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Updated: May 22, 2026

Applying X-ray Imaging Crystal Spectroscopy for Use as a High Temperature Plasma Diagnostic
Published on: August 25, 2016
Tomographic inversion method to characterize coherent mode activity in the Wendelstein 7-X stellarator
C Büschel1, C Brandt1, H Thomsen1
1Max-Planck Institute for Plasma Physics, EURATOM Association, 17491 Greifswald, Germany.
Abstract:
A new method is presented for the analysis of coherent mode activity, using tomographic reconstruction of the soft x-ray emissivity of the plasma. The analysis includes the determination of the mode frequency, the poloidal mode number, the radial location, the propagation direction, and the ballooning character. The parameters of a constructed mode are fitted to the line-integrated frequency-filtered soft x-ray signals of the 360 photodiodes installed in the Wendelstein 7-X (W7-X) stellarator. Artificial line-integrated photodiode signals, based on experiment-relevant parameters, are used to explain the workflow of the mode analysis. It is demonstrated that the mode parameters can be successfully reconstructed, even for modes at or near the noise level of the measurements. The method is applied to experimental measurements on W7-X, where strong coherent mode activity has been observed in the frequency space during a phase with a core plasma beta of β0 = 4.8%. The mode analysis indicates an m = 14 ± 2 mode whose characteristics agree with kinetic ballooning modes that can be destabilized at high β.
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