EXPERIMENTAL-STUDY OF LOW-FREQUENCY TURBULENCE IN A TOROIDAL PLASMA

Low-frequency turbulence has been studied in a plasma embedded in a pu re toroidal magnetic field. A study of the variation of the spectral c haracteristics of the low-frequency turbulence with the variation in n u(in)/OMEGA(i) has been carried out with two different sets of density and potential profiles. The spectral relation between density and pot ential fluctuations indicates that the Rayleigh-Taylor instability exc ites fluctuations in the long wavelength region, while the Rayleigh-Ta ylor driven drift instability is responsible for the short wavelength regime for nu(in)/OMEGA(i) much less than 1. Spectral characteristics observed in regions where density gradients are parallel to gravity ar e similar to the case where density gradients are antiparallel to the gravity and support the view that the fluctuations generated in the ba d curvature region are transported to the good curvature region. The r esults are in agreement with theoretical predictions, numerical simula tions, in situ measurements, and other laboratory experiments on drift and Rayleigh-Taylor instabilities. For nu(in)/OMEGA(i) greater-than-o r-equal-to 1, the spectral characteristics change and may correspond t o the collisional gradient drift instability or the cross-field instab ility.