Measurement and modeling of hydrogenic retention in molybdenum with the DIONISOS experiment

1. TEC Measurement and modeling of hydrogenic retention in molybdenum with the DIONISOS experiment G.M. Wright University of Wisconsin-Madison, FOM – Institute for Plasma Physics Rijnhuizen D.G. Whyte, B. Lipschultz Plasma Science and Fusion Center, M.I.T. The Pilot-PSI Team FOM – Institute for Plasma Physics Rijnhuizen

2. Outline • The DIONISOS experiment • The influence of MeV-range ion irradiation • The influence of high flux plasma exposure • Diffusion and bulk retention • Consequences and solutions • Summary and conclusions

3. Outline • The DIONISOS experiment • The influence of MeV-range ion irradiation • The influence of high flux plasma exposure • Diffusion and bulk retention • Consequences and solutions • Summary and conclusions

4. Dynamics ofIONImplantation and Sputtering Of Surfaces DIONISOS = • Simultaneous ion beam irradiation and plasma exposure of target. • Real-time, in-situ measurement of D concentrations using 3He(d,p)a NRA. • Helicon source yields high flux (~1021 D/m2s), low temperature (Te ~ 5 eV) plasmas • Target can be biased to 500 V and heated to ~750 K.

5. Trapped deuterium depth profiles measured in-situ emphasize the importance of diffusion Flux density ~ 1 x 1021 D/m2s Total fluence ~ 1.5 x 1024 D/m2 3.5 MeV 3He irradiating surface Trapped D concentrations as high as 3 at.% • Trapped D profiles are highly sensitive to Mo temperature: • low TMo diffusion limited • high TMo  flat profile, trap limited

6. Indications that retention for 600-700 K extends much deeper than 5 mm. Implies total retention is lowest at 300K. D retention in Mo after exposure to 3.5 MeV 3He beam and D plasma in DIONISOS No dependence on incident ion energy.

7. Outline • The DIONISOS experiment • The influence of MeV-range ion irradiation • The influence of high flux plasma exposure • Diffusion and bulk retention • Consequences and solutions • Summary and conclusions

8. 3.5 MeV 3He irradiation of the target produce lattice displacements that can become traps TMo = 500 K Vbias = 100 V D flux = 1021 m-2s-1 3He flux = 3×1017 m-2s-1 texposure = 1500 s For these conditions, displacements due to the 3He irradiation account for ~85 % of the total trapped D. Displacements account for a larger fraction of trap sites deeper into the Mo bulk near the end of the 3He ion range.

9. Trap density increases non-linearly with 3He ion fluence (and displacements) TMo = 500 K Vbias = 100 V Same plasma parameters Trap density (3He fluence)1/4(dpa)1/4 Non-linearity indicates an approach to a saturation level but also a large increase in trap density after only a small amount of 3He fluence.

10. Scaling of trap production from irradiation may depend on several factors Takagi et al, Fusion Sci. & Technol. 41 (2002) 897. 0.8 MeV 3He beam TMo = 493 K Indicates a dependence on irradiating ion energy. Trap production may also depend on radiating species (ions/neutrons), target composition, and target temperature.

11. Outline • The DIONISOS experiment • The influence of MeV-range ion irradiation • The influence of high flux plasma exposure • Diffusion and bulk retention • Consequences and solutions • Summary and conclusions

12. Plasma exposure also produces trap sites well beyond the implantation range rimplant~ 10 nm TMo = 500 K Vbias = 100 V D ~ 1021 m-2s-1 High rate of low-energy ion implantation into a target with very low natural hydrogenic solubility (<10-7 D/Mo for conditions in DIONISOS) Indicates a trap production mechanism that extends beyond rimplant or the production of traps in rimplant that subsequently mobilize into the bulk.

13. Vacancy clustering and void/blister formation How does surface trap production scale? Implanted D super-saturates The implantation zone. W atom is displaced and a vacancy is formed. • What experimental factors influence strength and formation of stress fields? • Plasma flux density – sets rate of implantation (source) • Hydrogenic solubility – sets saturation limit (boundary condition) • Diffusion/surface recombination – rate-limiting process removes D from implantation zone (sink).

