| dc.description.abstract | The nonlinear growth and breaking of waves in nature are fundamental phenomena. For example, the waveform of growing waves driven by the strong wind on the water surface will steepens and breaks finally. These nonlinear phenomena not only occur in transverse waves but also in longitudinal waves. In plasmas, the electron plasma wave generated by a pulse laser grows nonlinearly as it propagates from higher plasma density region to low plasma density region. As the electron plasma wave breaks, higher order harmonics, X-ray emission and fast electrons are generated. Therefore, the wave breaking which is applied to accelerate electrons by laser wake field in plasmas is an important issue. However, to our knowledge, the experimental study of wave breaking in microscopic discrete level is not conducted. Namely, the micro-origin that causes the changing of growing waves to breaking waves is still a mystery.
The dust acoustic wave propagates in dusty plasmas composed of negatively charged micro-meter dust particles can be observed directly through a high speed CCD camera (sampling rate = 500 Hz). By tracking the motion of dusts which oscillate in the dust acoustic wave field, the particle-wave dynamics and the micro-origin of wave breaking are investigated. In the experiment, the dust particles introduced to the plasma are charged negatively due to the higher electron mobility. The dust particles suspended in the vicinity of the bottom electrode are confined by the electric field of a hollow cylindrical glass cell. The floating dust particles driven by the downward ion flow line up vertically and form a dust Coulomb liquid. The ion stream also provides a free energy source to spontaneously excite the dust acoustic wave. In the system, the dust acoustic wave field affects the dust particle motion which constitutes the waveform evolution of the dust acoustic wave. The main subjects of this work are answering the following questions: (1) How the wave field affects the particle motion? (2) How the particle motion constitutes the wave propagation and growing. (3) What is the microscopic origin of the breaking of growing waves.
In order to find out the answers, the waveform evolution and the corresponding particle motion are analyzed. It is found that the dust acoustic wave amplitude grows as the dust oscillation amplitude or the inter-dust phase lag is increased. However, the wave onsets breaking as the inter-dust topological relation is changed during the increasing of dust oscillation amplitude/ inter-dust phase lag. On the other hand, the growing wave also heats up the dust particles and causes chaotic dust motion. The chaotic motion opposes the growing trend of dust acoustic wave. Finally, it is found that, after wave breaking, the wave also generates fast particles. The corresponding fast particles, wave heating phenomena can be clearly observed in phase space.
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