Correlation reflectometry in fusion plasmas—an application at TEXTOR

Abstract

Correlation reflectometry (CR) is based on the simultaneous measurement of two or more spatially separated antennae at one or more frequencies with a high spatial and temporal resolution. It is sensitive to plasma density fluctuations at a reflection layer deduced from reflections of a probing wave with a frequency in the mm range. In fusion plasmas it is an appropriate tool to investigate instabilities based on small scale structures or wave phenomena. Furthermore, it provides the measurement of plasma rotation perpendicular to the local magnetic field under the assumption of frozen turbulence.An overview of CR diagnostics installed at different devices is presented. The different applications principles as measurement of turbulence properties e.g. decorrelation time and correlation length are highlighted. Also the measurement of the pitch angle to deduce the local magnetic field is discussed. Furthermore, the often used application to determine the turbulence rotation is presented.Recent measurements at TEXTOR with two reflectometers and three distributed antennae arrays will be presented. The long range correlations of density and velocity oscillations of the geodesic acoustic mode are investigated. But also properties of the ambient turbulence are studied. Measurements of radial correlation length of the ambient turbulence are performed for two poloidal positions, too. From these measurements the scaling of the radial turbulence correlation length is found to be gyro-Bohm-like.