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(EX/C3-5Ra) Impurity Transport in Internal Transport Barrier Discharges on JET

R. Dux¹⁾, C. Giroud²⁾, K.-D. Zastrow³⁾, JET EFDA Contributors^4)
 
1) Max-Planck-Institut für Plasmaphysik, EURATOM Association, Garching, Germany
²⁾ FOM Rijnhuizen, Ass. EURATOM-FOM, TEC, Nieuwegein, Netherlands
³⁾ EURATOM-UKAEA Fusion Association, Culham, United Kingdom
⁴⁾ see annex 1 in J. Pamela, OV/1-4, FEC 2002

Abstract.  In JET plasmas with internal transport barrier (ITB) the behaviour of metallic and low-Z impurities (C, Ne) was investigated. In ITB discharges with reversed shear, the metallic impurities accumulate in cases with too strong peaking of the density profile, while the concentration of low-Z elements C and Ne is only mildly peaked. The accumulation might be so strong, that the central radiation approximately equals the central heating power followed by a radiative collapse of the transport barrier. The radial location with strong impurity gradients (convective barrier) was identified to be situated inside (not at!) the heat flux barrier. Calculations of neo-classical transport were performed for these discharges, including impurity-impurity collisions. It was found, that the observed Z-dependence of the impurity peaking and the location of the impurity ``barrier'' can be explained with neo-classical transport. ITB discharges with monotonic shear show less inward convection and seem to be advantageous with respect to plasma purity.

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