Measurements of fluctuations by reflectometry

Density fluctuations are expected to play an important role in the anomalous transport that degrades confinement in toroidal magnetic confinement devices. So far some interesting information on the characteristics of the edge turbulence have been obtained by Langmuir probes but few diagnostics are able to give information on density fluctuations in the plasma bulk. Due to the small coherence lengths of the fluctuations only diagnostics with high spatial and temporal resolution can be used. In addition some passive methods suffer a lack of sensitivity to the fluctuations due to the poor photon statistics. Reflectometry is a relatively new technique based on the reflection of microwave radiation at the plasma cutoff layer. The phase delay between the launched and the reflected waves gives information on the location and the movements of the reflecting layer. (Author)