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. (author)