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Features and possible mechanisms of long range D retention in PFM’s

Features and possible mechanisms of long range D retention in PFM’s (based on investigations carried out in RF). Presented by Valery Kurnaev. ITPA DIV SOL meeting 7-10 May 2007, Garching, Germany. 7-10 May 2007. Contributions presented by:.

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Features and possible mechanisms of long range D retention in PFM’s

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  1. Features and possible mechanisms of long range D retention in PFM’s (based on investigations carried out in RF) Presented by Valery Kurnaev ITPA DIV SOL meeting 7-10 May 2007, Garching, Germany 7-10 May 2007

  2. Contributions presented by: • Yu.Martynenko, B.N.Kolbasov A.A.Skovoroda, A.Spitsin Kurchatov Research Centre • A.Airapetov L.Begrambekov, A.Golubeva, O.Fadina, A.Pisarev, P.Shigin MEPhI • V.Alimov, Institute of Physical Chemistry, RAS

  3. The evidence of long range D retention Polycrystalline & mixed materials: Long range effect at ion implantation was seen more than 40 years ago M.Guseva 6 keV D is captured in the hot rolled W at depth of several microns. In single crystal W D ins not captured in the bulk V.Alimov et al in Hydrogen and helium recycling at PFC, ed by A.Hassanein Kluwer Academic Publishers, 2002, p.131-143 In the CVD coatings of W2C and WC exposed to the D plasma at T> 400 K, D atoms diffuse into the bulk and accumulate to 2 at.% at depths of several micrometersV.Alimov Carbon based materials Deep penetration of deuterium into carbon fibre composite CF222 (up to 2-3 mm) after exposure to the PISCES-A plasma.- B. Emmoth, M. Rubel, E. Franconi, Nucl. Fusion 30 (1990) 1140. Tritium depth profiles in divertor tiles of JET have revealed that in the 2D (two-dimensional) CFC tiles about 40% of tritium was retained at depths larger than 1 mm, to only few percent found at these depths in the 4D CFC tile from TFTR R.-D. Penzhorn, N. Bekris, U. Berndt, J.P. Coad, H. Ziegler, W. Nägele, J. Nucl. Mater. 288 (2001) 170., R.-D. Penzhorn, J.P. Coad, N. Bekris, L. Doerr, M. Friedrich, W. Pilz, Fus. Eng. Des. 56&57 (2001) 105. , N. Bekris, C.H. Skinner, U. Berndt, C.A. Gentile, M. Glugla,  B. Schweigel, J. Nucl. Mater. 313-316 (2003) 501.

  4. Surer deep hydrogen penetration in vanadium alloy Hydrogen isotopes penetrate through the 0.7 mm sample of V-3,49Ga alloy to the non irradiated backside after stationary, pulsed power plasma and 6-keV ions irradiation Hydrogen concentration profiles in V-3,49Ga alloy after exposition in stationary plasma of PLAST installation (D= 2∙1025 m-2, Т = 4500С, Е = 100 eV). 1 – irradiated side, 2 - backside (ERDA, 2-MeV He+) Preprint # 6452/7 Kurchatov Institute

  5. Surer deep hydrogen penetration in vanadium alloy (2) Deuterium concentration profiles in V-3,49Ga alloy after irradiation with pulsed deuterium plasma (15 pulses, 0.28 MJ/m2); 1 – backside, 2 – irradiated side. 3- non irradiated (ERDA, 2-MeV He+) Hydrogen concentration profiles in V-3,49Ga alloy after irradiation with 6-keV H+ ions in ion accelerator ILU at dose 1,0 ·1023 м-2, and target temperature Т = 4500С 1 – irradiated side, 2 - backside, 3 – non irradiates sample (ERDA, 2-MeV He+)

  6. Hydrogen irradiation induced deepstrengthening Stationary, pulsed power hydrogen plasma and 6-keV H+ ions irradiationresults in deep strengthening – a reason is hydrates formation, which create compressive stress in material. backside Microhardness as a function of depth measured on samples cross section after stationary plasma irradiation (● – irradiated side; ▼ - back side, ▲- before irradiation): а) V-10Ti-6Cr-0,05Zr – irradiation time 20 min. ( D= 2,2 ·1024 m-2), b) V-10Ti-6Cr-0,05Zr – irradiation time 1 hour. (D= 6,4 ·1024 m-2), c) V-15Ti-10Cr-0,05Y - irradiation time 1 hour. (D= 6,4 ·1024 m-2). Before irradiation Preprint # 6452/7 Kurchatov Institute

  7. NRA analysis of CVD W2C & WC coatings(V.Alimov et al) At temperatures above 550 K D concentration in the bulk starts to decrease. Presumably, deuterium is retained in carbon precipitates.

  8. Mechanisms of long range D transportYu.Martynenko et al Shock wave initiation ion Cascade of displacements defects Physical interpretation connected withgeneration and transport ofdislocation loops underinfluenceof surface layer tension, generation of thermoelastic tension and shock waves, that force diffusion of admixtures along interstitials and grain borders.

  9. But physical mechanisms are far from full understanding. For adequate explanation versatileinvestigations using novel experimental methods as well as intimate theoretical analysis are necessary For chemical active C based materials with compound structure (CFC) the task is much more complicated and need very careful investigations

  10. Channels of D transport in CFC V.Alimov et al Migration in the bulk obeys diffusion equation with D=D(Fluence, Temp,) Values for the deuterium migration derived from the D depth profiles are in good agreement with data for intrinsic hydrogen diffusivity in the fibres. Migration through pores along fibres with low activation energy may be considered as mechanism of deuterium penetration into the bulk of the CFC materials.

