Plasma physics is the branch of science
that studies the properties and applications of plasma. A plasma is an assemblage
of positive ions and unbound electrons in which the total number of positive
and negative charges is almost exactly equal. In general, the plasma will
also contain some proportion of neutral atoms or molecules. The properties
of plasma are sufficiently different from those of solids, liquids, and
gases for it to be considered to be a fourth state of MATTER.
HISTORICAL DEVELOPMENT
Electrical discharges in gases were studied as
early as the 1830s by the English physicist Michael FARADAY and later by
Sir William CROOKES, who first suggested (1879) that the ionized gas in
the discharge constituted a fourth state of matter. In 1926, radio-frequency
oscillations were discovered in a low-pressure discharge in mercury vapor.
These oscillations were further studied by American chemist Irving LANGMUIR
and coworkers. In 1929 Langmuir applied the term plasma to the regions of
the discharge in which the oscillations occurred. The terminology was derived
from a belief that the ions acted like a rigid jelly through which the electrons
were free to move. Using electrostatic-probe techniques that now bear his
name, Langmuir established that the electrons pervading the plasma had a
characteristic temperature. Since the 1920s the study of the plasma state
has grown into a major research discipline.
OCCURRENCE OF PLASMA
Near the Earth's surface, gaseous plasmas either
are transient natural phenomena, such as LIGHTNING, or they occur in the
operation of certain devices such as fluorescent tubes and electric arcs.
Beyond the Earth's dense lower atmosphere, however,more than 99% of the
matter in the universe appears to exist as plasma. In the upper atmosphere,
the ionosphere contains plasma that reflects terrestrial radio transmissions.
Driven from the surface of the Sun, the SOLAR WIND
is a hydrogen plasma having a density of 10 particles per cu cm. This plasma
sets up a bow shock wave, known as the magnetopause, upon encountering the
Earth's magnetic field. Within the Earth's magnetic field, plasma from the
solar wind is confined within bands known as the VAN ALLEN RADIATION belts
at distances up to several Earth radii. At high latitudes, where these belts
approach the upper atmosphere, they give rise to the AURORA.
The Sun and stars are dense, highly ionized plasmas
that have central temperatures in excess of 10,000,000 K and densities of
(10 to the power of 25) particles per cu cm. The vast space between stars
is filled with a tenuous, weakly ionized plasma that may range in density
from about 1 particle per cu cm near stars to about 10 to the minus 5 power
particles per 3 cu cm in intergalactic space (see INTERSTELLAR MATTER).
Thus most of the universe exists in plasma form. Some plasma physicists,
such as Hannes ALFVEN, have proposed a plasma-based cosmological theory
to replace the currently dominant BIG BANG THEORY. According to these scientists,
gravity together with the electromagnetic forces of plasmas were sufficient
to shape the universe as it now known, with no need for an initiating event
(see COSMOLOGY).
Plasmas can also exist in solids. For example,
the fixed ions and free electrons in an electrically conducting METAL constitute
a plasma, as do the free electrons and mobile positive "holes"
in a SEMICONDUCTOR.
PROPERTIES OF PLASMA
The electric field of an isolated charged particle
diminishes as the square of the distance from the particle. In a plasma,
however, this field is modified because the electrons are free to move into
the vicinity of positive ions and away from other electrons. The field of
each isolated particle is thus partially shielded by its immediate neighbors.
Over a sufficiently large distance--wherein the fields of many individual
charges are able to cancel each other--this shielding becomes complete.
This distance, called a Debye length, is a measure of the distance over
which an individual charged particle can exert an effect. Volumes greater
in radius than a Debye length must be approximately neutral. The Debye length
is equal to (6.9 times the square root of (T/n))centimeters, where T is
the temperature of the electrons in Kelvins and n(e) is the number of electrons
per cubic centimeter. For a body of particles to behave as a plasma, its
dimensions must be large compared to the Debye length.
Any displacement of the electrons relative to the
ions over a distance of the order of a Debye length gives rise to strong
electrostatic fields that accelerate the electrons back toward the ions.
Upon reaching their original positions, the electrons will have kinetic
energy equal to the potential energy acquired in their displacement and
will consequently overshoot, continuing to oscillate about an equilibrium
position until this energy is dissipated. This simple harmonic oscillation
is common to all plasmas. A particular frequency called the plasma frequency
is determined by the time it takes for a particle with thermal speed to
travel a Debye length. For electrons, the plasma frequency (in cycles per
second) is approximately 9,000 times the square root of the number of electrons
per cu cm.
Where a plasma comes into contact with a solid
object, a region of charge separation develops in which charge neutrality
is not preserved. The thickness of this region, called the sheath, is about
equal to the Debye length. The sheath arises because, at the same temperature,
the electrons in a plasma have a much higher average velocity than do the
ions (most of which have positive net charge). If an object in contact with
a plasma has no net current flowing through it, then electrons and positive
ions must be reaching it at the same rate. This balance can occur only if
the average velocity of the approaching electrons is reduced and that of
the approaching positive ions increased by the formation of a layer of net
negative charge near the object and a layer of net positive charge one Debye
length into the plasma. If an electric field is applied across a plasma
formed by an electron-emitting cathode, then, in the absence of a magnetic
field, essentially the entire potential drop occurs across the sheath at
the cathode. The bulk of the plasma remains at a uniform potential.
