DENSITY, TEMPERATURE, AND POTENTIAL FLUCTUATION MEASUREMENTS BY THE SWEPT LANGMUIR PROBE TECHNIQUE IN WENDELSTEIN 7-AS

In the Wendelstein 7-AS stellarator (W7-AS) [Plasma Phys. Controlled F usion 33, 1591 (1991)], current-voltage characteristics of the Langmui r probe at sweep frequencies in the range 400 kHz to 1 MHz were measur ed and it was found that the mean and fluctuation values of the ion sa turation current, floating potential, and electron temperature were in dependent of the sweep frequency. A radial scan in the vicinity of the velocity shear layer was performed. The simultaneous sweeping of 3 pr obe tips showed a statistically significant spatial coherence of the f luctuations in the poloidal direction and a decrease in spatial cohere nce of the fluctuations with increasing tip separation could be demons trated. The observation of a change in the propagation direction of fl uctuations as the shear layer was crossed and a calculation of the tra nsport spectrum show that the swept probe method is capable of reprodu cing known results. Apparent temperature fluctuations, due to variatio ns of density and potential during a sweep, are shown by simulations t o be only of importance at frequencies above half the Nyquist frequenc y.