(EX/P8-21) MHD issues in Tore Supra steady-state fully non-inductive scenario

P. Maget1), F Imbeaux1), G Giruzzi1), V S Udintsev2), G T A Huysmans1), H Lütjens3), J-L Ségui1), M Goniche1), Ph Moreau1), R Sabot1), X Garbet1)
 
1) Association Euratom-CEA, CEA/DSM/DRFC, Centre de Cadarache, 13108 Saint-Paul-lez-Durance, France
2) Association Euratom - Confédération Suisse, CRPP/SB/EPFL, Station 13, CH-1015 Lausanne, Switzerland
3) Centre de Physique Théorique, Ecole Polytechnique, Route de Saclay, 91128 PALAISEAU Cedex, France

Abstract.  Impact of MHD modes on non-inductive discharges: a crucial issue Fully non-inductive tokamak plasma discharges are attractive for a fusion reactor, and the understanding of their MHD limitations beyond the linear stability analysis is therefore essential. Specific features are due to the enhanced impact of non-linear effects: one is due to the bootstrap current perturbation, and the other is linked to the intrinsic physics of plasma current generation by the external system used. In the Tore Supra tokamak, a Lower Hybrid wave launcher provides the external source, which has non-linear dependences on the plasma current and electron temperature profiles. Such complex interplay is causing spontaneous temperature oscillations, but also a regime with permanent MHD activity, after the triggering of a double-tearing mode. We report observations and interpretations of MHD modes in such non-inductive discharges in Tore Supra. The double-tearing mode and its non-linear impact: experiment and simulations Due to the slightly hollow deposition profile of LH waves in Tore Supra, the magnetic configuration exhibits a flat or reversed q-profile. The plasma is therefore prone to the triggering of double-tearing modes. In its non-linear development, this mode can either produce a regime of periodic crashes (the q=2 sawteeth), or degenerate into the MHD regime where islands are produced on q=3 and q=4. With its large radial extension, the double-tearing mode could have dramatic effect on a scenario with important bootstrap fraction, as confirmed by numerical simulations. MHD modes: a powerful diagnostic for investigating oscillating regimes In cases where oscillating regimes are present, MHD modes give valuable information about the dynamics of the current profile, as will be shown. Safe operation diagram for non-inductive discharges: preforming and steady-state phases Only the full reconnection of a double tearing mode has an important impact on a Tore Supra non-inductive discharge. On this basis, a diagram for safe operation can be drawn for fully non-inductive discharges. In the ohmic (preforming) phase, MHD modes can also undermine the development of a successful non-inductive phase when the edge safety factor is too high. This aspect will also be addressed.

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