Short communication Tungsten dust nanoparticles generation from blistering bursts under hydrogen environment in microwave ECR discharge K. Ouaras * , K. Hassouni, L. Colina Delacqua, G. Lombardi, D. Vrel, X. Bonnin 1 LSPM-CNRS, Universit e Paris 13 Sorbonne Paris Cit e, 99 Avenue J. B. Cl ement, 93430 Villetaneuse, France article info Article history: Received 9 March 2015 Received in revised form 24 June 2015 Accepted 24 July 2015 Available online 26 July 2015 Keywords: Blistering burst Tungsten dust Nanoparticles Hydrogen plasma abstract Blistering burst induced tungsten dust nanoparticles were observed for the first time when a tungsten sample is submitted to a hydrogen low-temperature discharge under low flux and low incident energy values (20, 120 and 220 eV) at a surface temperature of 500 K. Tungsten nanoparticles (~50 nm) were organized in 2D domains with diameter that is well correlated to the blister volume losses by burst. These observations suggest that dust nanoparticles were generated from blistering burst. © 2015 Published by Elsevier B.V. In ITER, the plasma facing divertor parts are intended to be made of tungsten or tungsten based materials [1,2]. A phenomenon inherent to the interaction of W with hydrogen and helium plasma is the formation of blisters [3e11] on the surface even if the inci- dent energy is too low to create displacement damage. Blister for- mation results in the reduction of isotopes inventory and tungsten embrittlement. With such morphological changes due to ion loading, blister may be easily eroded, e.g. overheating of the ther- mally poorly connected structure and are therefore more sensitive to transient event like ELMs than standard tungsten [12e15]. Tungsten may also undergo melt splashing [16e18] and cracks formation on the surface [19]. It is therefore prone to eject a sub- stantial amount of matter in the plasma. Dust formation is another major problem inherent in the operation of tokamaks [20e29]. Dust investigation becomes an important topic considering its impact on safety issues. Various hazards are to be considered: (i) plasma contamination; (ii) ex- plosion in case of air ingress LOVA (loss of vacuum accident); (iii) dusts containing tritium or activated materials may be highly radioactive. Concerning the safety limits, the dust inventory for ITER during the DT phase must be maintained below approximately one tone. Dust formation was closely related to the erosion and re- deposition of materials during both the standard discharge condi- tions (eg. Sputtering inter alia) and the transient event described above. Moreover, the net sputtering yield of tungsten is expected to be reduced by the ‘prompt redeposition’ during the first Larmor gyration of W þ [30] and thus reduce the possibility to form W dust. Nevertheless, in this paper a new mechanism of tungsten dust formation under hydrogen plasma load is experimentally evi- denced; it is based on a blister bursting scenario that leads to production of tungsten nanoparticles. Blister bursting were already observed by Shu et al. [31,32], who exposed tungsten samples at 520 K and 315 K to deuterium plasmas that provide low energy (~38 eV), high flux (~10 22 m À2 s À1 ) ions and high fluence exposure (~10 25 to10 27 m À2 ). These authors showed in particular that blister bursting does not occur after the plasma exposure but only after the samples were heated during temperature-programmed desorption (TPD) experiments and the burst of the blister could be associated to the release of D 2 . Blis- tering burst is closely related to the concentration of deuterium inside the blister and the surface temperature. Blistering burst was also observed by Manhard et al. [33] after TPD of polycrystalline tungsten samples which was initially exposed to a deuterium plasma maintained at lower temperature, i.e., 320 K, and submitted to lower fluence, i.e., 5 Â 10 24 m À2 , but at similar ion fluxes as compared with Shu et al. [31,32]. Manhard et al. [33]. also attrib- uted bursts of D 2 to the rupturing of blisters filled with high- pressure D 2 gas. All these studies showed that blistering burst * Corresponding author. E-mail address: ouaras@lspm.cnrs.fr (K. Ouaras). 1 Current address: ITER Organization, Route de Vinon-sur-Verdon, F-13067 St- Paul-lez-Durance, France. Contents lists available at ScienceDirect Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat http://dx.doi.org/10.1016/j.jnucmat.2015.07.035 0022-3115/© 2015 Published by Elsevier B.V. Journal of Nuclear Materials 466 (2015) 65e68