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(EX5/2) Improved Energy Confinement at Trans Greenwald Densities in Discharges with a Radiating Edge in the Tokamak TEXTOR-94

B. Unterberg1), G. Mank1), A. Messiaen2), J. Ongena2), S. Brezinsek1), V. Dreval4), P. Dumortier2), R. Jaspers3), D. Kalupin2), H. R. Koslowski1), A. Krämer-Flecken1), A. Kreter1), M. Lehnen1), A. Pospieszczyk1), J. Rapp1), U. Samm1), B. Schweer1), G. Sergienko1), S. Soldatov4), M. Z. Tokar1), G. Van Wassenhove2), R. R. Weynants2) and the TEXTOR Team 1)2)3)
 
1) Institut für Plasmaphysik, Forschungszentrum Jülich, Ass. ``EURATOM- FZ Jülich'',
D-52425 Jülich, Germany
2) Laboratoire de Physique des Plsamas - Laboratorium voor Plasmafysica, Ass. ``EURATOM- Belgian State'', ERM- KMS, B-1000 Brussels, Belgium
3) FOM Instituut voor Plasmafysica Rijnhuizen, Ass. ``EURATOM-FOM'', NL-3430 BE, Nieuwegein, The Netherlands
4) Nuclear Fusion Institute, Russian Research Centre ``Kurchatov Institute'', Kurchatov Square 1, 123182 Moscow, Russia

Abstract.  Confinement quality as good as in the ELM-free H-mode at plasma densities substantially above the Greenwald density (up to $ \overline{n}_{e}^{}$/nGW = 1.4) has been obtained in discharges with a radiating boundary in the tokamak TEXTOR-94. This is achieved by optimising the gas fuelling rate of RI-mode discharges to avoid both a confinement back transition at the beta limit or a confinement degradation to L-mode levels as a consequence of a too strong gas puffing. A successful increase of the density to values well above nGW without degradation is obtained if the plasma density and the neutral pressure at the edge can be kept low as a result of a moderate gas fuelling. In discharges with a strong gas fuelling, high plasma edge density and neutral pressure builds up and the toroidal plasma rotation at the edge just inside the LCFS is reduced. Furthermore, measurements of density fluctuation spectra at the plasma boundary indicate a qualitative change of edge turbulence with a significant increase of fluctuations below 50 kHz. Under these conditions the edge density and the recycling flux at the main limiter start to increase prior to the global degradation. Modelling of the profile evolution after strong gas fuelling with a 1-D particle transport code shows the re-appearance of the ion temperature gradient driven mode in the plasma bulk, which is first suppressed in the transition from L- to RI-mode after impurity injection, and supports the experimental finding that the strong gas fuelling is the reason for the degradation.

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IAEA 2001