25B08
Dust in Magnetized Plasmas
Alex Samarian
School of Physics, the University of Sydney, NSW 2006, Australia

Overview of recent progress in studies of magnetized dusty plasmas is presented. Dust dynamic under theinfluence of an applied magnetic field in various types of discharges and conditions (including dust
dynamic in the magnetized plasma sheath) is examined. The various dust driven mechanisms in magnetizedplasma are considered and comparison experimental and theoretical data is performed. Influence ofmagnetic field on dust structures are analyzed as well as dust stabilization (freezing) in the alternating
magnetic field.

1. Introduction
Dusty (complex) plasma is a complex object
capable of self-organization, which has been
subjected to intensive investigation during the lasttwenty years. Under typical laboratory conditions,the grains generally carry negative charge owing
to various processes in the ambient plasma. A
typical feature of dusty plasma is strong
interaction between charged dust particles, which
may result in formation of ordered structures ofliquid and crystal types. The presence of themassive dust grain in the plasma introduces
spatial and temporal scales over which kinetics ofindividual dust can be investigated [1-3].
One of the ways of studying the dynamic(kinetic) characteristics of a dust is studying its
response to various external excitations. The
laboratory dusty plasma in the presence of
external perturbation is a useful experimental
model for investigating of dusty plasma structuresin space and industrial and power facilities. Dusty
plasma may be affected, in particular, by an
external magnetic field. Dust dynamics under theinfluence of a magnetic field in various types of
dusty plasmas had long been studied in the last
decade [4-26].
For example, the structure and the motion of theSaturn rings such as spokes on the B rings or everchanging structure of the F rings are believed to
be influenced not exclusively by the gravitational
field but also by the magnetic field [27]. Studying
the effect of magnetic field is further of interest
when considering the behavior of dust particles in
the wall plasma of tokamaks. It is well known thatsignificant amount of dust particles are present infusion devices due to flaking, blistering, andviolent arcing on chamber walls. Recent studies
showed that dust particles can acquire very highspeeds due to accelerated plasma flow in magnetic
field and contaminate the core plasma, which
ultimately lowers the performance of fusion
devices [28,29].

2. Dust Dynamic in Magnetic Field
In the laboratory, dust dynamic under theinfluence of an applied magnetic field in various
types of discharges and conditions have beenstudied. Effects of the azimuthal ion flow inducedby E × B drift on single dust particle in amagnetized cylindrical electron cyclotron
resonance (ECR) plasma was reported by
Nunomura [4]. The first observation of dust
crystal rotation using an axial magnetic field ofvariable strength was made in a dc argon glow
discharge by Sato [6]. Sydney Complex Plasma
Lab (CPL) reported the experimental investigationin the rotation of large crystals and threedimensional clusters in an inductive rf argon
discharge [10,11]. Later CPL began to investigate
planar dust clusters under the influence ofmagnetic field [12]. Because planar dust clusters
are two-dimensional and have a smaller numberof particles, their simplicity allows for easier
analysis and hence a better understanding in thedriving mechanisms for dust rotation. Yokota
rotated a UV-induced annular crystal consisting offine aluminum particles about the magnetic dipole
axis of a spinning magnetized miniature spherewith field B ≈ 0 to 50 G in order to mimic some of
the mechanisms in planetary ring formation [26].
Konopka [5] observed rotation of monolayer
dust crystals induced with a cylindrical permanent
magnet of strength B = 140 G in a capacitive rf
helium discharge. Ishihara studied the precessionof single dust particle placed in the plasma withthe presence of an external magnetic field [7]. In
his model, the dust particles were suggested torotate about its axis due to the ion drift in the E ×
B field. Sato extended their dust rotation
experiment to not only dc but also rf argon glow
discharges with much stronger variable magnetic
field up to B ≈ 10000 G [6]. The motivating point
about the use of stronger magnetic field is that thedust particles become partially magnetized whenB > ≈ 4000 G. As well, this group managed torotate dust crystals with particles of different
diameters (a ≈ 0.1 to a few tens µm), but
qualitatively did not observe any effect of particle
size on the rotation. Interestingly, Sato [6] also
observed the phenomenon of “angular velocitysaturation” at B ≈ 4000 G, the value predicted in
the following year by Kaw using collisional fluid
theory for magnetized ions [8]. Karasev and
Vasil’ev continued studies of dust rotation inmagnetized plasma in DC discharge plasma[16,17,21,25];
while Konopka performedexperiments in magnetic fields up to flux densitiesof 2.3T [15,24] and reported a dust particles
pattern formation similar to one observed byHuang [19].
The most accepted explanation of dust
rotation in magnetic field is that the dust particles
are rotated by ion drag force [4,7,8]. Indeed as fora magnetized dusty plasma, particles are locatedin a region with a radial confining electric field, a
vertical magnetic field applied will result in an
azimuthal E × B ion drift (the ion rotation
direction is in turn determined by a competitionbetween E × B and diamagnetic rotations of theions in the collisional magnetized plasma) andtherefore a rotation of particles on the horizontal
plane. However, the experimentally observed
motion of plasma-dust structures cannot be
explained by the forces of ions only. So manyvarious mechanisms are discussed in literature:from ambipolar diffusion [17, 22] tomagnetomechanical effect [20], the coupling
between harmonic oscillations and the Lorentz
force on the dust particles was considered by
Ishihara [9].
3 Dust Dynamic in Magnetized Sheath.
The presence of dust in the plasma provides
an important diagnostic tool. For example, inbounded plasma, sheath characteristics can bestudied in considerable detail by using fine dust
probes. This is due to the fact that the charge on
the grain is a function of local plasma conditions
and thus, study of the dust dynamics in such asurrounding can provide information on theelectron and ion fluxes, on the sheath potential,
and on the electric field. Therefore, it is important
to investigate the charged dust behavior inside amagnetized sheath, which will provide a useful
guide in measuring the sheath characteristics.
It was found that the charge on the dust
determines its trajectory and dust performs thespiraling motion inside the sheath. The location of
the turning spiral is determined by the number ofnegative charge on the dust, which in turn is afunction of the dust radius. The back and forthspiraling motion finally causes the dust to move ina small, narrow region of the sheath. The temporal
behavior of the spiraling dust motion appears likea damped harmonic oscillation, suggesting thatthe plasma drag force causes dissipation of theelectrostatic energy.
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