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Measurement of Nonlinear Mode Couplings in the Large Mirror Device-Upgrade

Experimental study of plasma turbulence has been proving theoretical predictions by using multi-channel diagnostics and analyzing techniques such as bi-spectral analysis. We performed a fluctuation measurement of the ion saturation-current with a 64-channel poloidal Langmuir probe array in the Large Mirror Device-Upgrade. A number of fluctuation peaks in the poloidal wave number–frequency space were observed, and they satisfied the matching conditions of wave number and frequency with each other. The nonlinear mode couplings among the fluctuation peaks were confirmed by two-dimensional bi-coherence analysis. By amplitude correlation technique, three original parent modes were found. All the other fluctuation peaks were quasi-modes, which were successively generated by mode couplings from the three original parent modes.