Speaker
Gavin McCabe Weir
Description
See the full Abstract at http://ocs.ciemat.es/EPS2018ABS/pdf/P2.1107.pdf
A comparison of measurements from radial and poloidal correlation ECE
diagnostics on Wendelstein 7-X
G.M. Weir1 , T. Windisch1 , M. Hirsch1 , A. Card1 , O. Grulke1 , H.-J. Hartfuss1 , U. Hoefel1 ,
J.H.E. Proll2 , T. Schroeder1 , J. Smoniewski3 , T. Klinger1 and the W7-X Team1
1 Max-Planck Institute for Plasma Physics, Greifswald, Germany
2 Eindhoven University of Technology, Eindhoven, The Netherlands
3 HSX Plasma Laboratory, University of Wisconsin-Madison, Madison, WI. USA
The stellarator Wendelstein 7-X (W7-X) is designed to produce an approximately quasi-
omnigeneous magnetic configuration that is optimized to have low neoclassical transport under
steady state conditions relevant for a fusion reactor [1]. Electron temperature (Te ) fluctuations
driven by drift wave turbulence are below the sensitivity of a single Electron Cyclotron Emis-
sion (ECE) diagnostic, but they can be measured by correlating the signal between two inde-
pendent ECE measurements [2]. A two-sightline poloidal correlation ECE (CECE) diagnostic
has been developed and installed on W7-X that operates through spatial decorrelation of the
electron cyclotron emission. The poloidal CECE diagnostic is sensitive to long wavelength Te
fluctuations that are relevant for the study of Ion Temperature Gradient and Trapped Electron
Mode turbulence. Similarly, the ZOOM device [3] that is installed on W7-X functions as a ra-
dial CECE diagnostic through spectral decorrelation of the electron cyclotron emission. Over
a 1s long plasma discharge, both radiometers are sensitive to Te fluctuations of less than 0.3%.
The poloidal CECE has 8 fixed-frequency channels per sightline covering 10 cm on the high-
field side of the magnetic axis, while the radial CECE has 16 variable-frequency channels over
6 cm that can be shifted across the plasma minor radius. These correlation radiometers have
been used to measure the magnitude of Te fluctuations as well as the radial correlation length in
plasma with electron cyclotron resonant heating. The ECE radiation spectrum is measured to be
asymmetric across the magnetic axis of W7-X, and relativistically downshifted emission affects
measurements on the low-field side of the magnetic axis. In this contribution, Te fluctuation
measurements from the two radiometers will be presented and compared to gyrokinetic calcu-
lations of Trapped Electron Mode turbulence in W7-X, and the impact of relativistic downshift
on Te fluctuation measurements will be discussed.
References
[1] T. Klinger et al. Plasma Phys. Control. Fusion 59 014018 (2017).
[2] C. Watts, H.J. Hartfuss, and M. Häse, Rev. Sci. Instrum. 75 3177 (2004).
[3] Ch. Fuchs and H.J. Hartfuss, Rev. Sci. Instrum. 72 383 (2001).
