Reemission of neutral hydrogen molecules from tungsten Iztok C ˇ adez ˇ * , Sabina Markelj, Primoz ˇ Pelicon, Zdravko Rupnik Joz ˇef Stefan Institute, Association EURATOM-MHEST, Jamova cesta 39, 1000 Ljubljana, Slovenia article info PACS: 07.77.Àn 82.80.Gk 52.25.Tx abstract Vibrational distribution of H 2 and D 2 molecules emitted from tungsten surface exposed to the continuous flow of partially dissociated neutral hydrogen gas is studied. A special vibrational spectrometer is used for this study. Vibrational distributions can be well described by vibrational temperatures being T V = 3700 ± 100 K for H 2 and T V = 3400 ± 100 K for D 2 . To gain more detailed information on the state of the tungsten surface we employed ion beam analytical method ERDA for in situ determination of H/D concentration. Both experiments indicate high importance of impurity layer (presumably oxide) on the observed phenomena. Ó 2009 Elsevier B.V. All rights reserved. 1. Introduction Hydrogen recycling is an important process on the wall of a fu- sion reactor. Hydrogen ions that hit the wall surface after being neutralized may return into the plasma as neutral atoms or if they recombine with other atoms as neutral hydrogen molecules. Neu- trals are predominantly reemitted as molecules at low surface temperatures while atom reemission dominates at higher temper- atures [1]. Molecules produced at the surface can be vibrationally excited. These molecules are important for energy exchange be- tween the edge plasma and the reactor wall and together with other charged and neutral particles they determine the character- istics of the edge plasma and therefore plasma–wall interaction. Excited neutral hydrogen molecules are produced not only on the surfaces in direct contact with the plasma but also at more remote surfaces such as are the gaps of castellated structures and the wall of the vacuum vessel. Rate of production and relaxation of the ex- cited molecules as well as cross sections for different collision pro- cesses where they participate need to be known in order to enable detailed modelling of the edge plasma (e.g. [2]). Reaction rate con- stants often strongly depend on the internal excitation of partici- pating particles so that this excitation can even alter the series of dominant reactions occurring close to the wall. There is quite a lot of experimental data on hydrogen retention and release for W. On the other hand there is to our knowledge al- most no experimental data on vibrational distribution of hydrogen molecules emitted from the tungsten surface and very little data on interaction of hydrogen atoms and molecules, with low kinetic energies (few meV), with the tungsten surface. In order to acquire new quantitative data on processes with vibrationally excited hydrogen molecules we are studying interaction of hydrogen atoms and molecules with high-Z materials and some results ob- tained with tungsten are presented in this paper. Terms ‘hydrogen molecule’ and ‘hydrogen atom’ are used in the text as a general designation for all hydrogen isotopologues or isotopes and chemi- cal symbols H and D are used when isotopic composition of a par- ticle is relevant. 2. Experiment Experiments were performed using two diagnostic techniques on separate experimental set-ups [3]. For vibrational spectroscopy of H 2 and D 2 we employ a method based on properties of the lowest energy dissociative electron attachment (DEA) in hydrogen, e + H 2 ? H À + H or e + D 2 ? D À + D. Vibrational temperature of target gas is determined by deconvolu- tion of experimental negative ion (H À or D À ) yield spectra. These spectra represent dependence of negative ion yield on the energy of electron beam in the 0–5 eV range. A new set-up (DTVE-B) employing this method of vibrational spectroscopy was developed in order to facilitate studies of processes involving vibrationally ex- cited hydrogen molecules [3,4]. A special source of vibrationally excited hydrogen molecules (TS) was used for the present studies and it is shown in Fig. 1(a). Hydrogen gas is introduced in the cell where it becomes partially dissociated by a hot tungsten filament. Atoms that are created on the filament can eventually recombine on the cooled wall surface and vibrationally excited molecules are created depending on the conditions in the cell and wall mate- rial [5]. Gas containing vibrationally cold (v = 0) and hot (v > 0) molecules as well as atoms which did not recombine flows out of the cell through the exit orifice and is being intercepted by the electron beam of DTVE-B (Fig. 1(b)). A disk of the studied material (25 mm dia.) is placed close (5 mm) and parallel to the exit orifice 0022-3115/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jnucmat.2009.01.074 * Corresponding author. E-mail address: Iztok.Cadez@ijs.si (I. C ˇ adez ˇ). Journal of Nuclear Materials 390–391 (2009) 520–523 Contents lists available at ScienceDirect Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat