Hydrogenic retention in tungsten exposed to ITER divertor relevant plasma flux densities G.M. Wright a, * , A.W. Kleyn a,b , E. Alves c , L.C. Alves c , N.P. Barradas c , G.J. van Rooij a , A.J. van Lange a , A.E. Shumack a , J. Westerhout a , R.S. Al a , W.A.J. Vijvers a , B. de Groot a , M.J. van de Pol a , H.J. van der Meiden a , J. Rapp a,d , N.J. Lopes Cardozo a,e a FOM-Institute for Plasma Physics Rijnhuizen, Association Euratom-FOM, A member of the Trilateral Euregio Cluster, Postbus 1207, 3430 BE Nieuwegein, The Netherlands b Leiden Institute for Chemistry, Leiden University, Leiden, The Netherlands c Instituto Tecnológico e Nuclear, Estrada Nacional 10, 2686-953 Sacavém, Portugal d Institut für Plasmaphysik, Forschungszentrum Jülich, Association EURATOM, 52425 Jülich, Germany e Eindhoven University of Technology, Eindhoven, The Netherlands article info PACS: 52.40.Hf 52.75.-d 52.50.dg 28.52.Fa abstract Tungsten targets are exposed to the plasma conditions expected at the strike point of a detached ITER divertor (10 24 D/m 2 s, T e  2 eV). The surface temperature of the target is 1600 K at the center and decreased radially to 1000 K at the edges. A 2-D spatial scan of the W target using nuclear reaction anal- ysis (NRA) shows an asymmetric D retention profile with the lowest retention values at the center of the target and the highest 6 mm off-center. Even in the regions of larger retention, the D concentrations were 65  10 15 D/cm 2 as measured by NRA. Thermal desorption spectroscopy (TDS) is used to measure the global D retention. Very low retention with retained fractions ranging from 10 7 to 10 5 D retained /D incident were measured with TDS. Both NRA and TDS results show no clear dependence of retention on incident fluence possibly indicating the absence of plasma-driven trap production in W under these conditions. Ó 2009 Elsevier B.V. All rights reserved. 1. Introduction Hydrogenic retention in the walls of ITER can affect density con- trol and fuelling rates. Also, during deuterium(D)-tritium(T) oper- ation, there is a safety issue as only 350 g of mobilizable T are allowed to be stored in the ITER wall [1,2]. Tungsten (W) is marked for use as a plasma-facing components (PFC) material in the ITER divertor. The thermal properties of W allow it to survive the ex- pected heat loads at the ITER strike points but, perhaps more importantly, W also has low hydrogenic solubility and, in the ab- sence of a strong hydrogenic trap production mechanism, is ex- pected to have low hydrogenic retention levels. This is confirmed by numerous laboratory studies on hydrogenic retention in W [3–8]. From this it has been assumed that W has an advantage over carbon-based materials (i.e. graphite, CFCs) with respect to tritium inventory in the ITER divertor. However, studies have identified a mechanism for trap produc- tion in refractory metals, specifically W. It has been postulated that exposure of W to a high flux of low energy (6200 eV) ions leads to a build-up of stresses in the W lattice due to the low hydrogenic solubility of W [6–8]. These stresses are relieved through deforma- tion of the lattice and the creation of vacancies, dislocations or voids, which then represent hydrogen trapping sites. There are indications that this trap production mechanism is dependent on the incident ion flux density [8,9] but the relationship and how it extrapolates to ITER-relevant flux densities is not clear. The purpose of this study is to expose poly-crystalline tungsten samples to plasma flux densities and energies that are expected at or near the ITER divertor strike points. This allows the hydrogenic retention at these high flux densities (10 24 D/m 2 s) to be observed and measured experimentally rather than relying on extrapola- tions from ion beam or low-density plasma experiments. 2. Experiment W targets were exposed to ITER divertor relevant deuterium plasmas in the linear plasma device Pilot-PSI. The Pilot-PSI experi- ment uses a cascaded arc to produce high plasma densities (610 21 m 3 ) at low electron temperatures (T e 6 5 eV) [10,11]. The plasma is confined to a column of 15 mm diameter with the high- est densities and temperatures located at the center of the column. The plasma electron density and temperatures are measured with Thomson scattering [12]. For these experiments, the central 0022-3115/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jnucmat.2009.01.173 * Corresponding author. E-mail addresses: wright@rijnhuizen.nl (G.M. Wright), kleyn@rijnhuizen.nl (A.W. Kleyn). URLs: http://www.rijnhuizen.nl (G.M. Wright), http://wwwchem.leidenuniv.nl (A.W. Kleyn), http://www.itn.pt (E. Alves), http://www.tue.nl (N.J. Lopes Cardozo). Journal of Nuclear Materials 390–391 (2009) 610–613 Contents lists available at ScienceDirect Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat