17 Dusty plasmas
17.1 Introduction
Non-neutral plasmas had one less species than a conventional plasma (one species instead
of two);dusty plasmas have one more species (three species instead of two). Not sur-
prisingly, the addition of another species provides new freedoms which giverise to new
behaviors.
The third species in a dusty plasma, electrically charged dust grains, typically have a
charge to mass ratio quite different from that of electrons or ions. Several methods for
charging dust grains are possible. Electron bombardment is the usual means forcharging
laboratory dusty plasmas, but photoionization or radioactive decay could also be operative
mechanisms and may be important for certain space and astrophysical situations. Photoion-
ization would make dust grains positive because photoionization causes electrons to leave
dust grains. Radioactive decay of dust grains would make the dust grains develop apolar-
ity opposite to that of the particle emitted in the decay process, e.g., alpha particle emission
by dust grains would make the dust grains negative.
We shall consider here only the typical laboratory dusty plasma situation where the
plasma is weakly ionized and dust grain charging is due to electron bombardment. Negative
charging occurs because the electrons, being much lighter than the ions andusually much
hotter, have a much larger thermal velocity than the ions. As a result, when a dust grain
is inserted into the plasma, it is initially subject to a greaterflux of impacting electrons
than impacting ions, thereby causing the dust grain to become negatively charged. This
negative charge eventually becomes sufficiently large to repel incident electrons and thus
attenuate the incoming electronflux. On the other hand, the negative charge of the dust
grain accelerates incident ions thereby increasing the ionflux to the dust grain. The net
charge on the dust grain reaches equilibrium when the electron and ionfluxes intercepting
the dust grain become equal. This is a dynamic equilibrium because it involvesa continuous
flow of plasma to the dust grain. Some kind of external source is thus required to replenish
the plasma electrons and ions continuously, because otherwise all the plasma electrons and
ions would eventually collect on the dust grain. The dust grain charging process is quite
similar to the development of afloating potential on an insulated probe immersed in a
plasma (see Eq.(2.124)).