Speaker
Christian Brandt
Description
See the full Abstract at http://ocs.ciemat.es/EPS2018ABS/pdf/P4.1056.pdf
Tomographic inversion of Wendelstein 7-X stellarator plasmas
C. Brandt1 , H. Thomsen1 , T. Andreeva, N. Lauf1 , U. Neuner1 , K. Rahbarnia1 , J. Schilling1 ,
T. Broszat1 , R. Laube1 and the Wendelstein 7-X Team
1 Max-Planck-Institute for Plasma Physics, Greifswald, Germany
In the operational phase OP1.2a (Aug-Dec 2017) of the Wendelstein 7-X (W7-X) stellarator
experiment the soft X-ray tomography diagnostic (XMCTS: soft X-ray multi camera tomogra-
phy system) has been commissioned. Soft X-ray tomography systems are powerful diagnostics
for high temperature plasmas measuring spatiotemporal X-ray emissivity profiles. The XMCTS
consists of 20 poloidally arranged pinhole cameras at one toroidal location observing a triangu-
lar shaped up-down symmetric plasma cross section. X-ray radiation is mainly emitted in the
hot plasma core (electron temperatures > 1 keV). In the pinhole cameras the plasma radiation is
filtered by a beryllium foil of 12.5 µm thickness being transmissible for X-ray radiation above
1 keV. Taking into account the detector silicon thickness of 100 µm the detectable energy range
is limited to approximately 1 − 10 keV. With 18 available cameras in OP1.2a the soft X-ray
emissivity has been recorded along 324 lines-of-sight with a time resolution of 0.5 µs.
The presentation concentrates on the preparation and first results of the tomographic inver-
sion. For correct calculation of the tomograms, both, the knowledge of the exact geometry of
the lines-of-sight and the sensitivity of each photodiode are of crucial importance. The as-built
coordinates of the cameras have been measured after the in-vessel installation. The deviations
from the designed lines-of-sight are smaller than 1◦ . Changes of the lines-of-sight geometry
according to mechanical deformations of the vacuum vessel in the pumped down state and
its systematic effects on the tomography are discussed. The amplitude responses of all cam-
eras measured before and after in-vessel installation are considered. The tomography code used
bases on the regularization applying a minimum Fisher constraint. First tomography results of
selected discharges from OP1.2a are discussed.
