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The comparison of permeation and TDS experiments with polycrystalline tungsten

The comparison of permeation and TDS experiments with polycrystalline tungsten Yu. Gasparyan a,b , A. Rusinov a , S. Stepanov a , S. Yarko a , N. Trifonov a , A. Pisarev a , A. Golubeva c , A. Spitzyn c , S.Lindig b ,M . Mayer b , J. Roth b

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The comparison of permeation and TDS experiments with polycrystalline tungsten

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  1. The comparison of permeation and TDS experiments with polycrystalline tungsten Yu. Gasparyana,b, A. Rusinova, S. Stepanova, S. Yarkoa, N. Trifonova, A. Pisareva, A. Golubevac, A. Spitzync, S.Lindigb,M. Mayerb, J. Rothb a Moscow Engineering and Physics Institute, Moscow, Russia b Max-Planck-Institut für Plasmaphysik, EURATOM Association, Garching, Germany c RRC „Kurchatov Institute“, Moscow, Russia

  2. Motivation • W will be used in the divertor region of ITER and operate at elevated temperatures. • It is important to know the hydrogen behavior in W at elevated temperatures for fuel balance and reactor safety (tritium inventory). • The modeling of our IDP experiments gave defects with a detrapping energy of (2.05 ± 0.15) eV • The existence of such defects should increase hydrogen retention in W at high temperatures.  Verify the existence of high energy defects using TDS

  3. Sample Tungsten, cut from 50 μm thick foil with purity of 99.97 % (Plansee) After experiment at 600 C for several days, Ar cleaned side Polished unannealed W • Grain size of virgin sample: ~ 1μm • Before IDP samples are annealed at 900 K for 10 hours • Increasing of grain size after annealing?

  4. Ion-driven permeation results Temperature: 873-973 K Ion energy: 200 eV/D Incident flux: 1017-1018 D/m2sec D2 pressure: ~ 7×10-4 Pa • Observed “effective diffusivity” were 4 orders of magnitude less than Frauenfelder’s diffusivity • Activation energy is ~ 2.05 eV  too much for diffusion barrier • It is supposed due to trapping at defects

  5. Modeling of IDP Experimental details T = 893 K PD2 = 7.7×10-4 Pa Finc = 0.36×2.0×1017 D/m2sec Variable parameters: Et = 2.0 eV nt = 16.5 ppm Krec= 3.4×10-21 m4/sec Kd = 2.9×1018 1/m2/Pa • Good agreement in assumption of uniformly distributed defects with detrapping energy of Et = (2.05±0.15) eV and the concentration of 1-20 ppm • GDP gives (S×D) ~ 10*(S×D)Fr

  6. TDS setup sample Setup for TDS measurements with fast loading. Base pressure: <10-7 Pa. Linear heating up to 1700 K.

  7. TDS after gas exposure • Exposure to D2 at 10-3 Pa for 1 hour at T=873 K in TDS chamber • The sample was moved out to another chamber; TDS chamber was baked out for the night • The hot region was annealed to the maximal temperature • TDS measurements • Deuterium release starts from 850 K (~exposure temperature) with a peak at 1000 K. One can expect that the activation energy is rather high. • Release after 1200 K (HD signal) was attributed to background

  8. Calculations • Frauenfelder’s value for diffusivity and solubility • Uniform distribution of defects • Saturation of the sample • Equilibrium between trapped and dissolved deuterium • Different combination of trap concentration and binding energy can give same position of peak. • Lower binding energies need unrealistically high concentrations of defects and give low amplitude. • The detrapping energy above 2.0 eV should be considered.

  9. TDS after plasma exposure Particles: mainly D3+ (D3+-72,5%, D2+-21%, D+- 6,5%) Energy: 300 eV Temperature: 330 K Flux: 1020 D/m2sec Fluence: 3.5×1022 D/m2 • Most part of deuterium desorbed until 800 K • One can see the peak at 1000 K for both annealed and unannealed samples • Annealing decrease retention significantly at such fluences

  10. High temperature defects A. van Veen, et al., J. Nucl. Mater. 155-157 (1988) 1113-1117 Vacancies: Et = 1.4-1.5 eV Decoration of voids: Et = 1.8-2.1 eV • Uniformly distributed in the bulk voids can be a case of our experiment • Voids can be formed during “rolling” process as well as during annealing at 900 K • Vacancies cluster formation during annealing at such temperatures was observed in H. Eleveld (J. Nucl. Mater. 212-215 (1994) 1421-1425)

  11. Summary • Uniformly distributed defects with detrapping energy of ~ 2 eV and concentration of 1-20 ppm can explain both IDP and TDS experimental results • The existence of the high energy defects can increase the tritium inventory at high temperatures and make problems for tritium removal • This defects were attributed to small voids or vacancies clusters • Estimated concentration is small, but these clusters can be centers of bubbles formation

  12. PSI 2008 • D.Nunes(P3-47). SEM of the tungsten probe cross section after exposition in tokamak. 10-30 μm bubbles on grain boundaries were observed • R.A.Causey(P3-69). 2 mm Plansee tungsten annealed at 1273K and exposed to high deuterium fluences. SEM and TEM don’t detect bubbles. < 2 nm bubbles could not be detected. • N.Yoshida(I-18). Bubbles formation after neutron irradiation at 600C • Bubbles are formed after exposure to helium plasma • G.M.Wright(P1-65). Deuterium retention after deuterium exposure at 1000 C. • O.Ogorodnikova(J.Appl. Phys., 2008). TDS after 200 eV ion exposure at 773 K, peak at 1100 K; 2.1 eV was used in calculations

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