| dc.description.abstract | With increasing driving, the transition from the ordered state to the weakly disordered state before entering the turbulence state is a universal phenomenon in many nonlinear wave systems such as acoustic, plasma, optical, and dusty plasma systems. Through modulation instability, the waveforms in the weakly disordered states are spatiotemporally modulated, causing the generation of defect filaments with null amplitudes and undefined phases. It also leads to the name defect mediated turbulence (DMT).
In the weakly disordered plane traveling waves such as acoustic and optical waves, defect filaments are winded by helical waves named acoustic vortices (AVs) and optical vortices, respectively. Previous studies in acoustic type waves found that the modulation instability causes waveform undulation and induces sequential rupture and reconnection of adjacent wave crest surfaces. It is the key to generate a pair of AVs with opposite helicities winding around a pair of defect filaments with opposite topological charges. Nevertheless, the transition from the ordered plane wave state to the DMT state with many fluctuating defect filaments and the generic dynamical behaviors of defect filaments, remain unexplored fundamental issues.
In this work, those issues are experimentally addressed in a self-excited dust acoustic wave (DAW) in the dusty plasma system composed of micro-size particles with negatively charged oscillating longitudinally and suspended in a low pressure discharge, by monitoring the spatiotemporal waveform evolution in the 2+1D space-time space.
It is found that, with increasing driving in a small size system, the sequential ruptures of the crest surfaces from the cluster boundary followed by their reconnection with adjacent ruptured crest surfaces, or repelling one of the pairwise generated defects out of the boundary is the key for the single AV generation. The gyration motion of this single AV in 2+1D space-time space modulates the amplitude and phase of neighbor distorted waveforms, causing the emergence of side band peaks at the main peak and its harmonics in the power spectrum. Further increasing driving makes the system enter the state with few short-lived AV and the DMT state with multiple AVs. Gradually increasing defect filament fluctuations and defect number in the transition to the DMT more strongly distort the nearby waveforms. It leads to the transition from the emergence of the distinct side band peaks to the broadened peaks in the power spectra of temporal dust density fluctuation. In the DMT state, two defect filaments with the same (opposite) topological charges repel (attract) each other within a wavelength scale. The local filament curvature in the space is also positively correlated in that scale. Furthermore, the local wave amplitude and phase gradient caused by the wave crest surface stretching due to waveform undulation can be used as an early indicator for the subsequent propagating velocity of the defect. | en_US |