Spatially Localized mm-WVE Backscattering Measurements of High-k turbulence in the DIII-D Tokamak

Summary form only given. A collective Thomson backscattering system has recently been installed on the DIII-D tokamak with the goal of measuring high-k turbulence such as expected from electron temperature gradient driven drift waves. The diagnostic is based on microwave backscattering in X-mode polarization at 96 GHz to probe wavenumbers in the range of ~30-40 cm -1 . The second harmonic cyclotron resonance layer is used as an incident beam dump and to absorb radiation scattered in the forward direction from long wavelength turbulence. This system utilizes a DIII-D steerable ECH antenna for launching and complements a similar system located at the tokamak midplane as well as an array of other turbulence diagnostics. Fluctuation localization, beyond that typically achievable through collective scattering, is accomplished by taking advantage of wavenumber matching criteria. By continuously changing the position along the ray path at which the incident wavevector is aligned to the plane in which turbulent fluctuations are occurring, the radial distribution of ~35 cm -1 turbulence in an ohmic L-mode plasma is obtained. Application of a genetic algorithm to de-convolve the spatial distribution of fluctuations from scattered power measurements using a calculated instrument function will also be discussed and preliminary results presented.