Overview of current density measurements and sawtooth studies on TEXTOR

The analysis of Faraday rotation measurements shows that the safety factor on the magnetic axis stays well below one during the full sawtooth cycle. The current density profile has a unique shape during the sawtoothing plateau phase of ohmic discharges, independent of plasma current and magnetic field, when it is plotted in suitable normalized coordinates. The observation of short lasting spikes on the Faraday rotation signals can be interpreted as the formation of a transient current sheet during the sawtooth crash. Direct measurements of the magnetic amplitude of the internal kink mode with a fast polarimeter channel show that the displacement of the core because of the growing precursor mode stays much smaller than the q = 1 radius just prior to the sawtooth collapse. The versatile neutral beam heating system on Tokamak EXperiment for Technology Oriented Research (TEXTOR) allows the study of the influence of the plasma rotation on the sawtooth period, which has a minimum when the magnetohydrodynamic rotation in the tokamak frame is stopped.