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In TCV, the study of the energy confinement time for extreme plasma shapes and the approach to the Troyon beta-limit at intermediate values of the normalized current, IN=Ip/aB~2 MA/mT, are the main motivations for developing scenarios with high elongation and low current, using Electron Cyclotron Heating (ECH). At very high elongation, low current operation in ohmically heated discharges is limited by vertical stability and current profile broadening is needed to extend the range of stable equilibria. Using far off-axis 2nd harmonic, X-mode, ECH to broaden the current profile, highly elongated discharges (k ~ 2.4) with low current (Ip ~ 300 kA) and a high safety factor (qedge ~ 12) have been created and stabilized in TCV. A model enables us to distinguish between the effect of the current profile and that of the plasma shape on the value of the internal inductance. Knowing the plasma shape, the internal inductance then gives information about the current profile. The analysis of the discharges shows that the current profile broadening and consequently the plasma elongation depend strongly on the location of the ECH power deposition. The maximum effect is seen for rdep ~ 0.7 at the beginning of the EC heating. To keep the normalized deposition radius constant during an elongation process, the launchers are progressively moved outward. In this case the plasma elongation reaches higher values and increases slightly faster than in a scenario with fixed mirrors. The rate of increase of plasma elongation however is shown to be limited by current diffusion.

Additional information

Authors: CAMENEN Y ET AL, Centre de Recherches en Physique des Plasmas, Ecole Polytechnique Fédérale de Lausanne, Association EURATOM-Confédération Suisse (CH)
Bibliographic Reference: An oral report given at: 12th Joint Workshop on Electron Cyclotron Emission and Electron Cyclotron Resonance Heating Organised by: Association Euratom-CEA, Cadarache Held at: Aix-en-Provence (FR) May 13-16, 2002
Availability: Workshop papers are available online at: This paper is also published in report LRP 726/02, available free of charge from: École Polytechnique Fédérale de Lausanne, Ecublens, CH-1015 Lausanne Fax +41-21-69347
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