On many tokamaks the reconstruction of the magnetic field structure in the plasma is supported by polarimetric measurements. Recent proposed and realized methods are based on a far‐infrared laser beam with a rotating polarization ellipse. The same instrument usually performs as an interferometer measuring the line integrated plasma density. It has been shown that the rotating polarization ellipse disturbs the interferometric measurements. A method based on the principle of a rotating polarization in which the interferometric measurement is unaffected is proposed. Bench test results are presented which show the feasibility of this method.

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