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10-15 May 2021
Nice, France
Europe/Vienna timezone
The Conference will be held virtually from 10-15 May 2021

Continuous electron temperature tracking via X3 mode of electron cyclotron emission for high-density plasmas in Wendelstein 7-X

13 May 2021, 14:00
4h 45m
Nice, France

Nice, France

Regular Poster Magnetic Fusion Experiments P6 Posters 6


Neha Chaudhary (Max-Planck-Institut für Plasmaphysik, Greifswald)


Wendelstein 7-X (W7-X) stellarator with a major and minor radius of 5.5 m and 0.53 m respectively is a high aspect ratio magnetic confinement device that is optimized for steady-state operations aiming at 30 min of detached plasma confinement. Steady-state heating is performed by up to 7.5 MW continuous-wave electron cyclotron resonance heating (ECRH) 1 provided by 10 gyrotrons and up to 1.7 MW neutral beam injection is available for 10 s duration. Stellarator operation becomes possible at high plasma densities due to the lack of a plasma current related Greenwald limit. In W7-X, the high-density plasma scenarios above the cut-off of the X2 mode of electron cyclotron emission (ECE) have been demonstrated by successively switching gyrotron beams to O2 polarisation for which the cut-off is as high as 2.4 $\times$ 10$^{20}$ m$^{-3}$. Detached O2 ECRH heated high-density plasmas could be controlled up to 28 s which was the technically allowed limit for the uncooled divertor installed in the operational phase 1.2. The absorption of radiation from O2 ECRH is directly proportional to T$_{e}$$^{2}$ and, therefore, continuous measurement of T$_{e}$ is essential to keep track of the absorption of incident power and hence the heating of the plasma.
In W7-X with the central magnetic field of 2.5 T, the classical ECE using optically thick X2 mode around 120-160 GHz is shielded by cut-off for the densities beyond 1.2 $\times$ 10$^{20}$ m$^{-3}$. Therefore X3 mode typically around 180-220 GHz is explored for continuous measurement of T$_{e}$ in high-density plasmas with an upper-density limit at 3.65 $\times$ 10$^{20}$ m$^{-3}$ resulting from the cut-off of X3 emission. The high aspect ratio of W7-X resulting in a flat magnetic field gradient along the line of sight results in spectrally well-separated X2 and X3 emission which facilitates the study. For experiments at W7-X, a Martin Puplett Interferometer (MPI) was used for Fourier transform spectroscopy to probe the broadband ECE in the spectral range 50-500 GHz. The stray radiation [2] from non-absorbed ECRH particularly in O2 heated plasmas is a problem in investigating these ECE harmonics as this radiation lies in the middle of emission from X2 mode at 140 GHz. A multimode stray radiation notch filter [3] based on multiple dielectric disk structure was constructed to get an unpolluted ECE spectrum.
The two ECE diagnostics at W7-X consist of the heterodyne radiometer [4] for measurement of classical ECE and MPI for measurement of broadband ECE and they share the same line of sight which is perpendicular to the applied magnetic field to reduce the Doppler broadening of spectral emission lines. This also provides the opportunity to validate the results from MPI in the spectral range of X2 mode by comparison with radiometer measurements. The absolute calibration of MPI was done with a blackbody source emitting at a maximum temperature of 600°C. The experimentally measured broadband ECESpectra covering X2 and X3 mode shows that the X2 emission used for classical ECE becomes hidden by cut-off with increasing plasma density. However, the X3 emission first increases with density indicating that the emission is still optically thin. And for densities exceeding approximately 1 $\times$ 10$^{20}$ m$^{-3}$, this dependency on density vanishes indicating optical thickness such that a local temperature may be derived.
The comparison of X3 emission with Thomson scattering diagnostic shows that for T$_{e}$ ≥ 3 keV the emission from X3 follows the electron temperature indicating that X3 emission is optically thick and represents the core T$_{e}$. These experimental results are supported by the radiation transport calculation done with ray-tracing code TRAVIS [5]. Additionally in order to quantify the X3 mode of ECE with respect to T$_{e}$ profile information a forward modeling of the Fourier transform spectrometer measurement process has been implemented. The modeling and profile inference have been done in the Bayesian Minerva modeling framework [6].


1 Wolf, R. C., Bozhenkov, S., Dinklage, A., Fuchert, G., Kazakov, Y. O., Laqua, H. P., et al. (2019). Electron-cyclotron-resonance heating in Wendelstein 7-X: A versatile heating and current-drive method and a tool for in-depth physics studies. Plasma Physics and Controlled Fusion, 61: 014037.
[2] Oosterbeek, J. W., Chaudhary, N., Hirsch, M., Beurskens, M. N. A., Höfel, U., Card, A., et al. (2019). Michelson Interferometer design in ECW heated plasmas and initial results. Fusion Engineering and Design, 146, Pt. A, 959-962.
[3] Chaudhary, N., Oosterbeek, J. W., Hirsch, M., Höfel, U., Wolf, R. C., & W7-X Team, Max Planck Institute for Plasma Physics, Max Planck Society (2019). Investigation of Optically Grey Electron Cyclotron Harmonics in Wendelstein 7-X. EPJ Web of Conferences, 203: 03005.
[4] Hirsch, M., Höfel, U., Oosterbeek, J. W., Chaudhary, N., Geiger, J., Hartfuss, H.-J., et al. (2019). ECE Diagnostic for the initial operation of Wendelstein 7-X. EPJ Web of Conferences, 203: 03007.
[5] Marushchenko, N. B., Turkin, Y., & Maaßberg, H. (2014). Ray-tracing code TRAVIS for ECR heating, EC current drive, and ECE diagnostic. Computer Physics Communications, 185(1), 165-176.
[6] Höfel, U., Hirsch, M., Kwak, S., Pavone, A., Svensson, J., Stange, T., et al. (2019). Bayesian modeling of microwave radiometer calibration on the example of the Wendelstein 7-X electron cyclotron emission diagnostic. Review of Scientific Instruments, 90: 043502.

Affiliation Max-Planck-Institut für Plasmaphysik, Greifswald
Country or International Organization Germany

Primary author

Neha Chaudhary (Max-Planck-Institut für Plasmaphysik, Greifswald)


Matthias Hirsch (Max-Planck-Institut für Plasmaphysik) Dr Johan Oosterbeek (Max-Planck-Institut für Plasmaphysik, Greifswald) Dr Udo Hoefel (Max-Planck Institut für Plasmaphysik) Dr Nikolai Marushchenko (Max-Planck Institut für Plasmaphysik) Dr Jakob Svensson (Max-Planck-Institut für Plasmaphysik) Robert Wolf (Max-Planck-Institute for Plasma Physics) W7-X Team

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