Capacitively coupled rf discharge with a large amount of microparticles: Spatiotemporal emission pattern and microparticle arrangement

Heartbeat instability as auto-oscillation between dim and bright void regimes

We investigated the self-excited as well as optogalvanically stimulated heartbeat instability in RF discharge complex plasma. Three video cameras measured the motion of the microparticles, the plasma emission, and the laser-induced fluorescence simultaneously. Comprehensive studies of the optogalvanic control of the heartbeat instability revealed that the microparticle suspension can be stabilized by a continuous laser, whereas a modulated laser beam induces the void contraction either transiently or resonantly. The resonance occurred when the laser modulation frequency coincided with the frequency of small breathing oscillations of the microparticle suspension, which are known to be a prerequisite to the heartbeat instability. Based on the experimental results we suggest that the void contraction during the instability is caused by an abrupt void transition from the dim to the bright regime [Pikalev et al., Plasma Sources Sci. Technol. 30, 035014 (2021)PSTEEU0963-025210.1088/1361-6595/abe0a2]. In the bright regime, a time-averaged electric field at the void boundary heats the electrons causing bright plasma emission inside the void. The dim void has much lower electric field at the boundary and exhibits therefore no emission feature associated with it.

Long-term evolution of the three-dimensional structure of string-fluid complex plasmas in the PK-4 experiment

Microparticle suspensions in a polarity-switched discharge plasma of the Plasmakristall-4 facility on board the International Space Station exhibit string-like order. As pointed out in [Phys. Rev. Research 2, 033314 (2020)2643-156410.1103/PhysRevResearch.2.033314], the string-order is subject to evolution on the timescale of minutes at constant gas pressure and constant parameters of polarity switching. We perform a detailed analysis of this evolution using the pair correlations and length spectrum of the string-like clusters (SLCs). Average exponential decay rate of the SLC length spectrum is used as a measure of string order. The analysis shows that the improvement of the string-like order is accompanied by the decrease of the thickness of the microparticle suspension, microparticle number density, and total amount of microparticles in the field of view. This suggests that the observed long-term evolution of the string-like order is caused by the redistribution of the microparticles, which significantly modifies the plasma conditions.