Modelling of carbon dust formation by cluster growth in argon plasmas X. Bonnin a, * , G. Lombardi a , K. Hassouni a , A. Michau a , F. Be ´ne ´dic a , C. Arnas b a LIMHP-CNRS, UPR 1311, Institut Galile ´e, Universite ´ Paris XIII, 99 Avenue Jean-Baptiste Cle ´ment, F-93430 Villetaneuse, France b LPIIM-CNRS, UMR 6633, Universite ´ de Provence Aix-Marseille I, F-13331 Marseille cedex 03, France Abstract In tokamaks with carbon plasma-facing components, one can observe the presence of nano-sized dust particles. Under- standing such dust particle formation is a prerequisite to any attempt to limit or avoid this dust that may be responsible for tritium retention and pollution of the plasma. We report on coupled modeling of carbon chemistry and dust particle nucle- ation, growth, and transport in a plasma discharge. The chemical model used for carbon cluster dust growth is described in detail. The results are consistent with measurements made at LPIIM from low-pressure argon DC discharges in a stainless steel reactor with a graphite cathode [C. Arnas, C. Dominique, P. Roubin et al., J. Nucl. Mater. 337–339 (2005) 69], serving as a proxy for the tokamak plasma edge. The time evolution of the ‘large’ dust particles consists of a nucleation phase followed by an accretion phase. These reach a dust grain size of 40 nm on a timescale comparable to the experimental observations (minutes to hours). Ó 2007 Elsevier B.V. All rights reserved. PACS: 82.40.Ra; 52.25.Tx; 52.40.Hf; 52.65.y Keywords: Carbon impurities; Dust; Erosion and deposition; Sputtering 1. Introduction The aim of this paper is to investigate the coupled phenomena of carbon dust particle nucleation, growth, and transport, in the low-pressure DC argon discharges developed at the LPIIM in Mar- seille [1]. These take place inside a stainless steel reactor, where the only possible source for carbon is the graphite cathode used to initiate and maintain the discharge. Carbon dust (tens of milligrams) is found to collect on the anode for discharges lasting more than a few minutes. The discharge ions travel through the collisional sheath and impact the cath- ode with an average energy of order 20 eV, enough to cause sputtering. Energetic charge exchange neu- trals may also impact the cathode. Carbon is then sputtered from the cathode by the incident argon flux, and then transported inside the discharge, where it experiences collisions with electrons, ions, resulting in charge transfer, electron attachment/ detachment, ionisation, etc. Collisions also take place between carbon species, which leads to the formation of carbon clusters up to a critical size at 0022-3115/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jnucmat.2007.01.161 * Corresponding author. Fax: +33 149403414. E-mail address: bonnin@limhp.univ-paris13.fr (X. Bonnin). Journal of Nuclear Materials 363–365 (2007) 1190–1194 www.elsevier.com/locate/jnucmat