Interaction of particles with complex electrostatic structures and 3D clusters

Particles of micrometer size externally introduced in plasmas
usually find their positions of levitation in the plasma sheath,
where the gravity force is compensated by the strong electric
field. Here due to electrostatic interaction they form different
structures, which are interesting objects for the investigation of
strongly coupled systems and critical phenomena. Because of the low
damping (e.g. in comparison to colloidal suspension) it is possible
to measure the dynamics up to the relevant highest frequency (e.g.
Einstein frequency) at the most elementary level of single particle
motion. The task of this work was to analyze the three dimensional
structure, dynamical processes and the limit of the cooperative
behavior in small plasma crystals. In addition to the study of the
systems formed, the immersed particles themselves may be used for
diagnostics of the plasma environment: estimation of parameters or
monitoring of the processes inside plasma. The laboratory
experiments are performed in two radio-frequency (RF) plasma
reactors with parallel plate electrodes, where the lower electrode
is a so-called "adaptive electrode". This electrode is segmented
into 57 small "pixels" independently driven in DC (direct current)
and/or RF voltage. When RF voltage is applied to one of these
pixels, a bright localized glow, "secondary plasma ball", appears
above. Three dimensional dust crystals with less than 100 particles
are formed inside this "plasma ball" - the ideal conditions for the
investigation of the transition from cluster systems to collective
systems. The investigation of the particle interactions in crystals
is performed with an optical diagnostic, which allows determination
of all three particle coordinates simultaneously with time
resolution of 0.04 sec. The experimental results are: 1. The binary
interaction among particles in addition to the repelling Coulomb
force exhibits also an attractive part, which is experimentally
determined for the first time. 2. Analysis of the dynamical
evolution shows the tendency of the systems to approach the state
with minimum energy by rearranging particles inside. 3. The
measured 63 particles' crystal vibrations are in close agreement
with vibrations of a drop with surface tension. This indicates that
even a 63 particle crystal already exhibits properties normally
associated with the cooperative regime. The possibility to use
levitated particles as a new powerful diagnostic of the plasma
sheath region is proposed. The existence of different equilibrium
positions of microparticles suspended in an Oxygen discharge
provides evidence of a structured electronegative plasma sheath, a
feature so far only mathematically and numerically investigated.