Abstract
Recently a new improved confinement regime was found in the Wendelstein 7-AS (W7-AS) stellarator (Renner H. et al 1989 Plasma Phys. Control. Fusion 31 1579). The discovery of this high density high confinement mode (HDH-mode) was facilitated by the installation of divertor modules. In this paper, measurements of short wavelength density fluctuations in the HDH-mode using collective scattering of infrared light are presented. These measurements will be contrasted to fluctuations during normal confinement operation (NC-mode). The autopower spectra of the measurements show a consistent increase of the fluctuation level associated with the transition from NC- to HDH-mode. Correlation calculations on a 20 μs timescale between magnetic and density fluctuations lead to the result that the fluctuations are correlated in NC-but not in HDH-mode. Finally, a comparative analysis between the enhanced Dα H-mode (EDA H-mode) found in the Alcator C-Mod tokamak and the HDH-mode in W7-AS is carried out.
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