Phase contrast imaging interferometer for edge density fluctuation measurements on LHD

Abstract

A phase contrast interferometer employing a CO2 laser (wavelength λi=10.6 μm) is designed and installed in order to study density fluctuations on a large helical device. A 250×50 mm slab beam passes the edge of the plasma, where ρ=r/a>0.65, and provides observations of edge density fluctuations. A spatial image of the integrated fluctuations is measured by a multichannel detector array, of which the effective spacing is 5 mm in the plasma. Measured wave number components are dominated by radial components, and within the range of 7.2×10−2⩽k⩽0.63 mm−1 and within the frequency range of 5–125 kHz. A clear difference in the fluctuation levels and peak wave numbers are observed between different discharges, which differ in the energy confinement.