THEORETICAL AND EXPERIMENTAL INVESTIGATION OF THE PHASE-RUNAWAY IN MICROWAVE REFLECTOMETRY

Runaway of the reflectometer output phase can be produced by transvers ely propagating density fluctuations if the antenna system is operated in the tilt-mode, i.e. away from perpendicular incidence. The observe d characteristics of the phase runaway are explained with an analytica l model. The two-dimensional (2D) effects resulting from poloidally pr opagating density fluctuations are calculated using a time-independent full-wave 2D code. Numerical simulations for both monochromatic and t urbulent density fluctuations are presented. Good agreement with the c haracteristics of phase runaway measured with reflectometry in the W7- AS stellarator is obtained. The potential of a reflectometer in the ti lt-mode to measure the radially resolved poloidal propagation velocity of density fluctuations is analysed.