The paper deals with JET polarimeter measurements and in particular it presents a study of the Faraday rotation angle, which is used as a constraint in equilibrium codes. This angle can be calculated by means of the rigorous numerical solution of Stokes equations. A detailed comparison of calculations is carried out with the time traces of measurements, inside a limited dataset representative of JET discharges: in general, it is found that the Faraday rotation angle and Cotton–Mouton phase shift measurements can be represented by the numerical solution to Stokes equations. To obtain this agreement in particular for Faraday rotation, a shift of the magnetic surfaces must be included. This results in an improvement of the position of the magnetic surfaces as calculated by the EFIT equilibrium code. The approximated linear models normally used can be applied only at low density and current. The Cotton–Mouton is calculated at high plasma density including the contribution by the Faraday rotation angle. For high plasma current the non-linear terms in the propagation equations can be important. These conclusions have some impact on the mathematical form of the polarimetric constraints (Faraday and Cotton–Mouton) in equilibrium codes.
EAST equilibrium current profile reconstruction using polarimeter-interferometer internal measurement constraints