Far‐infrared heterodyne scattering to study density fluctuations on the DIII‐D tokamak

A far‐infrared collective scattering system has been installed and is operational on the DIII‐D tokamak. The system consists of a 245 GHz (λ= 1.22 mm) twin frequency laser and uses heterodyne detection techniques which enables us to determine the propagation direction of fluctuations in the laboratory frame of reference. At present, three receiver channels are available to study poloidal propagating fluctuations along the entire midplane of DIII‐D in a wave number range of 2≤ k ≤16 cm−1. The beam geometry of the system dictates a wave number resolution of Δk ≊ 0.7 cm−1 and a radial spatial resolution of ± 20 cm at k=10 cm−1. Fluctuation spectra and propagation direction measured during Ohmic operation were found to depend on plasma parameters. Large plasma toroidal rotation was measured which correlated with modifications in the fluctuation spectra. During H‐mode operation, a dramatic drop of low‐frequency fluctuations (< 250 kHz) is observed coincident with the drop in D‐alpha light. This is followed by ...