Abstract
Far-infrared laser scattering measurements have been carried out in the STP-3(M) reversed field pinch (RFP) plasma in order to study the nonlinear dynamic behaviour of density fluctuations. Density fluctuations with toroidal, poloidal and radial wave-numbers from 4.4 to 8.8 cm −1 are measured at the central and edge plasma regions. Spectral distributions of the scattered waves show several coherent peaks on the bulk spectrum broadened up to ∼ 500 kHz. Frequencies of these coherent peaks seem to correspond to those of MHD modes, because of the analogy with the spectra of magnetic fluctuations. Experimental results show the following features; first, that density fluctuations having a toroidal wave-number, say, k Φ = 4.4 cm −1, seem to be spatially localized near the toroidal field reversal surface and, secondly, spectral components of coherent modes are nonlinearly coupled with each other to generate higher frequency components. Bispectral analysis of the scattered wave signals has been utilized to investigate the nonlinear coupling phenomena of density fluctuations.
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