Hannover 2020 – scientific programme

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P: Fachverband Plasmaphysik

P 4: Helmholtz Graduate School 1 and Magnetic confinement 1

P 4.3: Talk

Monday, March 9, 2020, 14:55–15:20, b305

ECCD-induced electron temperature crashes at W7-X — •Marco Zanini, Heinrich Laqua, Torsten Stange, Henning Thomsen, Tamara Andreeva, Christian Brandt, Matthias Hirsch, Udo Höfel, Kian Rahbarnia, Robert C. Wolf, Alessandro Zocco, and W7-X Team — Max-Planck-Institut für Plasmaphysik, 17491 Greifswald, Germany

The plasma in the superconducting optimized stellarator Wendelstein 7-X is mainly heated by an electron cyclotron resonance heating (ECRH), which allows up to 7.5 MW of injected power. ECRH itself can also be used to drive net toroidal current in the plasma (electron cyclotron current drive, ECCD). Toroidal current is not necessary for plasma confinement in stellarators, but the small amount of intrinsic toroidal current makes W7-X a perfect testbed for ECCD experiments. During ECCD experiments, fast and repetitive crashes of the electron temperature have been detected. A 1-D model for current evolution shows that the current drive deforms the rotational transform profile in such a way that low order rational values are crossed, leading the plasma in a condition where instabilities can be triggered. An initial attempt of mode analysis suggested an odd poloidal and toroidal number, thus being coherent with sawtooth oscillations in tokamaks. The pattern of collapses changes in time for long discharges, as the toroidal current evolves, and it was observed that, for relatively high toroidal currents, the change of magnetic topology coupled with these crashes can significantly affect plasma performances.