Analysis of the polarisation rotation of far infrared beams in the Tore Supra plasmas

On Tore Supra, a far infrared interfero-polarimeter diagnostic has been routinely used for diagnosing the electron and current density by measuring the Faraday rotation angle. As this angle is a few dozen of degrees on present plasma fusion devices, a high precision is needed to correctly reconstruct the current profile. In this article we have analysed the way the electronic cards deal with the noisy signal coming from the detectors, and the consequences on the Faraday angle measurement. We have measured the evolution of this noise in the successive stages of the calculation carried out within the electronic cards, and we discuss of the interest of a numerical treatment to increase the precision of the polarisation measurement.