Tests of a double-detector technique for the FIR polarimetry on Tore Supra

On the Tore Supra (TS) tokamak, a coupled FIR interfero-polarimeter diagnostic has been routinely used for electron density measurements [1-2]. For the more difficult current density evaluation, the used polarimetric technique is classical, with two detectors for each channel. It is a direct method to fully characterize the polarisation of the signal. As it needs amplitude measurements, it is very sensitive to the differential variations of the alignment during plasma, which are induced by moves of mirrors or refraction. To avoid this last inconvenience and thus improve the precision of the measurements, a prototype of a compact double detector has been designed and tested in the diagnostic environment without plasma, then during a TS campaign. The results showed that it was a very easy device to use, but that spurious depolarisations can strongly damage the rotation measurements. As a consequence, the mechanical design of the mounting needs to be reviewed.