Some Researches on Dusty Plasmas

Bohm velocity, which is the same as the ion-acoustic speed. Thus wake effects
associated with this flow are to be expected.
One of the first experiments carried out at Oxford related to probe-induced particle
circulation in a plasma crystal [1]. The apparatus is illustrated in Figure 1; a
capacitively coupled R.F. discharge in argon was employed, the lower electrode being
driven at 13.56 MHz, through a matching network. A Langmuir probe was used to
determine the plasma parameters prior to inserting the dust particles. On introducing
the dust particles, however, the plasma crystal was significantly affected by the probe.
Particle circulation was induced around a stable crystalline island, as shown in Figure
2 (and the associated video). A complete theory of the phenomenon has not been
carried out. It would involve modification of the electric field in the sheath and also a
change in the momentum of the positive ions arriving at the plasma crystal. An
estimate was made, however, of the bulk modulus of the crystal
K = – Δ P/( Δ V/V 0 ), knowing the momentum carried by the impinging dust particles
and the increased density of the crystal. This enabled us to calculate the sound speed
in the crystal, c s = (K/ ρ ) ½ = 0.019 m/sec, which was comparable to other published
measurements.
3. Effect of laser light on a plasma crystal.
Another experiment performed in Oxford was carried out using a focussed low power
(maximum power ~150 mW/mm 2 ) HeNe laser beam (632.8 nm) to fracture a crystal
[2, 3]. The laser beam was normally used to provide illumination for the dust
particles, via a lens to produce a fan of light. When the lens was inadvertently omitted
from the system, however, it was found that the focussed beam could drill a hole
through the plasma crystal, as shown in Figure 3 (see also the video). A certain
threshold power was required to knock particles out of the plasma crystal, not
surprisingly. The power of the beam could be changed by using a calibrated variable
neutral density filter. Measurements were made of the velocities acquired by the
particles when higher laser powers were employed, see Figure 4.
The radiation pressure of the beam, (E 2 /Z)/c, although small, was sufficiently high to
explain the observed results. When first reported, at a small meeting at Bad Honnef,
the audience did not accept this explanation. But within a few months other
laboratories were employing such techniques, sometimes using a light chopper to
produce a series of radiation pulses.
4. Damped dust oscillations as a plasma sheath diagnostic.
It has already been mentioned that many experiments have been carried out for dust
particles residing in the space charge sheath adjacent to the plasma. In equilibrium the
most important forces on the charged dust particle are the electric force and the
weight, the ion drag being less important. The method that we have employed at
Oxford to measure the charge was to observe damped oscillations around the
equilibrium position [4]. Other workers have employed forced oscillations, where a
low-frequency voltage is applied either to a probe or to the adjacent electrode, we
believed, however, that the use of such techniques must disturb the plasma. In our
case the dust particles were simply dropped into the plasma. When the dust particle is
in motion the damping is due to collisions with neutral atoms and the expression for
the drag force due to Epstein has been employed. The potential distribution in the
sheath was assumed to be parabolic in nature, leading to the following simple
equation of motion for the dust particles.
2