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Abstract

During plasma excitation by high power Alfvén waves at 2.5 MHz in TCA, harmonics have been observed on the wavefields in the plasma edge. Magnetic probe data reveal that the dispersion properties and excitation efficiency are closely linked to the fundamental Alfvén wave spectrum. In addition, these are little affected by the type of exciting antenna; similar harmonic fields are observed with earthed-shielded, floating-shielded and unshielded antennae, but they are strongly affected by antenna mode number. However, the harmonics are too large with respect to the fundamental at the powers employed to be straightforwardly explained by non-linear terms in the MHD equations. Threshold phenomena have been observed in which harmonics only occur above a certain level of the fundamental signal or antenna current. Theoretical modelling is combined with these experimental facts to characterise and to determine the origin of the harmonics. Analysis of magnetic probe signals has also revealed the existence of side-bands around the fundamental wave signal in Alfvén wave heating. The nature and the characteristics of these side-bands have been investigated using fast sampling and demodulation techniques. In particular, it has been possible to show evidence of a coupling between the low frequency (f approximately 10-20 kHz) Mirnov activity and the Alfvén waves. The symmetry of these side-bands above the fundamental can change depending on plasma conditions.

Additional information

Authors: BORG G G, Centre de Recherches en Physique des Plasmas, Ecole Polytechnique Fédérale de Lausanne, Lausanne (CH);DUPERREX P A, Centre de Recherches en Physique des Plasmas, Ecole Polytechnique Fédérale de Lausanne, Lausanne (CH);LISTER J B, Centre de Recherches en Physique des Plasmas, Ecole Polytechnique Fédérale de Lausanne, Lausanne (CH)
Bibliographic Reference: Paper presented: 18th European Conference on Controlled Fusion and Plasma Physics, Berlin (DE), June 3-7, 1991
Availability: Text not available
Record Number: 199110448 / Last updated on: 1994-12-02
Category: PUBLICATION
Original language: en
Available languages: en