  11. Fine grain graphite (as well as CFC?) –transparent for gases in principle Flux, mol/s Measured gas flux density j ~ σ P σ A/d, P –pressure, A –area, d- thickness, σ– specific gas permeability σ = 5·1015 mol/s for MPG-8 Pressure, Pa Penetrating gas flow via gas pressure for MPG-8 graphite (thickness 1.26 mm, diameter 30.5 mm), room temperature. Graphite sheet behaves as capillary: A=1m2, d =1cm, P =1Pa 3,5·1017 D2/s

  12. Influence of tokamak T-10 exposure on MPG-8 graphite permeability 4 mm Place of limiter tile used for the membrane MPG-8 membrane 1,8 mm thick cut off T-10 limiter demonstrated hydrogen permeability ~9·1015 mol·c-1m-1Pa-1 - 2 times more than for “virgin” graphite. Possible reason – graphite porosity increase after long term expose in tokamak. All attempts to increase MPG-8 permeability in lab plasma experiments failed!

  13. Comparison of lab experiments and tokamak exposed tile retention in CFC TDS investigation of the samples CFC N11 implanted by deuterium ions in the laboratory setups (MEPhI) and high field side tile of Tore Supra [1] Begrambekov L., Brosset C., Bucalossi J., Delchambre E., Gunn J.P., Grisolia C., Lipa M., Loarer T., Mitteau R., P.Moner-Garbet P., Pascal J.-Y., P.Shigin P., Titov N., Tsitrone E., Vergazov S., Zakharov A. “Surface modification and hydrogen isotope retention in CFC during plasma irradiation in the Tore Supra tokamak”.17th Internal Conference on Plasma Surface Interactions in Controlled Fusion Devices (PSI17). Hefei, China, May 22 – 26, 2006

  14. Hydrogen retention via fluence in lab exp. Possible underestimation of tokamak tile irradiation fluence hardly could seriously influence the presented data -hydrogen retention will not increase 30 times even for ~100 times higher fluence [2] L. Begrambekov, O. Buzhinsky, A. Gordeev, E. Miljaeva, P. Leikin, P. Shigin. TDS investigation of hydrogen retention in graphites and carbon based materials. Physica scripta, N108 (2004), p.72-75.

  15. Possible mechanisms of enhanced D trapping under tokamak plasma irradiation 1. Presence of hydrogen in the Tore Supra tiles TDS spectra of H2 and D2 for the as prepared CFC N11 samples irradiated in deuterium plasma Lab. experiment Ei = 100 eV/at, Ji = 1,21020 at/m2/s Ф = 4,51023 at/m2 Presence of hydrogen in the CFC twice increases deuterium trapping at the expense of filling the hydrogen traps by deuterium due to isotope exchange mechanism

  16. 2. Trapping of deuterium activated by electron irradiation Thermal desorption of deuterium as D2 and as CD4 from CFC graphite under ion and electron irradiation. Ei=100 eV/at, Ji=1,21020 at/m2/s Ф=4,51023 at/m2 Increase of deuterium retention along with irradiation time as well as deuterium retention under electron irradiation support an assumption about participation of D2 molecules from surrounding atmosphere in deuterium trapping into graphites. Ion and electron irradiation act as a driving forces of the process. Plasma electron irradiation of CFC increased deuterium trapping 1.2 – 3 times depending on energy distribution of impinging deuterium particles [ Collaboration: DRFC, CEA-Cadarache – MEPhI, ROSATOM, 2007]

  17. 3. Transportation of deuterium into the material and trapping in the bulk of the tile TDS analysis of the samples cut from different depth of the tile shows, that more 10% of cumulative trapping collected in the bulk of tile. The samples of 1 – 2 mm thickness are usually used in the laboratory experiments. The laboratory results could be compared only to trapping in surface region of tokamak tile. 4. Deuterium trapping in graphites irradiated by ion flux with wide energy distribution Peculiarities of deuterium trapping in graphites exposed to plasma allows to expect an enhancement of retention in graphites irradiated by ion flux with wide energy distribution in tokamaks in comparison with graphites subjected to mono energy ion irradiation in laboratory setups.

  18. 5. Graphite surface irradiation during Helium Glow Discharge Conditioning Helium ion bombardment leads to development of the surface relief and destruction of near surface layer. As result, trapping of deuterium during tokamak discharges should be enhanced 6. Graphite surface irradiation by oxygen impurities Graphite irradiation by ions and atoms of oxygen impurities leads to accelerated surface destruction and develop the surface. As a result, trapping of deuterium ions and neutrals by graphite surface is probably more enhanced.

  19. Conclusion • The long range D retention in general is the very complicated physical (& chemical) problem. Many observed features of this phenomenon is not clear now. • Complex and multiple-factor impact of PFM in tokamaks makes difficult distinguishing its influence on the LR hydrogen isotopes retention. • Laboratory experiments with well defined impact parameters are crucially necessary. • During last two years we mounted and calibrated a series of high vacuum installations with well controlled parameters for measurement of D retention in plasma exposed (as well in situ in simulators) samples and started comparative studies of samples radiated at different tokamaks and laboratory installations.

  20. Plans • Investigations of samples irradiated in LENTA, PR-2, PIN laboratory plasma facilities as well as in T-10, TEXTOR and Tore Supra tokamaks are now in progress. • As there is very large scattering of the experimental data on D retention, careful measurements at D retention via fluxes (at high fluxes) and temperature in different irradiation facilities are under way. • To pick up contribution of different factors on D retention step by step experimental investigations at varying definite factors (i.m. flux density, consequent and simultaneous irradiation with deuterium and helium, broad energy spectra influence, etc) are planned. • Experimental measurements of neutron irradiation influence on hydrogen isotopes retention in PFM is now discussed.

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