A broadband light collection system for ECE diagnostics on the FTU tokamak

Electron temperature profiles are currently measured on the FTU tokamak by spectral analysis of electron cyclotron emission. Light pipes are used for collecting the light and transporting it to a Fourier transform spectrometer and to a grating polychromator. Use of a light pipe facing the plasma allows one to obtain a large étendue in spite of the restricted diagnostic access, while a narrow field of view is defined by a quasi optical system placed outside the tokamak vacuum. The reliability of this combination of light pipes and quasi optical elements has been checked in the laboratory and has been confirmed by more than 3 years of routine operation on the FTU tokamak.