Two-dimensional phase contrast imaging for local turbulence measurements in large helical device (invited)

Two-dimensional phase contrast imaging (2D) installed on the large helical device (LHD) is a unique diagnostic for local turbulence measurements. A 10.6 μm infrared CO2 laser and 6x8 channel HgCdTe 2D detector are used. The length of the scattering volume is larger than plasma size. However, the asymmetry of turbulence structure with respect to the magnetic field and magnetic shear make local turbulence measurements possible. From a 2D image of the integrated fluctuations, the spatial cross-correlation function was estimated using time domain correlation analysis, then, the integrated 2D k-spectrum is obtained using maximum entropy method. The 2D k-spectrum is converted from Cartesian coordinates to cylindrical coordinates. Finally, the angle in cylindrical coordinate is converted to flux surface labels. The fluctuation profile over almost the entire plasma diameter can be obtained at a single moment. The measurable k-region can be varied by adjusting the detection optics. Presently, k=0.1-1.0 mm-1 can be measured which is expected region of ion temperature gradient modes and trapped electron mode in LHD. The spatial resolution is 10%-50% of the minor radius.