Polarimetric measurement ofm= 1 sawtooth precursor oscillations in the TEXTOR tokamak

The HCN-interferometer/polarimeter on TEXTOR has been extended by a horizontal line of sight in the equatorial plane. The achieved signal to noise ratio and the time resolution of this polarimeter channel are sufficient to resolve the m = 1 magnetic precursor oscillation before the sawtooth collapse. The growth rate of the precursor oscillation in a sawtoothing ohmically heated discharge has been determined to be . The magnetic perturbation, determined from the Faraday rotation integral, assuming a kink-mode eigenfunction, is 13 mT just before the sawtooth crash and the `displacement' of the core is about 0.9 cm or . The amplitude of a stationary m = 1 mode observed in an additionally heated discharge with balanced neutral beam injection amounts to 26 mT.