Measurement of nonlinear mode coupling of tearing fluctuations

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Abstract
Three-wave nonlinear coupling of spatial Fourier modes is measured in the MST reversed field pinch by applying bispectral analysis to magnetic fluctuations measured at the plasma edge at 64 toroidal locations and 16 poloidal locations, permitting observation of coupling over 8 polodial modes and 32 toroidal modes. Comparison to bispectra predicted by resistive MHD computation indicates reasonably good agreement. However, during the crash phase of the sawtooth oscillation the nonlinear coupling is strongly enhanced, concomitant with a broadened (presumably nonlinearly generated) k spectrum.