Particle-In-Cell Simulations of Shaped and Non-shaped Gaps in TEXTOR Test Limiter

In this paper, we present results of Particle-In-Cell simulations of the plasma behavior in the vicinity of gaps in castellated plasma-facing components. We studied gaps in the test limiter of TEXTOR, which is a plasma-facing component designed for studies of impurity transport and fuel retention, mainly via hydrocarbon layer formation. The castellated limiter tiles have been analyzed using the postmortem techniques and first attempts of impurity transport modelling were done using the 3DGAP code [Matveev et al., 2008]. In order to improve the agreement between modeling and experiment, realistic plasma particle flux distribution needs to be included in the code. For this purpose, a series of PIC simulations for varying plasma conditions was performed by using the SPICE2 code [Dejarnac et al., 2008]. Plasma behavior inside the gaps is discussed and semi-empiric characterization via generic fitting procedure is proposed. Introduction Fuel retention in nuclear fusion devices is a subject of intensive research, especially because of implications for ITER and future reactors [Loarer, 2009]. For safety reasons, the amount of tritium inside the vessel is restricted (in case of ITER to approximately 1 kg), and so its accumulation leads to a limitation of the number of discharges, which can be performed before the safety limit is reached. The amount of retained fuel varies between 2 20 percent depending on the machine and method used for analysis. Measurements at DIII-D [Walsh et al., 1992] show that up to 40% of the retained fuel can be trapped in gaps between castellated plasma-facing components (PFCs). The gaps are in particular difficult to access by cleaning techniques. Studies of the deuterium plasma transport inside the gaps was performed at the TEXTOR tokamak in the so-called test limiter [Litnovsky et al., 2005]. The limiter tiles were exposed to plasma and then analyzed using post-mortem techniques, such as the Secondary Ion Mass Spectrometry (SIMS), Nuclear Reaction Analysis (NRA) and Electron Probe Microanalyzer (EPMA). The amount of deposited hydrocarbons and its fuel content has been measured as well as the material mixing inside the gaps. In order to explain the deposition mechanisms, a numerical model 3DGAP, a 3D Monte-Carlo neutral impurity transport code, has been developed [Matveev et al., 2008]. Modeling of transport in gaps involves a number of parameters, which are often not known from experiment. The aim of the calculations is to determine such parameters by matching the calculation results with experimental data. In order to make the simulations realistic, the distribution of particle fluxes coming from the main plasma is needed. This can be simulated by means of Particle-In-Cell simulations. The aim of this work is to provide such fluxes for the range of plasma conditions typical for the TEXTOR tokamak by using the SPICE2 code. 169 WDS'09 Proceedings of Contributed Papers, Part II, 169–175, 2009. ISBN 978-80-7378-102-6 © MATFYZPRESS