Single Cord far-infrared polarimetry experiment on RFX

This paper summarises the work of the authors in the realisation and testing of a pilot single chord polarimetry experiment on the RFX (reversed field experiment) machine, at the far-infrared wavelength of 118.8 µm. The fundamental purpose of this experiment is to demonstrate the feasibility of a Faraday rotation angle measurement on RFX, with the required resolution, and to clarify all the aspects which may be useful for the final six-chord version of the diagnostic. The apparatus which has been developed is described, with emphasis on some of the modifications which proved essential in order to obtain a satisfactory measurement, in spite of the hostile environment of the machine. Typical results of Faraday angle measurements are presented and discussed, and the reliability of the measurement is demonstrated. In particular, the required +0.2o accuracy is achieved and the results are consistent with magnetic field profiles similar to the ones predicted by the Bessel function model. Suitable design considerations are formulated in order to guarantee the required performance in the six-chord version, even when the machine moves into full regime.