Turbulence in high-density H-mode plasmas

Nils P. Basse1,2
1 Association EURATOM - Risø National Laboratory, Roskilde, Denmark
2 Ørsted Laboratory, Niels Bohr Institute for Astronomy, Physics and Geophysics, Copenhagen, Denmark

In 2001 a new improved confinement regime was found in the Wendelstein 7-AS (W7-AS) stellarator. The discovery of this high density H-mode (HDH-mode) was facilitated by the installation of divertor modules.

In this talk measurements of short wavelength (20 cm-1) density fluctuations in the HDH-mode using the localised turbulence scattering (LOTUS) diagnostic are presented. These measurements will be contrasted to fluctuations during normal confinement operation (NC-mode).

The measurements show a consistent increase of the fluctuation level associated with the transition from NC- to HDH-mode. There is a very strong similarity between the global parameters of the HDH-mode and the enhanced D-alpha H-mode (EDA H-mode) found in the Alcator C-Mod tokamak. This motivates us to make a comparative analysis of the fluctuation behaviour in these two improved confinement regimes to determine whether the two are identical modes.