PPPS-2013: Synchronization of the dust acoustic wave

Summary form only given. Synchronization is a nonlinear phenomenon where an oscillation adjusts its frequency in response to an external driving. We investigate waves in a dusty plasma consisting of micron-size particles of solid matter as well as electrons, ions and neutral gas atoms. The dust acoustic wave is a compressional wave analogous to an ion acoustic wave. Differently from previous experiments,¹ in our experiment² we vary not only frequency but also amplitude of the external driving to characterize synchronization. We introduce dust consisting of 4.83 micron polymer microspheres into an argon rf glow discharge plasma. The microspheres become negatively charged and are electrically levitated and confined. To help shape the dust cloud into a 3D volume, we insert a glass box into the plasma to enhance the natural horizontal dc electric fields that help confine the negatively-charged dust. Due to an ion streaming instability, a dust acoustic wave is self-excited with a natural frequency of 22 Hz. This low frequency is due to the inertia of the heavy dust particles. We observe the wave´s propagation using a high-speed camera. To synchronize the wave, we apply an external drive by sinusoidally modulating the overall ion density using a frequency that is different from the wave´s natural frequency. In the presence of this driving, we observe that the wave has a frequency of 1,2,3 or ½ times the driving frequency, and this indicates synchronization to the external drive. We find that synchronization of our wave has a threshold amplitude. We prepare an Arnold tongue diagram, which is a traditional method of characterizing synchronization. We find that this diagram has a branching feature. We also observe an unexpected nonharmonic state, which we find to be a nonlinear oscillation that is not in synchronization with the external drive².