Determination of both the electron density and the poloidal magnetic field in a tokamak plasma from polarimetric measurements of phase only

It is shown that it is possible to measure both the plasma density and the poloidal magnetic field simultaneously by polarimetry, purely from phase measurements. The latter are advantageous compared with previously proposed amplitude measurements. The phase measurements can be performed either by modulating the input polarization (two alternative configurations for this modulation are considered), or by modulating the output polarization. The merits of the three schemes are discussed. The polarimeter required for these measurements is simpler and more robust than the conventional interferometer - polarimeter presently used for the combined density and magnetic field measurements.