Speaker
Lukas-Georg Böttger
Description
See the full Abstract at http://ocs.ciemat.es/EPS2018ABS/pdf/P2.1109.pdf
Experimental Investigation of Turbulence in the Wendelstein 7-X
Stellarator with Phase Contrast Imaging
L.-G. Böttger1,2 , O. Grulke1,2 , A. von Stechow1 , J. Alcusón1 , P. Xanthopoulos1 ,
E. Edlund3 , Z. Huang4 , M. Porkolab4 and the W7-X Team1
1 Max-Planck Institute for Plasma Physics, Greifswald, Germany
2 Technical University of Denmark, Kgs. Lyngby, Denmark
3 SUNY Cortland, Cortland, USA
4 MIT Plasma Science and Fusion Center, Cambridge, MA, USA
In the recent experimental campaign OP 1.2a of Wendelstein 7-X (W7-X), currently the world’s
largest optimized stellarator with a plasma volume of 30 m3 , high-performance discharges with
a maximum stored energy of 1MJ were achieved. As one of the optimization criteria of W7-X
was the reduction of neoclassical transport, turbulent transport mechanisms are believed to play
a much more important role now. Numerical gyrokinetic simulations suggest a significantly dif-
ferent appearance of turbulence in stellarators than in tokamaks. However, a systematic experi-
mental investigation of turbulence in optimized stellarators has not been done, yet. To address
this topic the phase contrast imaging (PCI) diagnostic was installed at W7-X and successfully
put into operation in the recent experimental campaign OP 1.2a. The PCI diagnostic allows for
non-invasive spatiotemporal measurements of electron density fluctuations. It is sensitive to ion
temperature gradient turbulence and trapped electron modes – in the hot core up to the colder
edge.
Gyrokinetic simulations have shown that density fluctuations depend strongly on geometrical
effects. One particular aspect is the influence of the elongation or the correlated inverse safety
factor iota on the development of ion temperature driven turbulence [1]. In W7-X different mag-
netic field configurations were used which allow for an experimental comparison. Since W7-X
uses as main heating source electron cyclotron resonance heating, the ion-temperature is influ-
enced by the coupling between ions and electrons which depends on the plasma density. With
this in mind ion temperature gradient driven turbulence is expected to be dependent on the ex-
perimentally accessible plasma density. In order to gain insights in the underlying mechanisms,
the experimental analysis is accompanied by comparisons to data obtained from GENE. This
gyrokinetic code allows for investigating numerically ion temperature gradient driven instabili-
ties based on actual experimental configurations and profiles.
References
[1] P. Xanthopoulos et al., Phys. Rev. Lett. 113, 155001 (2014)
