Threshold phenomena in a throbbing complex plasma

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.

Merging and splitting of plasma spheroids in a dusty plasma

Dust particle growth in a plasma is a strongly disturbing phenomenon for the plasma equilibrium. It can induce many different types of low-frequency instabilities that can be experimentally observed, especially using high-speed imaging. A spectacular case has been observed in a krypton plasma where a huge density of dust particles is grown by material sputtering. The instability consists of well-defined regions of enhanced optical emission that emerge from the electrode vicinity and propagate towards the discharge center. These plasma spheroids have complex motions resulting from their mutual interaction that can also lead to the merging of two plasma spheroids into a single one. The reverse situation is also observed with the splitting of a plasma spheroid into two parts. These results are presented for the first time and reveal new behaviors in dusty plasmas.