Experimental setup for the investigations of the heartbeat instability. Microparticles are levitated in the discharge volume with the help of the thermophoretic force. A 532 nm laser is used for the illumination of the microparticles. A 772 nm tunable laser, modulated by a chopper, provides additional means of exciting the heartbeat instability. Attenuators of different transparency are used to vary the tunable laser beam power. Two photomultipliers are recording time-resolved emission intensity of the plasma. Longpass filter in front of them rejects the scattered light of 532 nm illumination laser. A CMOS camera records the dynamics of microparticles.
Example of the stability diagram showing the parameter range where the heartbeat instability is triggered (inside the dashed area) for a given number of levitating microparticles. Circles represent the measured stability boundary which can vary significantly between different experimental series.
Evolution of the integral light intensity measured during the self-excited heartbeat instability (a) outside the void and (b) inside the void. The curves are obtained by averaging over 200 consecutive cycles of the instability.
Dependence of the frequency of the laser-excited heartbeat instability, , on (a) the frequency of the laser modulation and (b) the tunable laser power . The results are for (a) and (b) . Laser beam crosses the center of the void.
(a) Positions of the laser beam with respect to the void, at which (b) the dependence of the frequency of the laser-excited heartbeat instability, , on the fraction of the void volume occupied by the laser beam, , was investigated. The void of volume is approximated as an oblate spheroid with the large and small semiaxes 6.5 and 2.6 mm, respectively. Laser beam is a cylinder with the diameter of 2.4 mm, V is defined as a volume ofintersection of the laser beam and the void. For the shown example, and . Movies of the microparticle cloud with thelaser beam in the center of the void () and for the laser beam outside the void () can be viewed as respective multimedia (enhanced online). [URL: http://dx.doi.org/10.1063/1.4757213.1] [URL: http://dx.doi.org/10.1063/1.4757213.2]10.1063/1.4757213.110.1063/1.4757213.2
Schematic representation of the sheath formation near the void boundary. Abrupt steepening of the electrostatic potential profile occurs when the ion velocity at the boundary exceeds the critical (Bohm) velocity . This leads to the acceleration of the negatively charged microparticles towards the center of the void. The electrostatic potentials on the void boundary before and after the critical transformation are and , respectively.
Fourier spectra of the light intensity inside the void for the experimental points from Fig. 4(a). At (when the intensity of the heartbeat is close to maximum), the spectrum is peaked at about the half of the laser frequency, suggesting the parametric instability as underlying mechanism of the laser-excited heartbeat.
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