Temporal sequences in the evolution of fluctuations on LMD-U

We investigate the dynamics of turbulence in linear magnetized plasma confined in LMD-U device. By use of poloidal array of probes and 2D-movable probe, spatio-temporal structure of drift wave fluctuations are studied. Abrupt changes of the density fluctuations are observed, and temporal evolution of Fourier-decomposed elements (both in poloidal direction and in frequency space) at the onset of transition is studied in detail. Primary components that lead the transition are identified. Then, delay times of higher harmonics after the sudden jump of primary components are measured using the conditional averaging and the cross correlation technique. This reveals the coupling between many Fourier components and yields the time for dynamical evolution in cascades. This experiment provides quantitative information of nonlinear interactions.