We demonstrate experimentally that the void in capacitively-coupled RF complex plasmas can exist in two qualitatively different regimes. The ‘bright’ void is characterized by bright plasma emission associated with the void, whereas the ‘dim’ void possesses no detectable emission feature. The transition from the dim to the bright regime occurs with an increase of the discharge power and has a discontinuous character. The discontinuity is manifested by a kink in the void size power dependencies. We reproduce the bright void (mechanically stabilized due to the balance of ion drag and electrostatic forces) by a simplified time-averaged 1D fluid model. To reproduce the dim void, we artificially include the radial ion diffusion into the continuity equation for ions, which allows to mechanically stabilize the void boundary due to very weak electrostatic forces. The electric field at the void boundary occurs to be so small that it, in accordance with the experimental observation, causes no void-related emission feature.
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)]. 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.
Gas temperatures in an argon low-power capacitively-coupled rf discharge were measured by means of tunable diode laser absorption spectroscopy on 1s 5 metastable states. It was shown that the uncertainty of the measurements in dust-free plasma could be minimized to ±1 K. In the presence of dust, due to instabilities in the dust subsystem (e.g. dust density waves), the uncertainty of the temperature measurements increased several times.Nevertheless, an evidence of the gas temperature increase in the presence of dust was detected. This result suggests that the microparticle suspensions, immersed in the low-pressure plasmas, significantly modify the temperature fields of the neutral gas.
Low‐pressure gas discharge plasmas are known to be strongly affected by the presence of small dust particles. This issue plays a role in the investigations of dust particle‐forming plasmas, where the dust‐induced instabilities may affect the properties of synthesized dust particles. Also, gas discharges with large amounts of microparticles are used in microgravity experiments, where strongly coupled subsystems of charged microparticles represent particle‐resolved models of liquids and solids. In this field, deep understanding of dust–plasma interactions is required to construct the discharge configurations which would be able to model the desired generic condensed matter physics as well as, in the interpretation of experiments, to distinguish the plasma phenomena from the generic condensed matter physics phenomena. In this review, we address only physical aspects of dust–plasma interactions, that is, we always imply constant chemical composition of the plasma as well as constant size of the dust particles. We also restrict the review to two discharge types: dc discharge and capacitively coupled rf discharge. We describe the experimental methods used in the investigations of dust–plasma interactions and show the approaches to numerical modelling of the gas discharge plasmas with large amounts of dust. Starting from the basic physical principles governing the dust–plasma interactions, we discuss the state‐of‐the‐art understanding of such complicated, discharge‐type‐specific phenomena as dust‐induced stratification and transverse instability in a dc discharge or void formation and heartbeat instability in an rf discharge.
We explore the characterization of melamine formaldehyde resin (MF‐R) micron‐sized particles, immersed in argon, neon and argon–oxygen plasmas, using Raman spectroscopy. It is shown that plasma treatment of MF‐R results in modification of its chemical composition. Particularly, a decrease in the intensities of the Raman scattering bands, corresponding to both formaldehyde C―H and melamine C―N and N―H bonds, is observed. The band at 980–990 cm−1, associated with breathing vibrations of the triazine rings, undergoes the most significant changes, and the greatest modifications of the spectra are observed after exposure to Ar and Ar–O2 plasma, whilst for the MF‐R particles exposed to Ne plasma these modifications are less pronounced. Copyright © 2016 John Wiley & Sons, Ltd.
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