Beam pattern measurements and comparison with Gaussian optics for the Correlation Electron Cyclotron Emission diagnostic at the ASDEX Upgrade Tokamak

The Correlation Electron Cyclotron Emission (CECE) diagnostic is used to study electron temperature turbulence at the ASDEX Upgrade (AUG) tokamak. The CECE diagnostic utilizes three lenses to focus electron cyclotron emission from the plasma onto an antenna. The optics configuration aims to minimize the beam spot size because the poloidal beam radius determines the minimum detectable turbulence length-scale. In order to gain a better understanding of the CECE poloidal resolution over the diagnostic line of sight, we developed a laboratory setup that mirrored the CECE optics in AUG. To determine how different antennas and antenna positions impact the diagnostic’s resolution, we conducted microwave beam pattern experiments using a fixed 94 GHz microwave source and a detector mounted on a moving track adjustable in all three dimensions. Through these measurements, we observed that beam patterns are well-approximated by Gaussian fits and that different antennas do not have a major impact on beam spot size. These measurements are important for understanding the turbulence measured by CECE, and understanding how CECE measurements can be compared to turbulence simulations.