Fusion Engineering and Design 86 (2011) 2458–2461 Contents lists available at ScienceDirect Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes Hydrogen retention in gallium samples exposed to ISTTOK plasmas R.B. Gomes a,∗ , R. Mateus a , E. Alves b , H. Fernandes a , C. Silva a , P. Duarte a a Associac ¸ ão Euratom/IST, Instituto de Plasmas e Fusão Nuclear – Laboratório Associado, Instituto Superior Técnico, Av. Rovisco Pais, 1049-001 Lisboa, Portugal b Instituto Tecnológico e Nuclear, Estrada Nacional 10, 2686-953 Sacavém, Portugal article info Article history: Available online 6 May 2011 Keywords: Liquid metals Plasma–surface interaction Gallium Hydrogen retention Tokamak ISTTOK abstract The use of liquid metals such as lithium and gallium have been pointed out as a suitable solution to solve problems related to the use of solid walls submitted to high power loads. A proper use of liquid materials in fusion reactors depends, among others, on their affinity to retain hydrogenic isotopes. While retention in lithium has been studied in detail, less is known for gallium. Taking into account the deep influence of this property on plasma behaviour it is deemed relevant to perform such studies in tokamak plasmas. An experimental setup has been developed to produce high purity gallium samples which were exposed to ISTTOK plasmas on both liquid and solid phases. Hydrogen retention and in-depth profiles were simul- taneously measured by ERDA and RBS analytical techniques. Experimental data proved that most of the retention takes place in a thin layer near the surface. Liquid samples present higher retention values which may be understood if higher hydrogen diffusivity is assumed. Retained fraction (H retained /H incident ) around 0.3 and 1% were obtained for solid and liquid samples, respectively. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Liquid metals have been acknowledged as a suitable solution to solve the erosion problem faced by solid plasma facing components submitted to the high power loads of fusion devices and capable to provide, at the same time, a high efficiency heat exhaustion process. Since high retention rates may result in a significant impact on the tritium inventory in future reactor designs, it is imperative to eval- uate the liquid metals hydrogenic solubility. The most promising candidate materials are lithium and gallium. It has been shown [1] that liquid lithium is able to retain very large amounts of hydrogen by lithium hydrates formation (up to a 1:1 stoichiometric ratio). This effect has a deep influence on plasma behaviour by imposing a near to zero recycling operating condition in tokamaks. A signif- icant increase on the energy confinement time has been observed in this regime [2]. Additionally, a broader plasma region available to fusion reactions is expected owing to higher electron tempera- tures at the edge plasmas [3]. The main advantages of gallium are a very high liquid state temperature range and a low chemical reac- tivity. In a previous work it has been reported that ISTTOK could be operated in the presence of a liquid gallium jet without noticeable discharge degradation, severe effect on the plasma parameters or significant plasma contamination by liquid metal [4]. While retention in lithium has been studied in detail, very few to none information is available for this property when applied to ∗ Corresponding author Tel.: +351 218419104; fax: +351 218417819. E-mail address: gomes@ipfn.ist.utl.pt (R.B. Gomes). gallium. In despite of this, very low retention rates are expected due to the fact that gallium hydrides are known to be chemically unsta- ble. The aim of this work was to evaluate the hydrogen content in gallium samples (both solid and liquids) exposed to a large num- ber of operative discharges and at different radial positions within ISTTOK edge plasma. For that purpose an experimental setup was developed to produce high purity gallium samples. Similar plasma conditions and exposure times were used for individual irradia- tions. Hydrogen retention measurements were achieved by means of ion beam analysis. 2. Experimental setup and analytical procedure Two different setups were implemented to duly expose gallium samples to ISTTOK plasmas: (i) a sample preparation chamber and (ii) a sample positioning and conditioning system. ISTTOK is a large aspect ratio, circular cross-section tokamak with main parameters: R = 46 cm, a = 8.5 cm, B T = 0.5 T, I p ≈ 4–6 kA. A complex procedure has been used to ensure a high purity degree in the bulk of gallium samples. 6 N gallium grade (99.9999% purity) was degassed (metal heated above 250 ◦ C during 24 h until the background pressure returns to values around 10 -6 mbar) into a suitable container, which was itself previously degassed at UHV pressure. This procedure is required since gallium has a high affin- ity to produce oxides by recombining with oxygen desorbed by the walls of the container or atmospheric air. Upon liquefaction (above 30 ◦ C) gallium was introduced directly into the degassing tank (Fig. 1) taking advantage of differential pressure to pump the liquid metal contained in the bottles provided by the producer (PPM 0920-3796/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2011.03.055