By means of far-infrared polarimetry the variation in the internal magnetic field structure during sawtooth activity has been measured in the TEXTOR tokamak with high spatial resolution and improved signal-to-noise ratios. Utilizing the reproducibility of ohmic plasmas as well as the uniformity of sawtooth oscillations under appropriate quasi-stationary conditions, the given number of vertical probing beams has been multiplied by running discharges at different major radii, and the noise levels of the Faraday rotation signals have been reduced by forming a coherent average of many sawtooth periods. In agreement with previous investigations it is found that the safety factor on axis has a mean value q0=0.73+or-0.1 and undergoes a change delta q0 approximately=0.06 during the sawtooth ramp phase. A new observation is the periodic appearance of a poloidal held perturbation in correlation with the sawtooth collapse which is localized at the large major radius side of the q=1 surface. The persistence of the corresponding signal deformations after averaging some hundred sawtooth periods suggests that this perturbation is axisymmetric with principal mode numbers m=1 and n=0. In the presence of an m=n=1 island, it may give rise to magnetic field line stochastization and thus may play an important role in the fast sawtooth crash.
Modification of sawtooth crash behaviour during large error field experiments on the DIII-D tokamak