ELEMENTARY PROCESSES IN DUSTY PLASMAS

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hashi et al. 1998). Its modification for two particles was described by Melzer et al. (1999), which is discussed in some detail below. The essence of the experiment was the following. Two particles of di erent masses are used: the first one having a radius a ≈ 1.7 m, and the second one being a cluster of two such particles sticking together. The particles are introduced into a plasma of an r.f. discharge in helium at a pressure of p 50 −200 Pa. Because of the di erent charge-to- mass ratios, the particles are levitated at di erent vertical equilibrium positions in the anisotropic sheath region above the lower electrode – more massive particles levitate closer to the electrode. Due to the inhomogeneous electric field, the force balance (determined by the electrostatic and gravity forces) fixes their vertical positions. Both particles, meanwhile, are free to move in the horizontal plane. The first observation was the following. For su ciently low pressures, the particles tend to form a bound state, in which the lower particle is vertically aligned to the upper one. Note that similar structures (vertically aligned chains) are quite common for many experimental investigations of multilayer dust crystal formation in gravity conditions. With increasing pressure, the bound state can be destroyed, and the particle separation in the horizontal plane is limited only by a very weak horizontal confinement due to a specially concave electrode. The existence of these states indicates some mechanisms of attraction and repulsion between the particles, which change each other in response to changes in plasma parameters. It is also found that the e ect exhibits hysteresis (dependent on the pressure).

Next, to prove that the aligned bound state is due to attraction between the particles rather than its being forced by an external confinement, the particles were manipulated by laser radiation. The laser beam is focused either on the upper or the lower particle, causing its motion. It was found that when the upper particle is pushed by the laser beam, then the lower particle follows its motion and the bound state is not destroyed. This behavior proves that the lower particle is subject to an attractive horizontal force mediated by the upper particle. If the lower particle is pushed by the laser beam, then the upper particle’s response is much weaker and the bound state can be easily destroyed. Hence, the interaction between the particles is asymmetric. It is clear that the attraction between the particles situated along the ion flow can be attributed to the wake e ect. However, the question of whether it has an electrostatic nature or is associated with momentum transfer from the ions scattered by the upper particle (Lapenta 2002) still needs to be investigated.

11.2.6Formation and growth of dust particles

In laboratory conditions, the dust particles are usually introduced into a plasma deliberately. On the other hand, they may, in principle, be self–formed in plasmas. There are several possible sources of dust particles. First is condensation leading to the appearance of solid particles or droplets. This process is typical for expanding plasmas, e.g., adiabatic plasma expansion in a vacuum or expansion of plasma in the channel of an MHD generator (Zhukhovitskii et al. 1984; Yakubov