A phase contrast interferometer on DIII‐D

A novel imaging diagnostic has recently become operational on the DIII‐D tokamak for the study of density fluctuations at the outer edge of the plasma. The phase contrast imaging approach overcomes the limitations of conventional scattering techniques in the spectral range of interest for transport‐related phenomena, by allowing detection of long‐wavelength modes (up to 7.6 cm) with excellent spatial resolution (5 mm) in the radial direction. Additional motivation for the diagnostic is provided by wave‐plasma interactions during heating and current drive experiments in the ion cyclotron range of frequencies. Density perturbations of 4×107 cm−3 with a 1‐MHz bandwidth can be resolved. The diagnostic employs a 7.6‐cm‐diam CO2 laser beam launched vertically across the plasma edge. An image of the plasma is then created on a 16‐element detector array: the detector signals are directly proportional to the density fluctuations integrated along each chord. Wavelengths and correlation lengths can be inferred from the spatial mapping. The phase contrast method and its application to DIII‐D are described and tests and first plasma data are presented.