| 摘要: | 多體複雜系統受局部強擾而產生似疏密波之集體激發結構，為一有趣且重要之非線性動力學議題，如石子投入湖中激起湖面漣漪或脈衝雷射聚焦於水中產生氣泡皆為典型例子。當集體激發結構尺度約略為介觀(僅幾個粒子)尺度時，多體系統已不可再被視為連續體，而空間中粒子密度漲落之異質性將影響集體激發結構之形成。然因受限於實驗觀察尺度，我們無從得知介觀集體激發結構與組成粒子間之交互作用及微動力行為。 微粒電漿系統是由一群帶負電的微米尺度粒子懸浮於低壓弱游離電漿中所構成，透過光學顯微鏡可直接觀測並追蹤粒子軌跡，探究粒子之微動力行為。因粒子間強大庫倫排斥力彼此相互強耦合，微粒電漿系統可展現微粒電漿晶格、微粒電漿液體及微粒電漿汽體等狀態。在本論文中，藉由聚焦脈衝式雷射於介觀微粒電漿液體，產生微粒電漿微泡，並利用高速攝影機追蹤粒子軌跡，探討上述集體激發結構如何形成。 我們發現電漿微泡受系統上下對稱破缺影響，展現不對稱微泡膨脹及收縮。電漿微泡收縮所引發之尾跡場乃是由一向下超音速傳播之空乏波及一系列向下傳播之阻尼震盪疏密波所構成。透過追蹤粒子軌跡，我們建構一清楚「尤拉—拉格朗日」圖像描述粒子與尾跡波之相互作用及微動力行為。我們並探討在不同壓力及不同微泡大小所引發不同尾跡波之異同，更探討兩個微粒電漿微泡間複雜交互作用。最後，我們介紹一有趣現象，說明一活躍粒子墜入電漿微泡尾跡場，可增益原阻尼震盪尾跡波。 Subjected to a strong localized perturbation in an open-dissipative nonlinear extended media, the formation of the acoustic type nonlinear coherent localized structures, such as bubbles, cavities, and solitons, is an interesting nonlinear dynamical problem in our daily life. Down to the discrete level, however, the local heterogeneity of density fluctuations becomes prominent and seriously affects the formation and the dynamics of the coherent localized structures, especially when the spatial scale of the coherent localized structures is in only few inter-particle distances. How the localized collective excitations interact with the particle motions and how the particle motions in turn affect the formation of the localized collective excitations are important issues and poorly understood due to the difficulty of direct observation at discrete level. Dusty plasma liquid formed by micrometer sized dust particles negatively charged and suspended in a low-pressure weakly ionized discharge background is an inspiring system to mimic and understand the generic dynamical behaviors of the acoustic type nonlinear coherent localized structures in nature at the discrete level because of the capability of direct optical visualization. The nonlinear coherent structures excited by dust density perturbation in dusty plasma systems have been extensively studied previously, but rare studies have been done on the spatiotemporal responses of a dusty plasma liquid subjected to a spatially and temporally localized strong density perturbation. Our previous study done by Chu et al. in 2003 firstly demonstrates the formation of various kinds of nonlinear collective excited localized structures (soliton-like dusty plasma bubbles) in 3D dusty plasma liquid subjected to a spatially and temporally localized strong density perturbation via an intense focused pulsed laser. However, the detailed micro-dynamics of the dust particles constituting the soliton-like bubble cannot be well resolved at that time owing to the sampling limit of the slow CCD. In this work, using a high speed CCD, we experimentally study the detailed dust micro-dynamics of the dusty plasma bubble generated by an intense focused pulsed laser in the 3D dusty plasma liquid. Under the symmetry breaking by the downward ion flow, asymmetric bubble expansion and collapse, and a damped solitary wake field composed of a leading ultrasonic downward rarefaction wave trailed by a series of compressional dense crests with descending crest speeds and crest heights are observed. The dust density waves propagating not along the downward ion flow directions are quickly damped, leading to the formation of a narrow-width wake field. The micro-dynamics of dust particles are well resolved by directly tracking the trajectories for each dust. A clear Eulerian/Lagrangian picture is then constructed to correlate the dust motions and the associated dust density fluctuations during the bubble expansion and collapse, and in the later damped solitary wake field. The mutual support of the dust oscillations and the downward compressional crests is found. In addition, the spatiotemporal responses of a single plasma bubble with different bubble sizes and under different background pressures are also investigated to study the effect of different strengths of external perturbations and the effect of different excitabilities of the 3D dusty plasma liquid respectively. Furthermore, the interaction between two vertically aligned plasma bubbles is studied to understand how nonlinear collective excitations of each bubble interfere with each other. Finally, an interesting phenomenon regarding the falling energetic particle enhanced dust acoustic wave in the damped solitary wake field of a plasma bubble is briefly reported. |
