Phase Contrast Imaging: A Turbulence Diagnostic for the Wendelstein 7-X Stellarator

Lukas-Georg Böttger*

*Corresponding author for this work

Research output: Book/ReportPh.D. thesis

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Abstract

Classical stellarators suffer from strong neoclassical transport. The advanced stellarator Wendelstein 7-X is the first fully neo-classically optimized device posing the question about the role of turbulence in optimized stellarators. While there has been a growing number of theoretical investigations about turbulence in advanced stellarator geometry over the last two decades, the experimental investigation is a completely new field. The present thesis deals with the phase contrast imaging (PCI) diagnostic implementation on Wendelstein 7-X as a tool to address the lack of experimental research on the evolution of turbulent fluctuations in optimized stellarator geometry. Numerical simulations suggest the occurrence of microturbulence in the spatiotemporal range of ion scales such as the ITG and TEM instabilities. The PCI diagnostic allows for non-invasive measurements of line-of-sight integrated density fluctuations. It incorporates a flexible design to cover spatiotemporal scales from the ion down to the electron scale. The PCI uses a CO2 laser emitting light at 10.6µm. At this wavelength the plasma acts as an optical isotropic medium in which density fluctuations present phase objects causing a scattering of the laser. Key element of the PCI diagnostic is the phase plate which allows for a phase shift of the most energetic scattered light components in order to convert the phase modulation into a measurable intensity variation. An excellent alignment onto the phase plate is crucial for the linear diagnostic response on the density fluctuations. An optical beam path length of more than 20m imposes a challenge on this alignment. As a consequence, remote control capabilities are implemented in order to control and correct any misalignment during operation. Turbulent density fluctuations in the ion scale are analysed on the example of a typical electron cyclotron resonance heated discharge in Wendelstein 7-X for a moderate density and different heating steps. The spectral analysis reveals broadband fluctuations and a spectrum showing typical turbulent properties such as indications for selfsimilar fluctuations resulting in a power law scaling of energy. The dominant wave number of kρs ≈ 0.3 lies in the expected range of ion scale turbulence. A constant phase velocity is observed, which indicates a strong Doppler shift due to the plasma E×B-rotation. Indeed, measurements of the plasma E×Brotation are in general agreement with the observed phase velocities for both heating steps. In the context of ITG and TEM as paradigmatic instabilities in the ion scale diamagnetic drift velocities are expected to contribute to the fluctuation propagation. The measurement results are indicative that the observed fluctuation behaviour is predominantly governed by TEM driven turbulence. This interpretation is supported by the observed reduction of turbulence with reduced heating power, which correlates with a reduction of the radial gradient of electron temperature while the density gradient stays almost unchanged.
Original languageEnglish
Place of PublicationLyngby, Denmark
PublisherTechnical University of Denmark
Number of pages131
Publication statusPublished - 2019

Keywords

  • Phase contrast imaging
  • Plasma turbulence
  • Stellarator

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