14. Dynamic capabilities of DIONISOS yield insights into the rate limiting processes For a diffusion-limited release, DnDsurfa Eion (deeper implantation). Opposite effect observed in DIONISOS implying a recombination-limited release.

15. Outline • The DIONISOS experiment • The influence of MeV-range ion irradiation • The influence of high flux plasma exposure • Diffusion and bulk retention • Consequences and solutions • Summary and conclusions

16. Mobile traps and implanted D implies retention in the bulk material will be significant Simulated D retention profile: D = 1023 m-2s-1 No MeV ion irradiation Trapplasma  D 3 s pulse, 3 s cooling, 30 s wait time Repeated and rapid thermal cycling drives the trapped D into the bulk. Simulation results in linear increase in retention with fluence. High hydrogenic diffusion rates in Mo and assumption of mobile trap sites are key to this mechanism. I-14, B. Lipschultz et al.

17. Underlying physics apply to materials with similar hydrogenic properties Takagi et al. Fusion Science and Technology 41 (2002) 897. Oliver et al. Journal of Nuclear Materials 307-311 (2002) 1418. Tanabe et al. Journal of Nuclear Materials 191-194 (1992) 439. Frauenfelder et al. Journal of Vacuum Science and Technology 6 (1969) 388. Ogorodnikova et al. Journal of Applied Physics 103 (2008) 034902.

18. TEC Tungsten Pilot-PSI experiment allows for tests at ITER-relevant plasma flux densities P1-65, A.W. Kleyn, G.M. Wright, et al. W targets D~1023-1024 m-2s-1 Te ~ 2 eV TW ~ 1000-1600 K Retained fraction determined with TDS D retention is low and may possibly indicate saturation. High tungsten temperatures have mitigated plasma-driven trap production.

19. Outline • The DIONISOS experiment • The influence of MeV-range ion irradiation • The influence of high flux plasma exposure • Diffusion and bulk retention • Consequences and solutions • Summary and conclusions

20. What are the issues and how do we solve them? • Bulk displacements and high flux, low energy plasma exposure can both create trap sites in high-Z refractory metals. • High H diffusion and trap production in the bulk means retention is higher at moderately elevated target temperatures (400-600 K). • High H diffusion and mobile traps indicate retention deep in the bulk occurs with repeated thermal cycling (T removal concerns). • What are the solutions? • High ambient surface temperature (hot walls) mitigates plasma-driven trap production and anneals irradiation-produced traps (Oliver et al. J. Nucl. Mater. 329-333 (2004) 977). • Control access of implanted hydrogen to the bulk (diffusion barriers). Diffusion-limited regime has lowest retention. 0-19, D.G. Whyte

21. Summary & Conclusions • The DIONISOS experiment has yielded new insights into hydrogenic retention properties of high-Z materials in an irradiating environment. • Irradiation of Mo with MeV ions produces bulk displacements that lead to significant trap concentrations throughout the irradiating ion range. • Exposure of Mo to a high flux of low energy plasma ions can create trap sites extending much deeper than rimplant • High diffusion rates and mobile traps in combination with thermal cycling can lead to retention throughout the bulk of the material and a linear increase in retention with incident fluence. • Many of the underlying physics driving these mechanisms in Mo can also be applied to W.

22. Summary & Conclusions • In future fusion devices, plasma facing components will be exposed to high plasma fluxes and 14 MeV neutron irradiation. The combination could lead to high retention rates even in refractory metals • Plasma, radiative, and neutron heating activates diffusion. Trap production by n-irradiation means traps will be distributed throughout the bulk. Even small local D/T concentrations could lead to high overall retention. • Diffusion barriers can limit retention in the bulk. • Operating with hot walls (900-1000 K) would solve many problems with hydrogenic retention in refractory metals.