Studies of impurity migration in TEXTOR by local tracer injection

1. background - plasma Cx+,CHy0,+ re-eroded/ reflected particles CH4 surface surface (substrate C, CR, Be) • plasma-wall-interaction: • physical sputtering/ reflection • chemical erosion (CD4, BeD) • deposition from background • redeposition of eroded species • impurity transport: • ionisation, dissociation • friction, thermal force • Lorentz-force • cross field diffusion vertical observation horizontal observation toroidal limiter (46cm) limiter lock test limiter D+ re-deposit substrate Erosion: Y Re-erosion: fEnh×Y Deposition efficiency: #deposited 13C on test limiter #injected 13CH4 atoms Studies of impurity migration in TEXTOR by local tracer injection A. Kirschnera, P. Wienholda, D. Borodina, C. Björkasa,b, O. Van Hoeyc, D. Matveeva,c, S. Brezinseka, A. Kretera, M. Laengnera, K. Ohyad, V. Philippsa, A. Pospieszczyka, U. Samma, B. Schweera, and TEXTOR teama aInstitut für Energie- und Klimaforschung – Plasmaphysik, Forschungszentrum Jülich, Assoziation EURATOM-FZJ, Trilateral Euregio Cluster, 52425 Jülich, Germany, bDepartment of Physics, University of Helsinki, Finland, cDepartment of Applied Physics, Ghent University, B-9000 Ghent, Belgium, dInstitute of Technology and Science, The University of Tokushima, Japan. Motivation ERO modelling results ● Former tracer injection experiments in TEXTOR lead to very small local deposition efficiencies. According modelling needs assumption of enhanced re-erosion (factor fEnh) of re-deposits to match. ● Possibly enhanced re-erosion of re-deposits: determines resulting net-deposition and thus important for wall life time. ● Study the influence of flux and energy of depositing tracer species on resulting deposition efficiency ⇒ involved mechanisms? Modelled 13C deposition efficiencies fEnh=5, RN=1 RI=0.1, RN=1 The 3D Monte Carlo code ERO ● Assuming reflection for hydrocarbons according to MD (RI=0.1, RN=1) - to simulate observed 13C deposition efficiency: fEnh~35 for reference case, fEnh=10-15 for low injection case ● Assuming fEnh=5 for re-erosion and RN=1 - to simulate observed 13C deposition efficiency: RI=0.8-0.9 for reference case, RI=0.6-0.7 for low injection case Modelled and simulated profiles of 13C deposition Low Injection Case: Reference Case: 13CH4 tracer experiments Experimental set-up: Test limiter after exposure: Reference: 0.3% polished C surface ● Reference case: RI>~0.9 needed to reproduce measured profile ● Low injection case: profile shape reproduced also for smaller RI Low injection rate : 0.71% Conclusions polished C surface ● Measured 13C deposition efficiency increases with impact energy and reduced flux of depositing species ● “Standard” assumptions in ERO lead to large 13C deposition efficiencies (55% for reference, 34% for low injection, 42% for biased limiter case) ● ERO needs enhanced re-erosion and/or increased ion reflection – enhancement smallest for biased limiter. Biased test limiter: 1.7% FEnh = F(Ein, Gin) polished C surface