Nonlinear waves externally excited in a complex plasma under microgravity conditions

An experimental study of intense induced oscillations and nonlinear waves in a complex plasma is presented. The neon plasma is created in the PK-3 Plus set-up on board the International Space Station (ISS) by using a capacitively coupled radio frequency (RF) discharge at a low gas pressure of 16 Pa. The excitation conditions are controlled by a function generator creating a periodic variable modulation at the RF electrodes. In the experiments, comparatively large melamine–formaldehyde particles of 9.2 μm diameter are used to reduce the damping rate coefficient. The elliptic-shaped particle cloud (with a small central void) of size 65 × 14 mm² was stretched horizontally (parallel to the electrodes) and compressed vertically. Without excitation a weak 5 Hz global breathing mode and weak horizontal waves (approximately with the same frequency) are observed. Increasing the modulation frequency widens the spectrum. The modulation first excites a global vertical slashing mode (at the modulation frequency). At a frequency of 3 Hz intense shaking induces periodic nonlinear wave-ridges in the bottom and top parts of the cloud. The ridges travel at an approximately constant speed of 4–5 mm s⁻¹. The modulation also intensifies horizontal waves localized at the horizontal cloud edges, though the regions of intensive waves are visibly disconnected. At higher frequencies the intense wave activity involves all the cloud, and 'oblique' (quasi-sound) wave-ridges start to propagate through the cloud. Distributions of wave velocity and force fields are analysed.