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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. The 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. It is found that the safety factor on axis has a mean value of 0.73 +/- 0.1 and undergoes a change of about 0.06 during the sawtooth ramp phase. A new observation is the periodic appearance of a poloidal field perturbation in correlation with the sawtooth collapse which is localised 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 stochastisation and thus may play an important role in the fast sawtooth crash.

Additional information

Authors: SOLTWISCH H, Forschungszentrum Jülich GmbH (KFA) (DE);KOSLOWSKI H R, Forschungszentrum Jülich GmbH (KFA) (DE)
Bibliographic Reference: Article: Plasma Physics and Controlled Fusion, Vol. 37 (1995) pp. 667-678
Record Number: 199510884 / Last updated on: 1995-08-10
Original language: en
Available languages: en