AbstractThe TEXTOR tokarnak is equipped with a routinely operated nine-channel HCN interferometer-polarimeter. The line-integrated electron densities are measured with a time resolution of up to 50 μs. The Faraday rotation angles are detected using a lock-in technique with a time response of over 1 ms. The application of inversion algorithms yields the local electron density and the distribution of the poloidal magnetic field. From the distribution of this magnetic field the profiles of the safety factor and the current density are calculated. The current profile during stationary ohmic discharges exhibits a characteristically unique shape. The central q value in ohmic discharges remains below 1 during the sawtooth period. The interferometric phase shifts and the Faraday rotation angles measured in real time are applied for density and position feedback. A recent extension of the instrument with a fast polarimeter that probes the plasma along a horizontal chord in the equatorial plane with a time resolution of about 100 μs allows the investigation of the amplitude and growth rate of the m = n = 1 internal magnetic mode.