Measurements of the Suprathermal Electron Density Profile by Electron Cyclotron Emission at the Upper Cut-Off Layer in Tore Supra

The possibility of measuring the radial profile of fast-electron distributions from ECE spectra is discussed. The most usual ECE diagnostic set-up, i.e. the second-harmonic X-mode, observed by low-held side antennae in the equatorial plane, can be used by exploiting the reflection of spontaneously emitted radiation by the upper cut-off layer and the length of the sightline between the reflecting surface and the receiving antenna. The emitted intensity depends on this length, which introduces an element of spatial resolution in the suprathermal ECE spectra. Two methods are proposed. First, a dynamic method is described in which the length of the sightline is varied by density perturbations. Second, a spectral method is discussed that uses the length variation of the sightline with the observation frequency. A proof of the principle behind both methods is given for a lower hybrid current driven discharge in Tore Supra in which a pellet was injected.