4 5 Position (3mm) Boden Deckel Modell 71% 0.001 29% 0.6

Co-deposition in JET Cavity probes 10-4 10-3 Film thickness [norm.] 10-2 10 1 10 0 10-1 10-2 M. Mayer, Final Report Task FT-3.4 (2003) M. Mayer, et al., EPS St. Petersburg (2003) In JET and ASDEX Upgrade: Co-deposition predominantly due to high sticking species within few wall collisions 10-3 10-4 0 5 10 15 Position [mm] D content model 0.0005 x (β 1 = 10-3 ) model 0.9995 x (β 2 = 0.92) model 0.0005 x β 1 + 0.9995 x β 2

Redeposition: Deposition of hydrocarbon radicals N 2 (C 3 ) 2 10 13 s(c 3 ) = 10-2 2 growth rate (cm -2 s -1 ) 10 12 j C3 = 5*10 14 cm -2 s -1 C 3 ellipsometry, infrared 10 11 s(c 3 ) = 10-4 0.1 1 10 substrate C 3 flux density (10 15 cm -2 s -1 ) 2 W.Jacob et al. (2002)

Redeposition of carbon: Sticking coefficients Sticking coefficients for hydrocarbon molecules and radicals Sticking Coeff. Experimental for 1100 K [v. Keudell'01] C 4 0 n.a. 0.02 to 10eV [Ruzic'02] C 3 10-4 to 10-2 0.02 to 1 C 2 0.02 0.03 to 1 C n.a. n.a C 1 1 C 2 <0.92 0.15 to 1 C 2 3 <0.35 0.02 to 1 C 2 5 <10-3 n.a. Surface loss probability MD calculations D. Ruzic et al.(2002) 0 10 10-1 10-2 10-3 0.01 0.1 1 10 Incident Energy (ev) C3 C3Exp C2 C2Exp Surface loss probability depends on hybridization state of the radical (sp 1 > sp 2 >> sp 3 ).

Extrapolation to ITER Steady state divertor erosion and wall co-deposition Extrapolations from JET: Scaling of carbon deposition with power and/or ion fluences 600 500 JET Brooks/Kirschner S=0 Extrapolation Brooks: Erosion at divertor only, erosion yield = 1.5% Carbon deposition together with tritium at areas other than divertor plates Present extrapolations: Tritium co-depoition (g) 400 300 200 In-vessel limit present evaluation T max =800 C S=0.5 Full description of erosion yield, variation of sticking coefficient for radicals and carbon atoms 100 0 S=1 0 200 400 600 800 1000 Number of ITER pulses G. Matthews et al., ETP Workshop eraklion (2003) J. Roth et al., JNM 337-339 (2005) 970

Open Questions and Data Needs ow are hydrocarbons released from end of ion range? Are diffusional and breakup processes involved? What is the origin of the composition of hydrocarbons and its change with ion energy? What is the origin of the flux dependence? Are individual steps with time duration of the order of 0.1-1 ms necessary? More information is needed on sticking coefficients of radicals. CD 4 signal (a.u.) More information is needed on the reerosion of deposited a-c: layers. Surface loss probability Chemical Erosion Yield (at/ion) 10-1 10-2 10-3 10 10-1 10-2 10-3 0 MD calculations D. Ruzic et al.(2002) on off Ion Beams IPP(6) PSI1 (17,19) JT-60U outer div (25) ToreS 1999 (22) ToreS 2002 (24) TEXTOR (27) PISCES (14) JET 2001 (20) P. Franzen, thesis IPP9/92 (1993) E. Vietzke, A.A. aasz (Academic Press 1996) 10 19 10 20 10 21 10 22 10 23 10 24 Incident Time (sec) Energy (ev) Ion Flux (m -2 s -1 ) T=T max C3 C3Exp C2 C2Exp 0.01 0.1 1 10