K. Ono

K. Takahashi

M. Tanaka

T. Shirafuji

Y. Hayashi

Y. Setsuhara

National Institute for Fusion Science (NIFS-Repository)

§ 25. Molecular Dynamics Analyses onStrongly Coupled Dusty PlasmasTakahashi, K., Ono, K., Setsuhara, Y. (Dept.Aeronautics and Astronautics, Kyoto Univ.)Hayashi, Y. (Dept. Electronics and InformationScience, Kyoto Institute of Technology)Shirafuji, T. (International Innovation Center, KyotoUniv.)Tanaka, M. (NIFS)Coulomb crystal formed by solid particles in dusty plas-mas has attracted much interest of plasma scientists since itwas discovered in laboratories. In the crystal, such structuresas body-centered cubic (bcc), face-centered cubic (fcc) andsimple hexagonal appear, depending on the charge state of theparticle and the plasma conditions. Especially, the simple hex-agonal structure should be focused an attention on. In the struc-ture, the particles form hexagonal lattices in a horizontal plane.The lattices stack in the direction perpendicular to the planewithout alternation, i.e., the particles line up in a straight linein the vertical direction. The formation of the vertical align-ment implies presence of some attractive forces, since it isdifficult to understand that the alignment can be formed onlyby Coulomb repulsive force between negatively charged par-ticles. Hayashi et ai. found the structural transition from bccor fcc to simple hexagonal induced by the growth of the par-ticles in a plasma for chemical vapor deposition (CVD)I). Inthe plasma, the attractive forces come to act on the large-sized particles charged by more than a few thousand electrons.The origins of the attractive forces were suggested in severalpapers. One theory proposed that the forces can be caused bywake potential in ion flow. Besides, it may be possible to actthe forces on the particles because of the dipole moment in-duced by the deformation of the sheath around the particles.In the present study, we try to understand the interaction be-tween particles and ion, and the potential formation aroundthe particle in experiments with optical emission spectroscopy.In our experiments, dusty plasmas were generated on aplanar electrode in a vacuum chamber. The electrode was con-nected to an rf (13.56 MHz) power supply. For the discharge,nitrogen (Nz) and argon (Ar) were introduced into the cham-ber through mass flow controllers. The flow rates of N z andAr were set at 3.5 and 5 sccm, respectively. For injection ofacrylic regin particles of 5 J.lm into the plasmas, a dust dropperwas located above the electrode. The particles were trappedby a metal ring on the electrode during discharge on. Whenpressure and rf power were maintained at 200 mTorr and 0.034W/cmz, respectively, the behavior of particles and the spatialdistribution of optical emission from the plasmas were ob-served. In the observation, the particles were illuminated by asemiconductor laser, and a charge-coupled device (CCD) cam-era was employed to detect the scattering light from the par-ticles and the optical emission.Figure 1 (a) shows the spatial distribution of emission oflight from the plasmas, indicating that the particles were trappedaround the plasma-sheath boundary at the height between 4and 6 mm from the electrode. The particles lined up in straightlines perpendicular to the electrode. The structure implied thatthe particles formed simple hexagonal structure, and that at-tractive force acted between the particles in the direction par-allel to the line. Then emission of light from an ion specie ofNz+ was observed with an interference filter passing throughfirst negative system band of the ion at 391 nm (Fig. 1 (b)). Itcan be expected in the figure that the density of the ion aroundthe particle was comparable to that around bulk region. Al-though one should have been required to analyze the densitydistribution of the ion in the a few micrometer scale in orderto understand the potential structure around the particle, thepresent CCD camera system could not achieve such high per-formance measurements in resolution and sensitivity.Further experimental and theoretical studies are still hopedto be done for analyses of the particle interaction in dusty plas-mas. We will continue to study optical emission spectroscopyin the dusty plasmas, employing a CCD with an image inten-sifier unit (ICCD).Reference1) Hayashi, Y. et al. : Jpn. J. Appl. Phys. 36 (1997) 4976.Fig. 1. The spatial distributions of emission of light from theplasma obtained by CCD camera. The intensity of the lightwas expressed by gray scale, the highest corresponding to whitecolor. The scale bar shows height from the electrode. (a) emis-sion of light from the plasmas and scattering light from thepatticles, and (b) emission of light at 391 nm from Nz+ ions.393

Molecular Dynamics Analyses on Strongly Coupled Dusty Plasmas

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