Another important property of a plasma is its electrical
conductivity. Because of its large number of free electrons, a gaseous plasma
generally offers little resistance to the passage of an electric current.
The resistance that does occur arises from the collisions of electrons with
each other and with ions. Because the probability of such collisions occurring
decreases with increasing electron velocity, the electrical resistance of
a plasma decreases at higher temperatures--exactly opposite to the behavior
of a metal. This characteristic has profound effects when attempts are made
to heat a plasma by passing a current through it. As the plasma gets hotter,
there is a proportional decrease in the power input. Thus it is very difficult
to raise the temperature of a hydrogen plasma much above 10,000,000 K by
this method (called ohmic heating).
Plasma is often surrounded by or embedded in a
magnetic field. A charged particle moving in a magnetic field experiences
a force perpendicular to the directions of both the motion and the field.
Thus a charged particle in a uniform magnetic field moving with velocity
components perpendicular and parallel to the field traces out a helix centered
on a magnetic line of force. The frequency of oscillation is proportional
to the magnetic field strength times the particle's charge-to-mass ratio.
This is called the cyclotron frequency. For electrons it is equal to 1.76
X 10 million X B radians per second, where B is the magnetic field strength
in gauss. For ions, the equivalent quantity is called the ion cyclotron
frequency, but it is different for each species. The radius of the cyclotron
helix is called the Larmor radius and is equal to the velocity perpendicular
to the magnetic field divided by the cyclotron frequency. The cyclotron
motion of a charged particle produces a magnetic field parallel to but opposing
the initial field within its orbit. This effect, called diamagnetism, is
an important property of a plasma (see MAGNETISM). A plasma tends to exclude
a magnetic field suddenly applied at its periphery, so a magnetic field
pressure can be used to balance the thermal pressure and to contain a plasma.
If both electric and magnetic fields are present
in the plasma, a particle drift velocity perpendicular to both fields is
superimposed on the cyclotron rotation. Other types of drift motion occur
if the magnetic field is inhomogeneous, or if a gravitational field is acting
on the particles.
Plasmas are also characterized by waves--collective
phenomena in which many particles oscillating about their equilibrium positions
interact through collisions and fields. Some of the simplest types of waves
occur when a magnetic field line is disturbed (see MAGNETOHYDRODYNAMICS).
Charge displacements excite electrostatic waves at the plasma frequency
discussed earlier. Electromagnetic waves, such as light or microwaves, can
propagate through a plasma so long as the wave frequency is higher than
the plasma frequency.
PRODUCTION OF PLASMAS AND THEIR APPLICATIONS
In the laboratory, most plasmas are produced by
applying an electric field to a gas in order to accelerate the free electrons.
The free electrons are supplied either by an electron-emitting cathode,
by photoionization (a process in which a light is used to excite an atom
to emit an electron), or by the passage of ionizing cosmic rays. Through
collisions these electrons impart energy to the gas atoms, releasing more
electrons. When the rate of free-electron production equals or exceeds the
rate of electron loss, a plasma is formed.
Plasmas have come to play an important role in
processing materials in the semiconductor industry. Ions supplied by plasmas
are used to etch surfaces and are deposited in materials to alter their
physical properties. Plasmas also have many potential applications, of which
the most significant is controlled nuclear FUSION. If certain light nuclei
can be held together in a plasma for a sufficient time at high enough temperatures
and densities, enormous amounts of energy will be released as light nuclei
fuse to form heavier ones.
Plasma thrusters for propulsion in space can eject
their propellant at much higher velocities than a chemical rocket, so that
less mass need be expended. One type of thruster uses pulsed magnetic fields
to expel the plasma. Another type accelerates the ions from a plasma through
a set of electrostatic lenses and then adds electrons to maintain overall
neutrality.
A potentially important application of plasma physics,
and one that has been the subject of much research, involves the direct
conversion of thermal energy to electricity without steam turbines or other
moving mechanical parts. One such device is the thermionic converter, which
consists of a cathode heated by the energy source so that electrons are
emitted from its surface; a cooled anode; and an intervening cesium plasma
that allows large currents to flow by neutralizing the space charge of the
emitted electrons. Another possibility is the magnetohydrodynamic generator.
In this device, a plasma is heated by the energy source and allowed to expand
through a channel located in a magnetic field perpendicular to the direction
of expansion; this produces an electromotive force perpendicular to the
directions of both the magnetic field and the expansion. This electromotive
force can drive an electric current (the field of which decelerates the
plasma) across the plasma, converting part of the plasma's thermal energy
of expansion into electricity.
