A. Sagara

A. Taguchi

H.T. Lee

K. Miyata

K. Tsukatani

M. Tokitani

N. Ashikawa

T. Tanaka

T. Wada

Y. Torikai

Y. Ueda

National Institute for Fusion Science (NIFS-Repository)

It is well known that hydrogen retention andpermeation in tungsten materials are influenced markedlyby surface mixing layers such as a W/C layer and a heliumbubble layer near the tungsten surface. For evaluation oftritium behavior in plasma facing materials, quantitativeinformation on tritium diffusion, trapping and release fromthe mixed surface have to be known. Only qualitativeresults, however, have been known so far. In the presentstudy, surface mixed layers formed by D/C and D/He ionirradiation with a precise control of impurity concentrationand tungsten samples exposed to TEXTOR edge plasmaswere used. These samples were exposed to tritium gas.Then surface density of tritium retention was measured byIP (Imaging Plate) and depth profiles were measured by achemical etching and a liquid scintillator. Surface mixedlayers were well characterized by NRA (Nuclear ReactionAnalysis), XPS (Xray Photoelectron Spectroscopy), andTEM (Transmission Electron Microscope). By theseexperimental results, relations between surface conditions(a C/W ratio and a chemical state of C and W) and tritiumuptake can be clearly understood.In the experiments, tungsten samples made by A. L.T. M. Corp. with a purity of 99.99% were exposed to D/Tmixed gas (T concentration was 7.2%) at 300 ˚C for 3hours. Tungsten samples were annealed at 2000 ˚C for 1hour and polished to a mirror finish. Three types oftungsten samples without plasma exposure, with He plasmaexposure to the fluence of 5 x 1024 m-2, and with theexposure to TEXTOR edge plasmas. For the TEXTORplasma exposed samples, C/W mixed layers or carbondeposition layers with the thickness of 100-200 nm wereformed.Tritium surface density after T gas exposure wasmeasure by IP showed clear dependence of T retention onsurface mixed layers. Very little T retention on the surfacewithout plasma exposure was observed, while significantamount of T retention was observed on the surfacemodified layers (either He plasma or TEXTOR plasma).Especially for the He plasma modified surfaces, most Tretention was observed even more than that on C depositionlayer. Tritium could be trapped in helium bubbles moreeffectively than carbon deposition layers. Details(temperature dependence, C layer thickness dependence,etc.) need to be investigated. One more note is that tritiumretention on (or in) W/C mixed layers is relatively low(higher than non-exposed samples, but lower than Cdeposition layers). It is believed that surface recombinationon W/C mixed layer could be low, which tends to reducerecombination of T on the surface. This might lead to theincrease in T retention near the surface. But this theory maynot be true. More detailed investigation is necessary.Tritium diffusion in tungsten was studied by a depthprofile measurement with a chemical etching method.Figure 1 shows depth profile of tritium in tungsten with andwithout He plasma exposure. Since 2000˚C annealedsamples used in this experiment had very few defects,tritium trapping deep into the bulk (more than 50 μm forthe sample without helium plasma exposure) is very small(about D/W = 10-9). On the other hand, near-surface densityat the depth less than 50 μm is higher than this value,though tritium exposure did not make any recoil damage.These trapped tritium could be exist at interstitial sites.To estimate diffusion length by simple 1Ddiffusion model, it was estimated to be about 2.2 mm withthe frauenfelder’s diffusivity1). This number seems muchlarger than the experimental one, an order of μm. Even withZahkarov’s diffusivity2), the diffusion length is about0.11mm. Therefore, a depth profile of tritium in the non-treated sample cannot be explained by a simple diffusionmodel. For the He plasma exposed sample, tritium trappingaround 20-30 μm in depth is higher. The helium ionimplantation range was about 2-3 nm, which is muchsmaller than this range. At near surface region, heliumbubble layers, thickness of which could be a few tens of nm,was formed. One of the theories to understand T trappingaround 20-30 μm is that this layer produces internal stresswhich may induce trapping sites at much deeper area thanthe helium bubble layer. More investigation will benecessary. We will try these experiments with differenttungsten samples and at different exposure conditions.1) J. Frauenfelder, J .Vac. Sci. Technol. 6 (1968) 388.2) A.P. Zakharov et al., Fiz. Khim. Mekh. Mater. 9 (1973)29.Fig. 1 Depth profiles of Tritium for the sampleswith and without He plasma exposure.543§65. Effects of Surface Mixing Layers of Tungsten on Hydrogen Isotope BehaviorUeda, Y., Wada, T., Tsukatani, K., Miyata, K.,  Lee, H.T. (Osaka Univ.),  Torikai, Y., Taguchi, A. (Toyama Univ.),  Ashikawa, N., Tokitani, M., Tanaka, T., Sagara, A.

Effects of Surface Mixing Layers of Tungsten on Hydrogen Isotope Behavior

Effects of Surface Mixing Layers of Tungsten on Hydrogen Isotope Behavior

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