Geometrical Optics Interpretation of the Phase Runaway in Microwave Reflectometry

In this article we present experimental results obtained with a heterodyne/amplitude modulated (AM) reflectometer at W7-AS and simulations [two-dimensional (2D) (WKB)] of reflection on a rotating cutoff layer corrugated by turbulence for the W7-AS conditions. When the turbulence has a large amplitude and a small size relative to the illuminated spot, the calculations show the existence of a phase runaway and reproduce the typical behavior of the measured phase and amplitude signals. It is not possible to deduce the plasma rotation from the phase runaway value at “close to normal” incidence, as this value depends on the turbulence characteristics. When the illuminated cutoff surface is smooth, no runaway occurs, amplitude and phase signals are coherent and in certain poloidal wavelength ranges (∼1 to 2 cm for W7-AS conditions) their relative phase is π/2, conditions that have been observed experimentally during H mode. The phase runaway can affect the ne(r) measurements. However, the comparison between the...