Methods for the estimate of the magnetic axis, the last closed flux surface and the differential shift in an RFP plasma

We present a preliminary investigation of the plasma column displacement in the RFX reversed field pinch (RFP) experiment, carried out with different diagnostics. Determinations of the plasma position have been obtained by a set of external pick-up coils, a multichannel far infrared (FIR) polarimeter and a soft x-ray (SXR) tomographic camera. These diagnostics provide complementary information about the plasma position. In particular, the location of the polarimetric chords allows a direct estimate of the horizontal magnetic axis shift. The analysis of the isoemissivity SXR surfaces also allow investigations of the differential shift of the various magnetic surfaces. A comparison of the results of these three diagnostics is reported, and their agreement is found to be satisfactory and consistent with our expectations.