Low- and high-mode separation of short wavelength turbulence in dithering Wendelstein 7-AS plasmas

In this article measurements of small scale electron density fluctuations in dithering high confinement (H)-mode plasmas obtained by collective scattering of infrared light are presented. A scan of the fluctuation wavenumber was made in a series of similar discharges in the Wendelstein 7-AS (W7-AS) stellarator [H. Renner , Plasma Phys. Control. Fusion 31, 1579 (1989)]. The experimental setup and discharge properties are described. H-alpha-light observing an inner limiter was used to separate low confinement (L)- and H-mode phases of the plasma; the separated density fluctuations are characterized. It was found that L- (H-) mode fluctuations dominate at high (low) frequencies, respectively, and that they possess well-defined and distinguishable scaling properties. Wavenumber spectra for L- and H-mode measurements are calculated and fitted by power-laws and exponential functions. The separated measurements can be fitted with the same exponents in L- and H-mode. Correlations between the density fluctuations, the H-alpha-signal and magnetic fluctuations as measured by Mirnov coils were analyzed. Correlation calculations using 50 ms time windows (several dithering periods) with time lag steps of 100 mus showed that all the fluctuating quantities are highly correlated and that the maximum correlation occurs for high frequency density fluctuations. Performing separate L- and H-mode correlations on a 20 mus time scale between magnetic and density fluctuations leads to the result that the minimum correlation time scale in L-mode is of order 100 mus, while no correlation exists for H-mode. (C) 2002 American Institute of Physics.