Speaker
Henning Thomsen
Description
See the full Abstract at http://ocs.ciemat.es/EPS2018ABS/pdf/P1.1050.pdf
Analysis of MHD activity in Wendelstein 7-X stellarator
H. Thomsen, C. Brandt, K. Rahbarnia, T. Andreeva, S. Bozhenkov, H. Laqua, T. Klinger,
A. Koenies, N. Lauf, U. Neuner, J. Schilling, T. Stange, M. Zanini and the W7-X Team
Max-Planck-Institut für Plasmaphysik, 17491 Greifswald, Germany
The Wendelstein 7-X stellarator (W7-X) in Greifswald has started its second experimental
phase in October 2017. The installation of the uncooled divertor modules allowed for a higher
energy limit as compared to the first experimental phase conducted in limiter configuration.
The magneto-hydrodynamic (MHD) diagnostic systems had been extended prior to the start
of this second experimental phase and the Mirnov diagnostic and the X-ray tomography
system have been put into operation. Performance-wise, long discharge lengths of up to 30 s
have been achieved and a peak diamagnetic energy of 1 MJ has been measured.
In this specific high-performance experimental program, the density has been increased by
means of pellet fueling during the first phase. In the second phase without pellet fueling, the
temperature increased (input heating was kept constant) while the line-averaged density signal
decreased; the measured diamagnetic energy was constant. The second phase was ended by
MHD mode activity leading to an abrupt decrease in the stored energy by approx. 150 kJ.
Mirnov and X-ray diagnostics show an oscillatory pre-trigger prior to the crash.
These signatures resemble the observations obtained during ECCD current drive experiments,
where the driven currents lead to similarly fast crashes. In this contribution we present the
analysis of the crashes in these experiment programs.
