Millimeter-Wave Beam Scattering by Edge Turbulence in Magnetically-Confined Plasmas

High-power millimeter-waves (mmw) in the electron cyclotron range of frequencies are extensively used in fusion devices. They have become essential for a number of applications ranging from plasma heating and current drive to core confinement preservation. In ITER, 24 MW of power are planned to be injected through the equatorial launcher to perform bulk plasma heating and current drive; and through the upper-launcher, mainly to stabilize neo-classical tearing mode (NTM) and prevent plasma disruption. The narrowness of the power deposition is a key feature of NTM stabilization and thus a key requirement for the upper-launcher beam in ITER. Recent calculations have estimated that, because of the long path of the upper-launcher beam in ITER, electron density fluctuations associated with the turbulence in the edge of the plasma could be responsible for a broadening of the mmw-beam, possibly deteriorating NTM stabilization efficiency. In this thesis we investigate mmw-scattering by plasma turbulence experimentally and numerically.

In the first part of this thesis, millimeter-wave scattering by plasma turbulence is investigated in the simple magnetized toroidal configuration, which turbulence is known to exhibit universal properties of the turbulence present at the edge of tokamaks. Scattering experiments run on both the TORPEX device and the Tokamak à Configuration variable (TCV) reveal that field-aligned elongated structures of enhanced electron density (blobs) are responsible for fluctuations of the transmitted mmw-power. Using conditional sampling on Langmuir probe measurements, we show that blobs can either increase the locally measured mmw-power or decrease it, depending on their location. In the case of the TORPEX experiments, experimental 2D-time imaging of the electron density from the HEXTIP-U array of Langmuir probes is used to run full-wave simulations of the mmw-beam propagation. The full-wave simulations are found in agreement with the experiments and reveal that blobs have a defocusing effect on the mmw-beam in their wake, leading to local changes in the mmw-power.

In the second part of this thesis, mmw-wave scattering is investigated in L-mode confined TCV plasmas. A transmission diagnostic is installed to measured the mmw-power from the X3 beam reaching the floor of the vessel after propagation in the plasma. A set of wall-embedded Langmuir probes located in the inner-wall of the TCV vessel is used to perform conditional sampling on the detected mmw-power signal and identify the effect of blobs present in the scrape-off layer on the mmw-transmission. Similar results to those found in TORPEX are observed: Blobs located in the upper-part of the scrape-off layer are responsible for changes in the transmitted power which can be either positive or negative depending on the blob location. The effect of a blob on the mmw-transmission is investigated numerically. Conditional sampling is performed on the turbulence data from GBS to reconstruct the time-resolved 2D evolution of the electron density in the scrape-off layer associated with the propagation of a blob in TCV. The time-evolution of the detected mmw-power associated with the blob propagation from the full-wave simulations is successfully compared to the experiment. The numerical simulations show that negative fluctuations of the electron density preceding the blob focus the mmw-beam and the blob defocuses the mmw-beam, resulting in partial variations of the mmw-power.

Finally, a combination of simulations run with WKBeam and experimental measurements demonstrate that the mmw-beam is 50% broader in TCV due to the edge turbulence than the predicted mmw-beam after propagation in a non-turbulent plasma.