BISPECTRAL ANALYSIS OF SCATTERED WAVE SIGNALS FROM DENSITY-FLUCTUATIONS IN THE REVERSED-FIELD PINCH PLASMA

Far-infrared laser scattering measurements have been carried out in th e STP-3(M) reversed field pinch (RFP) plasma in order to study the non linear 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 distri butions of the scattered waves show several coherent peaks on the bulk spectrum broadened up to similar to 500 kHz. Frequencies of these coh erent peaks seem to correspond to those of MHD modes, because of the a nalogy with the spectra of magnetic fluctuations. Experimental results show the following features; first, that density fluctuations having a toroidal wave-number, say, k(phi) = 4.4 cm(-1), seem to be spatially localized near the toroidal field reversal surface and, secondly, spe ctral components of coherent modes are nonlinearly coupled with each o ther to generate higher frequency components. Bispectral analysis of t he scattered wave signals has been;utilized to investigate the nonline ar coupling phenomena of density fluctuations. (C) 1996 Published by E lsevier Science Ltd.