Experimental study of low-frequency turbulence in a toroidal plasma

Abstract

Low-frequency turbulence has been studied in a plasma embedded in a pure toroidal magnetic field. A study of the variation of the spectral characteristics of the low-frequency turbulence with the variation in νin/Ωi has been carried out with two different sets of density and potential profiles. The spectral relation between density and potential fluctuations indicates that the Rayleigh–Taylor instability excites fluctuations in the long wavelength region, while the Rayleigh–Taylor driven drift instability is responsible for the short wavelength regime for νin/Ωi≪1. Spectral characteristics observed in regions where density gradients are parallel to gravity are similar to the case where density gradients are antiparallel to the gravity and support the view that the fluctuations generated in the bad curvature region are transported to the good curvature region. The results are in agreement with theoretical predictions, numerical simulations, in situ measurements, and other laboratory experiments on drift and Rayleigh–Taylor instabilities. For νin/Ωi≥1, the spectral characteristics change and may correspond to the collisional gradient drift instability or the cross-field instability.