STUDY OF DENSITY-FLUCTUATIONS DURING MHD ACTIVITY, SOFT LANDING DISCHARGES AND MAJOR DISRUPTIONS IN TEXTOR USING CO2-LASER COLLECTIVE SCATTERING

A modulation of microturbulence is observed in TEXTOR during low mode number MHD activity using CO//2 laser collective scattering. This is accompanied by a strong enhancement of density fluctuations near ka//s approximately equals 3 at the end of soft landing discharges and a displacement of the frequency spectrum towards lower frequencies. The increase is most significant for rapid rampdown of the plasma current accompanied by strong MHD activity but also occurs when the latter is not detected. The evolution of microturbulence is also studied during major plasma disruptions. It was found that disruptions without MHD precursor oscillations are characterized by a rapid increase in the density fluctuations starting approximately 100 ms before plasma disruption.