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

Abstract

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.

Novel analysis technique for measuring edge density fluctuation profiles with reflectometry in the Large Helical Device

“…This profile agrees roughly with some of the PCI inversions in Ref. 22 (for example, the maximum entropy method), which shows a strong peak right near the edge of the plasma, FIG. 8.…”

“…It must be noted that these two diagnostics measure different turbulent wavenumbers, with reflectometry measuring a specific k up to 15 cm 15 and PCI measuring an integrated k-spectrum from 1 to 9 cm 1 . 22 For this reason, quantitative comparisons of turbulence levels are inappropriate, but qualitative comparisons of the shape and location of the peak of the fluctuation profile are still meaningful. As discussed before, the wide range of profiles that result from different PCI inversion methods motivate an additional method for measuring fluctuation profiles, even if the wavenumber spectrum is slightly different.…”

“…Previous work on LHD using inverted PCI measurements showed that the peaking of the density fluctuation profile depended heavily on the method of inversion [19][20][21][22] and that some inversions led to profiles that peaked much further in radially than is seen in typical tokamak measurements. The PCI diagnostic is also susceptible to systematic errors due to optical misalignment, which may impact the measured location of the fluctuation profile peak.…”

Role of Helium–Hydrogen ratio on energetic interchange mode behaviour and its effect on ion temperature and micro-turbulence in LHD

“…In addition, the symmetry of the turbulence signal changes. This is most likely to be due to a change in the eddy tilt angle, which results in constructive interference on the top and destructive interference on the bottom [34,35]. This is a natural consequence of a change of a strong change in the E × B shearing rate by the bursting mode.…”

Design of tangential viewing phase contrast imaging for turbulence measurements in JT-60SA

“…A CO 2 laser two color interferometer for electron density measurements is planned for the same port section but a different port window with a double pass arrangement. 1, 11 The two CO 2 laser diagnostics do not interfere with each other. Figure 3 shows a schematic view of the detection system as currently envisioned.…”