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7Complex Systems of Charged Particles and their Interactions with Electromagnetic Radiation 2016
COLLECTIVE MOTION IN COMPLEX PLASMAS EXCITED BY A PROJECTILE
D.I. Zhukhovitskii, V.I. Molotkov, A.M. Lipaev, V.N.Naumkin, H.M. Thomas*
Joint Institute for High Temperature RAS, Moscow, Russia, e-mail: dmr@ihed.ras.ru *Forschungsgruppe Komplexe Plasmen, DLR, Oberpfaffenhofen, Germany, e-mail:
Hubertus.Thomas@dlr.de
A low-temperature plasma containing small solid particles, typically in the micrometer range, is usually referred to as dusty or complex plasmas. Since the mobility of electrons is much greater than that of ions, particles acquire a significant negative electric charge. This leads to formation of strongly coupled plasma, in which a collective motion can be observed at the level of individual particles. Complex plasmas are studied in gas discharges at low pressures, e.g., in radio frequency (RF) discharges (PK-1, PK-2, PK-3, PK-3 Plus, and PK-4 setups). Under microgravity conditions, large clouds of three-dimensional complex plasma can be observed. A collective motion of the dust particles can be excited by a probe moving in the dust cloud or by a quiescent probe in the plasma flow. We consider the experiments, in which a large moving particle (projectile) plays the role of a probe. Such projectiles are generated using controlled mechanisms of acceleration, or they can appear sporadically. Projectiles moving with subsonic velocity produce localized disturbances of surrounding particles; supersonic ones lead to the formation of Mach cones. It is shown in [1] that a dust cloud can be treated as a fluid of soft spheres, where each sphere is the Wigner-Seitz cell around a dust particle. Thus, the hydrodynamic approach is applicable for a collective motion of the particles.
For the case of a subsonic projectile motion, we show that the dust particles move along characteristic alpha-shaped pathways near the projectile while the latter moves almost freely through the bulk of the dust cloud, which means the zero drag force of the potential flow of dust particles (d'Alembert's paradox). As the projectile velocity is increased, the threshold subsonic velocity is reached, at which a spherical cavity around the projectile becomes hardly deformed. We demonstrate that this occurs due to stall of the flow that streamlines the cavity [2]. The threshold velocity decreases as the cavity size is increased, in accordance with experiment.
A supersonic projectile motion gives rise to the dust number density perturbations that are observed in experiments as the Mach cones. In this case, a collective motion of the dust particles occurs as propagation of the contact discontinuity. Measurement of the Mach angle makes it possible to determine the velocity of dust acoustic waves (DAWs). For a dust cloud in the neon RF discharge, we found that this velocity was essentially lower than that for the argon discharge [3]. The theory of DAWs propagation is based on the ionization equation of state for the dust cloud, which takes into account the overlapping of scattering potentials of the charged dust particles and resulting decrease of the ion drag force. The hydrodynamic approach is employed to deduce the wave equation, which includes the velocity of DAWs. The latter proves to be almost independent of the projectile coordinate and weakly dependent on the particle diameter and gas pressure [4].
The authors gratefully acknowledge the support from the Russian Science Foundation, Grant No. 14-12-01235.
References
[1] Zhukhovitskii D.I., Fortov V.E., Molotkov V.I., Lipaev A.M., Naumkin V.N., Thomas H.M., Ivlev A.V., Schwabe M., Morfill G.E., Phys. Rev. E, 2012, vol. 86, no. 1. p. 016401.
[2] Zhukhovitskii D.I., Ivlev A.V., Fortov V.E., Morfill G.E., Phys. Rev. E, 2013, vol. 87, no. 6. p. 063108.
[3] Zhukhovitskii D.I., Fortov V.E., Molotkov V.I., Lipaev A.M., Naumkin V.N., Thomas H.M., Ivlev A.V., Schwabe M., Morfill G.E., Physics of Plasmas, 2015, vol. 22, no. 2, p. 023701.
[4] Zhukhovitskii D.I., Phys. Rev. E, 2015, vol. 92, no. 2. p. 023108.
