Improvements of CO2 Laser Heterodyne Imaging Interferometer for Electron Density Profile Measurements on LHD

After installation of CO2 laser (wavelength 10.6 μm) heterodyne imaging interferometer (CO2 HI) in 2001, continuous developments have been carried out to improve the measurements capability and stability of operation. The CO2 HI works almost without phase jumping at high electron density (> 1 × 1020 m-3), where the existing far infrared laser (wavelength 118.9 μm) interferometer suffers from fringe jump due to the reduction of signal intensity caused by refraction. However a second interferometer is required to compensate mechanical vibration. A YAG laser (wavelength 1.06 μm) heterodyne imaging interferometer (YAG HI) is presently used for the vibration compensation. In the 10th LHD experimental campaign (2006?2007), sixty four channels of CO2 HI to measure electron density profile and ten channels of YAG HI to measure mechanical vibration are working. A measurement example of a pellet fuelled high-density discharge is reported

Effects of Resonant Magnetic Perturbation on Particle Transport in LHD

In this study, the effects of resonant magnetic perturbation (RMP) on particle transport are investigated in Large Helical device (LHD). The magnetic configuration is selected to be the outwardly shifted configuration, for which the magnetic axis position (Rax) is 3.9 m. At Rax = 3.9 m, the main plasma is surrounded by a thick ergodic layer, with width of about 30% of the plasma minor radius. The perturbation mode m/n = 1/1, where m and n are poloidal and toroidal mode numbers, is applied. The resonant layer is around the last closed flux surface. With RMP, a region in which both the connection and Kolmogorov lengths are finite and the magnetic field is ergodic forms; this region extends inside the main plasma. In the low-collisionality regime, where νh* 1), a clear difference in particle transport is found. A clear difference in turbulence is also observed, suggesting that turbulence plays a significant role in particle transport in the high-collisionality regime both with and without RMP

Collisionality dependence and ion species effects on heat transport in He and H plasma, and the role of ion scale turbulence in LHD

Surveys of the ion and electron heat transports of neutral beam (NB) heating plasma were carried out by power balance analysis in He and H rich plasma at LHD. Collisionality was scanned by changing density and heating power. The characteristics of the transport vary depending on collisionality. In low collisionality, with low density and high heating power, an ion internal transport barrier (ITB) was formed. The ion heat conductivity (χi) is lower than electron heat conductivity (χe) in the core region at ρ  <  0.7. On the other hand, in high collisionality, with high density and low heating power, χi is higher than χe across the entire range of plasma. These different confinement regimes are associated with different fluctuation characteristics. In ion ITB, fluctuation has a peak at ρ  =  0.7, and in normal confinement, fluctuation has a peak at ρ  =  1.0. The two confinement modes change gradually depending on the collisionality. Scans of concentration ratio between He and H were also performed. The ion confinement improvements were investigated using gyro-Bohm normalization, taking account of the effective mass and charge. The concentration ratio affected the normalized χi only in the edge region (ρ ~ 1.0). This indicates ion species effects vary depending on collisionality. Turbulence was modulated by the fast ion loss instability. The modulation of turbulence is higher in H rich than in He rich plasma

Two-dimensional wave-number spectral analysis techniques for phase contrast imaging turbulence imaging data on large helical device

An analysis method for unfolding the spatially resolved wave-number spectrum and phase velocity from the 2D CO2 laser phase contrast imaging system on the large helical device is described. This is based on the magnetic shear technique which identifies propagation direction from 2D spatial Fourier analysis of images detected by a 6 × 8 detector array. Because the strongest modes have wave-number at the lower end of the instrumental k range, high resolution spectral techniques are necessary to clearly resolve the propagation direction and hence the spatial distribution of fluctuations along the probing laser beam. Multiple-spatial point cross-correlation averaging is applied before calculating the spatial power spectrum. Different methods are compared, and it is found that the maximum entropy method (MEM) gives best results. The possible generation of artifacts from the over-narrowing of spectra are investigated and found not to be a significant problem. The spatial resolution Δρ (normalized radius) around the peak wave-number, for conventional Fourier analysis, is ∼0.5, making physical interpretation difficult, while for MEM, Δρ ∼ 0.1

Particle transport of LHD

Particle confinement processes are studied in detail on the Large Helical Device (LHD). Diffusion coefficients (D) and convection velocities (V) are estimated from density modulation experiments. The magnetic configuration and collisionality are widely scanned in order to investigate parameter dependences of D and V. To study the effect of the magnetic configuration, magnetic axis positions (Rax) are scanned from 3.5 to 3.9 m. This scan changes the magnetic ripples quite significantly, enabling the effects of neoclassical properties on measured values to be widely elucidated. Dependences of electron temperature (Te) and helically trapped normalized collisionality are examined using the heating power scan of neutral beam injection. It was found that generally larger (or smaller) contributions of neoclassical transport in the core region, where normalized position p 0.7) diffusion and convection are dominated by anomalous processes. Measured edge turbulence shows a possible linkage

INTERFEROMETER SYSTEMS ON LHD

This paper describes the interferometer systems on the Large Helical Device (LHD). LHD is equipped with five interferometer systems, each of which has a different operational purpose and measurable electron density range. A single-channel millimeter-wav