Topic/Type: 1.5 Low-temperature, dusty and nano-plasmas, Oral

Molecular dynamics simulation of dynamic processes in complex (dusty) plasmas

C. Durniak1, D. Samsonov1, P. Harvey1, S. Zhdanov2, G. Morfill2

1 Dept. of Electrical Engineering and Electronics, The University of Liverpool, Liverpool, L69 3GJ, UK.
2 Max-Planck-Institut fur Extraterrestrische Physik, D-85740 Garching, Germany.

Complex or dusty plasmas are ordinary ion-electron plasmas with added
microparticles (or dust grains). The microparticles collect electrons
and ions and become highly charged. Their charges are usually negative
due to higher mobility of the electrons. The grains interact with
each other electrostatically via a Yukawa (Debye-Huckel,
screened Coulomb) potential and often form ordered structures.
Similar to colloids, complex plasmas can exist in solid, liquid or
gaseous states and exhibit phase transitions
[1]. They can be found in space, e.g. in planetary
rings, comets or interstellar clouds. In plasma technology, dust
contamination has negative effects on the yield of semiconductor
devices. As the grains are weakly damped by gas friction and
traceable individually, dynamic phenomena such as shocks
[2], Mach cones [3], solitons [4,5] and waves [6]
can be observed at the kinetic level in real time.

The molecular dynamics simulation code that we have developed solves
the equations of motion for each particle moving in the global
confinement potential and interacting with every other particle. The
code is based on an object-oriented multi-threaded programming. It
can be used to simulate various particle systems which can be
characterised by interaction forces or potentials such as some kinds
complex plasmas, colloids, granular media, plasma doping, ion beams,
film growth, ion implantation, etc. The equations of motion are solved
using the fifth-order Runge Kutta method with the Cash Karp adaptive
step size control. We use it to simulate two- and three-dimensional
(2D and 3D) systems.

In order to simulate monolayer complex plasmas we use the Yukawa
interaction potential. The particles are damped by the friction force
(equal to the neutral gas damping) and confined by a parabolic
potential. The particles are seeded randomly and the code is run to
equilibrate them until a crystalline structure was formed. By choosing
the vertical confinement much stronger than the horizontal monolayer
lattices can be formed in 3D space. Different excitation forces are
applied in order to launch waves, solitons, shocks or induce phase
transitions. The results of the simulation are validated by comparing
them to experiments.

The experiments are performed in a setup similar to that of
[4] by suspending micron-sized monodispersed
spherical micro-particles in a sheath of a capacitively coupled
radio-frequency discharge above a flat round electrode. The particles
are levitated and confined in a nearly parabolic potential. They are
excited by voltage pulses applied to wires stretched above the
electrode at approximately the same height as the particles. The
particles are illuminated by a thin sheet of laser light and imaged by
a video camera. Their positions and velocities are determined from
recorded video frames.

Here we report examples of dynamic phenomena in complex plasmas:
soliton steepening (tsunami effect) in an inhomogeneous complex
plasma, collision of two counter-propagating solitons, structural
changes and defect dynamics due to soliton influence. It was found
that the soliton amplitude can grow as it propagates even in the
presence of neutral damping. The solitons are delayed after a
collision however their shape is not affected. The solitons of
sufficient amplitude displace defects from their positions.

[1] H.M. Thomas and G.E. Morfill, Nature, 379, 806 (1996); C.A. Knapek et al., Phys. Rev. Lett., 98, 015004 (2007).

[2] D. Samsonov et al., Phys. Rev. Lett., 92, 255004 (2004).

[3] D. Samsonov et al., Phys. Rev. E, 61, (2000).

[4] D. Samsonov et al., Phys. Rev. Lett., 88, 095004 (2002).

[5] T.E. Sheridan et al., Phys. Plasmas, 15, 073703 (2008).

[6] D. Samsonov et al., Phys. Rev. E, 71, 026410 (2005).