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The additional power absorbed by the plasma can be determined from the time derivative of the total plasma energy, which can be estimated from the diamagnetic flux of the plasma using a DiaMagnetic Loop (DML). Compensation of the flux measurement sensitivity to poloidal magnetic fields is not always easy to adjust. A method based on the temporal variations of the diamagnetic flux of the plasma during Modulated Electron Cyclotron Heating (MECH) has been developed. Using MECH has the advantage that these poloidal fields are not significantly modulated and a good compensation of these fields is not necessary. However, a good compensation of the vessel poloidal image current is crucial to ensure a sufficiently large bandwidth. The application of this diagnostic to studies of the extraordinary mode (X-mode) absorption at the third electron cyclotron harmonic frequency (X3) has been performed on the TCV Tokamak in plasmas pre-heated by X-mode at the second harmonic (X2). A MECH frequency scan has allowed the determination of an optimum modulation frequency, situated at about 200-250Hz. Based on this diagnostic, full absorption of the injected X2 power was measured as well as full single-pass absorption of the injected X3 power was measured with the X2 preheating in co-current drive. The high X3 absorption is more than a factor of 2 higher than the one predicted by the linear ray tracing code TORAY. Experimental evidence indicates that a large fraction of the X3 power is absorbed by electrons in an energetic tail created by the X2 preheating.

Additional information

Authors: MANINI A 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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