Phase Runaway Effects on Wendelstein 7-AS

The phase runaway phenomenon has been studied using the W7-AS heterodyne reflectometer. The phase evolution is intermittent, changing rapidly in certain time intervals that show a strong level of amplitude fluctuations. There is a clear dependence of the value of the runaway phase with the local and global plasma parameters. Several authors have proposed that the phase runaway is due to rotation of turbulent structures provided that some asymmetry between these structures and the reflectometer axis exists. The relation between the phase runaway and plasma rotation has been confirmed at W7-AS. The radial profile of the runaway qualitatively corresponds with the plasma poloidal rotation profile. Besides, the observed phase drift can be inverted by inverting the toroidal magnetic field of the stellarator. thus changing the rotation direction of the plasma. This result implies that the neccessary asymmetry is independent on the direction of plasma rotation. 2D WKB simulations of the reflection on a poloidally rotating turbulent plasma for the W7-AS reflectometer and plasma parameters reproduce qualitatively the experimental features (intermittent phase drift, relation between phase and amplitude changes) of the measured phase signal.