Measurement of current‐density changes during sawtooth activity in a tokamak by far‐infrared polarimetry (invited)

Simultaneous measurements of plasma‐induced phase shifts and Faraday rotation angles along nine vertical chords by means of a far‐infrared interferometer/polarimeter have been used for some time to determine the electron density profile and the poloidal magnetic field distribution in the TEXTOR tokamak. While the sensitivity of the interferometric subsystem is sufficient to clearly detect sawtooth oscillations of the density profile, the noise level of the polarimetric signals usually obscures the appearance of sawteeth. However, owing to the high reproducibility of the sawtooth phenomenon under certain quasistationary discharge conditions, coherent‐signal‐averaging techniques can be applied to extract sawtooth‐correlated modulations of the Faraday rotation angles. After correction for density‐induced changes, these data represent the variation of the poloidal field pattern during a sawtooth cycle. Their analysis yields a periodic shift of the magnetic axis by about 1 mm, in good agreement with equilibrium calculations. For the axial current density a modulation amplitude of about 8% at an average central safety factor q 0≂0.77 is found.