Edge density profile measurements by X-mode reflectometry on Tore Supra

A broadband reflectometer operating in the frequency range 50-75 GHz has been developed on Tore Supra to measure electron density profiles at the edge. The system uses extraordinary mode polarization and performs routine measurements in 20 µs with heterodyne detection to ensure a high dynamic range sensitivity. It allows separate phase and amplitude information of the signal. The density profiles are fully automatically calculated from the raw phase. The initialization is done with automatic detection of the first cut-off from the amplitude of the reflected signal with an accuracy of up to ±0.5 cm. The profiles are now part of the public database of Tore Supra and can provide details of density structures better than the centimetre range. The high reliability of the measurements for various plasma conditions make this diagnostic an ideal tool to study specific edge plasma physics with given examples on detached plasma behaviour and antenna-plasma coupling processes for ion cyclotron resonant frequency and lower hybrid heating powers. It also is shown how the presence of suprathermal electrons may perturb the measurements.