Two-dimensional phase contrast imaging (2D) installed on the large helical device (LHD) is a unique diagnostic for local turbulence measurements. A 10.6 microm infrared CO(2) 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.

A high ion temperature (T i ) was achieved using a combination of perpendicular and parallel injected neutral beams in the Large Helical Device (LHD). Microturbulence spatial profiles in a high-T i discharge were measured by two-dimensional phase contrast imaging (2D-PCI) through almost the entire vertical central chord. The 2D-PCI microturbulence spectral ranges covered wavenumbers (k) of 0.1-1 mm −1 and frequencies ( f ) of 20-500 kHz. The ion thermal conductivity (χ i ) increased in the entire region with increasing T i . However, the difference between the experimental and neoclassical values of χ i became smaller at ρ < 0.5, where ρ is the normalized position, in the high-T i phase. Increasing fluctuation was not observed at this location, suggesting improved ion energy transport in this region. On the other hand, at ρ > 0.5, χ i deviated from the neoclassical value due to enhancement of the experimental χ i and reduction in the neoclassical χ i by a positive radial electric field. Increasing turbulence was observed at ρ = 0.6-0.8, with fluctuations likely propagated to the ion diamagnetic direction in the plasma frame, suggesting that the observed turbulence degrades the ion energy transport at this location in the high-T i phase.

A variety of electron density (n e ) profiles have been observed in the Large Helical Device (LHD). The density profiles change dramatically with heating power and toroidal magnetic field (B t ). The particle transport coefficients, i.e. diffusion coefficient (D) and convection velocity (V ) are experimentally obtained in the standard configuration from density modulation experiments. The values of D and V are estimated separately in the core and edge. The diffusion coefficients are found to be a function of electron temperature (T e ), and vary with B t . Edge diffusion coefficients are proportional to B −0.73±0.23 t . Non-zero V is observed, and it is found that the electron temperature gradient can drive particle convection, particularly in the core region. The convection velocity both in the core and edge reverses direction from inward to outward as the T e gradient increases. However, the toroidal magnetic field also significantly affects the value and direction of V . The density fluctuation profiles are measured by a two-dimensional phase contrast interferometer. It was found that fluctuations which are localized in the edge propagate towards the ion diamagnetic direction in the laboratory frame, while the phase velocity of fluctuations around mid-radius is close to the plasma poloidal E r × B t rotation velocity. The fluctuation level becomes larger as particle flux becomes larger in the edge region.

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