Studies of the influence of external hydrocarbon injection on local plasma conditions and resulting carbon transport R. Ding a,b,⇑ , A. Kirschner b , M.Z. Tokar b , M. Koltunov b , D. Borodin b , S. Brezinsek b , A. Kreter b , J.L. Chen a , J.G. Li a , G.-N. Luo a a Institute of Plasma Physics, Chinese Academy of Sciences, P.O. Box 1126, Hefei, Anhui 230031, PR China b Institut für Energieforschung – Plasmaphysik, Forschungszentrum Jülich, Association EURATOM-FZJ, Trilateral Euregio Cluster, D-52425 Jülich, Germany article info Article history: Available online 8 October 2010 abstract A one-dimensional fluid model, which calculates the modification of density and temperature along the magnetic field and the parallel electrical field in the presence of local impurity sources, has been imple- mented into the ERO code. The influence of impurity source strength on the local plasma parameters and resulting changes in impurity transport and deposition has been studied. Dedicated TEXTOR experiments of 13 CH 4 injection through roof-like test limiters are modelled for comparison. Modelling with high injec- tion rates (larger than about 4  10 18 s 1 ) results in too localized light emission pattern near to the injection hole, and therefore indicates an underestimation of the reduction of electron temperature in the model. Besides, the preliminary results indicate that possible modification of local plasma conditions cannot significantly reduce the modelled 13 C deposition and therefore cannot explain the measured low 13 C deposition efficiency. Ó 2010 Elsevier B.V. All rights reserved. 1. Introduction The use of carbon-based materials in fusion devices leads to the build-up of deposited layers, which can contain large amounts of fuel. Therefore, understanding the processes of carbon transport, re-deposition and re-erosion is critical for predictions of long-term tritium retention in ITER. The three-dimensional Monte-Carlo code ERO [1] which simulates erosion, impurity transport and deposi- tion has been used to estimate long-term tritium retention in the divertor of ITER [2]. However, further code development and benchmarking with existing experiment is still necessary. Hydro- carbon injection experiments have been performed on different tokamak devices such as TEXTOR [3], JET [4], ASDEX Upgrade [5] and DIII-D [6] to study carbon migration and resulting deposition. 13 C marked hydrocarbon injection experiments through test limit- ers at TEXTOR show very small local 13 C deposition efficiency [3]. ERO has been applied to such experiments for benchmarking [7,8]. The small 13 C deposition efficiency can only be reproduced by ERO modelling with assumptions of low (effective) sticking coefficients for hydrocarbons and a significantly enhanced re-ero- sion of re-deposited carbon. In previous modelling, the disturbance of edge plasma by external hydrocarbon injection was not consid- ered. It is expected that hydrocarbon injection into plasma can change the local plasma parameters through different physical mechanisms: cooling of electrons due to dissociation and ioniza- tion of the injected hydrocarbon molecules and impurity radiation, production of electrons owing to ionization of injected impurities, forces on electrons and background ions arising from collisions with impurity particles, etc. Such modifications of local plasma conditions can also affect the hydrocarbon transport and resulting deposition. This study attempts to reproduce the small 13 C deposi- tion efficiency without the ‘‘extreme assumptions” by taking into account the modifications of local plasma conditions. Within this work, a one-dimensional fluid model which calcu- lates the change of profiles along the magnetic field for the densi- ties, fluxes, temperatures of the main plasma components and parallel electric field. New modelling with locally changed plasma parameters has been performed in comparison with 13 CH 4 injec- tion experiments through roof-like test limiters at TEXTOR. 2. The model The ERO code [1] simulates the transport of externally injected impurities through a given background plasma and resulting depo- sition by means of the test particle approximation. A one-dimen- sional fluid model [9] has been used to calculate the modification of local plasma parameters due to external gas injection. The mod- el uses impurity data given by ERO calculations as additional source and sink terms. Quasi-neutrality, Coulomb collisions of background particles with impurity ions, radiation losses and par- ticle sink to limiter surface are taken into account. The electric field is calculated self-consistently from the electron force balance by 0022-3115/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jnucmat.2010.09.037 ⇑ Corresponding author at: Institute of Plasma Physics, Chinese Academy of Sciences, P.O. Box 1126, Hefei, Anhui 230031, PR China. E-mail address: rding@ipp.ac.cn (R. Ding). Journal of Nuclear Materials 415 (2011) S270–S273 Contents lists available at ScienceDirect Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat