Density fluctuations at the sawtooth crash in TFTR

Broadband transient density fluctuations at the sawtooth crash, propagating in the electron diamagnetic drift direction, have been observed in the interior of ICRH and low power NBI heated deuterium plasmas in TFTR. Density fluctuations during the sawtooth crash were measured using an X-mode scattering system operating at 60 GHz. Within the spatial and temporal resolution of the scattering system (Δζ ≅ 30 cm for kθ ≅ 7cm-1), transient (100μs) enhanced density fluctuations are modulated with the period of the sawtooth precursor suggesting fluctuations are enhanced in an an extended region around the x-point of the m=1 island. In a number of theoretical studies, it has been suggested that enhanced turbulence may account for the rapid crash times and anomalous heat pulse propagation rate observed on many tokamaks. In a previous study using CO2 laser scattering, enhanced density fluctuations were found to be correlated with the period of sawtooth oscillations (but not with the much faster period of the m=1 precursor), and it was argued that the heat pulse propagation may be enhanced by an increased level of drift wave turbulence. In a recent study on TFTR, it was suggested that Χe need only increase transiently in the core of the plasma to account for the rapid evolution of the heat pulse following the crash. The present observation of transiently enhanced fluctuations in phase with the m=1 precursor is qualitatively consistent with such a model. (author) 5 refs., 4 figs