Far infrared polarimetry for poloidal field determination in Tokamak plasma

The poloidal magnetic field distribution in a Tokamak plasma, determined on the basis of the Faraday effect, was investigated numerically and experimentally. Calculations based on appropriate plasma models, on the Appleton-Martree formula for the refractive index, and on the Poincare representation of the state of polarization yield optimum measurement wavelengths for actual Tokamak plasmas in the range 200 to 400 microns. Numerical simulations show the feasibility of the technique in terms of the experimental precision with which the Faraday profile and electron density distribution must be determined, considering also the propagation of errors in the evaluation procedure and the likely effect of disturbances. Using an HCN laser (lambda = 337 microns) a magnetooptical ferrite modulation technique was developed and tested which achieves the necessary precision and temporal resolution. This was verified by a single channel experiment on the French TFR Tokamak, where for the first time the Faraday rotation induced by the poloidal field was measured.