Speaker
Adrian von Stechow
Description
See the full Abstract at http://ocs.ciemat.es/EPS2018ABS/pdf/P2.1106.pdf
Phase contrast imaging of turbulent density fluctuations in Wendelstein
7-X
A. von Stechow1 , L.-G. Böttger1,2 , E. Edlund3 , Z. Huang4 , M. Porkolab4 , O. Grulke1,2
and the Wendelstein 7-X team
1 Max-Planck-Institute for Plasma Physics, Greifswald, Germany
2 Technical University of Denmark, Kgs. Lyngby, Denmark
3 SUNY Cortland, Cortland, USA 4 MIT PSFC, Cambridge, USA
The Wendelstein 7-X stellarator (W7-X) is optimized to reduce neoclassical transport. Thus,
turbulent transport is expected to play a significant role in the regulation of particle and heat
fluxes. Numerical simulations in the full W7-X magnetic field geometry indicate a range of in-
stabilities from electron (trapped electron and electron temperature gradient modes, TEM and
ETG) to ion scales (ion temperature gradient modes, ITG) that can contribute to turbulent trans-
port. Their growth rates at different radii are sensitive to the local gradients in plasma parameters
(electron and ion temperature, density), collisionality, ion mass as well as the specific (exter-
nally imposed) magnetic configuration. Full flux surface simulations also show a localization
of fluctuations both toroidally and poloidally, contrary to the toroidally symmetrical outboard
localization observed in Tokamaks.
W7-X has recently completed its second operation phase, the first in full divertor geometry.
Throughout the campaign, a wide range of electron cyclotron heating and fueling scenarios
have been tested, including pellet injection and possible divertor detachment in a range of dif-
ferent magnetic configurations. Of specific interest to turbulence characterization are matched
discharges across different magnetic configurations. TEMs have been shown analytically to be
suppressed in quasiisodynamic configurations (which W7-X can come close to), while ITG sim-
ulations show increased activity at high elongation. Turbulence diagnostics at W7-X include re-
flectometers, correlation ECE systems and the recently completed phase contrast imaging (PCI)
system that measures poloidally resolved electron density fluctuations along a sight line through
the plasma center. The project is a collaboration between the MIT PSFC and IPP.
This contribution aims to identify characteristic features of density fluctuations in W7-X
across its wide range of operating scenarios in both frequency and poloidal k-space, taking
advantage of the 32 poloidal measurement channels that cover a large wavenumber range from
0.5-23 cm−1 . In particular, the existence and modification of density gradient driven TEMs and
ion temperature gradient driven ITGs under changes to both the magnetic configuration and
plasma profiles will be investigated.
