Short wavelength turbulence generated by shear in the QH-mode edge on DIII–D

Abstract

A region of turbulence with large radial wavenumber (k[subscript r]ρ[subscript s] > 1) is found in the high-shear portion of the plasma edge in Quiescent H-mode (QH-mode) on DIII–D using the Phase Contrast Imaging (PCI) diagnostic. At its peak outside the minimum of the Er well, the turbulence exhibits large amplitude [~ over n]/n ~ 40%, with large radial wavenumber ∣[¯over k][subscript r]/[¯ over k][subscript θ]∣ ~ 11 and short radial correlation length L[subscript r]/ρ[subscript i] ~ 0.2. The turbulence inside the E[subscript r] well minimum is characterized by the opposite sign in radial wavenumber from that of turbulence outside the minimum, consistent with the expected effects of velocity shear. The PCI diagnostic provides a line-integrated measurement of density fluctuations, so data are taken during a scan of plasma position at constant parameters to allow the PCI to sample a range in k[subscript r]/k[subscript θ]. Analysis of the Doppler shift and plasma geometry allows the turbulence to be localized to a narrow region 3 mm inside the last closed flux surface, outside the minimum of the E[subscript r] well. The turbulence amplitude and radial wavenumber and correlation length are determined by fitting the PCI results with a simple non-isotropic turbulence model with two regions of turbulence. These PCI observations, made in QH-mode, are qualitatively similar to those made in standard edge localized modes (ELM)-free H-mode and between ELMs, suggesting a similar role for large k[subscript r] turbulence there.