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Abstract

It is sometimes observed that several Neoclassical Tearing Modes (NTMs) are present at the same time. This can happen both at the mode onset of a new NTM or during the non-linear saturated phase of a given mode. In order to analyse whether non-linear mode coupling plays any role before or after the onset of NTMs, we use a data analysis tool known as the bispectral technique. This technique has been used for identifying the non-linear three wave interactions in turbulent plasmas. We have used this technique for the study of NTMs in the Tokamak à Configuration Variable (TCV) and in the Joint European Torus (JET). The analysis during the saturated phase of a 3/2 NTMs in a JET discharge suggests a strong coupling between the 3/2, 4/3 and 7/5 modes. This coupling could be responsible for the observed stabilization effect of the 4/3 mode on the 3/2 mode. It also demonstrates that this analysis tool can be used to identify high m/n modes, beyond the capabilities of the standard method using poloidal or toroidal probe arrays. As this technique allows a more accurate evaluation of the existence of non-linear mode coupling, it has been used to study and confirm that no non-linear coupling occurred to trigger two specific NTM cases in TCV and JET, where a classical tearing mode and a crash after a long sawtooth period are responsible for the triggering of the necessary seed islands, respectively.

Additional information

Authors: RAJU D, Institute for Plasma Research, Bhat (IN);LISTER J B, Association EURATOM-Confédération Suisse, Centre de Recherches en Physique des Plasmas, École Polytechnique Fédérale de Lausanne (CH);SAUTER O, Association EURATOM-Confédération Suisse, Centre de Recherches en Physique des Plasmas, École Polytechnique Fédérale de Lausanne (CH)
Bibliographic Reference: A paper submitted to Plasma Physics and Controlled Fusion.
Availability: Available free of charge from: École Polytechnique Fédérale de Lausanne (EPFL), Ecublens, CH-1015 Lausanne Fax +41-21-6934747
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