Density fluctuations during the sawtooth crash in the Tore Supra tokamak

Using a CO₂ laser scattering experiment, transient density fluctuations related to the sawtooth crash were measured, for about 100 sawteeth, during similar and stationary ohmic discharges of the Tore Supra tokamak. Simultaneously, the evolutions of electron temperature, density and soft x-ray emission were recorded to analyse the plasma behaviour during the crash. In particular, tomographic reconstructions of soft x-ray signals were used to draw emissivity maps and to investigate in detail the sawtooth collapse. Two bursts of density fluctuations, spatially localized within the q = 1 surface, have been identified. A first burst appears together with the fast displacement of the hot core at the beginning of the crash (phase I), and a second stronger burst develops when the central temperature collapses (phase II). The levels and propagation directions of these fluctuations have been systematically compared with the direction of the initial hot core motion. From this analysis, it is deduced that fluctuations propagate from the hot core compression zone along the q = 1 separatrix with a velocity that is about five to 10 times larger than the hot core motion. Levels are strongly correlated to the hot core motion, indicating spatially localized fluctuations. Using these results and a numerical model, it is shown that fluctuations detected during phase I are principally located in the vicinity of the hot core compression zone while those correlated to phase II are in two more central regions, where the temperature collapses.