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Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
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OCR for page 283
Glossary
Ablation. Removal of material from an imploding pellet by melting
and vaporization.
Accretion. The process by which mass falls onto a condensed object.
The infalling gas is heated to high temperatures as it accretes onto
white dwarfs, neutron stars, and black holes. The x radiation from
the heated gas provides a characteristic observational signature for
these objects.
Activation. Process of making a material radioactive by bombard-
ment with neutrons or other nuclear particles.
Active galactic nuclei. Active galaxies, which radiate much of their
energy, intense continuum, and strong atomic lines in nonthermal
forms such as thermal and synchrotron emissions, are observed to
generate their power in very small regions called nuclei at their
centers. Many astronomers believe that mass accretion onto massive
black holes at the centers of active galaxies are responsible for the
enormous energy outputs of Seyfert galaxies, radio galaxies, and
quasars. It is possible that all galaxies have nuclei that are active to
some degree.
Adiabatic compression. Compression of plasma not accompanied by
gain or loss of heat from the outside. In adiabatic compression, the
plasma density and temperature should both increase.
Air shower. The shower of energetic particles created when a single
cosmic ray or gamma ray collides with the nucleus of an atom in the
283
OCR for page 284
284 GLOSSAR Y
upper atmosphere. By studying the number and types of particles
created in an air shower, and the particle trajectories, it is possible to
infer the energy and, with less accuracy, the direction of arrival of
the very-high-energy cosmic-ray primary.
Alive'n speed. The characteristic speed of low-frequency waves in
conducting magnetohydrodynamic (MHD) fluids. The Alfven speed
is proportional to the magnetic-field strength and inversely propor-
tional to the square root of the mass density. For MHD fluids, the
Alfven speed plays a role similar to the sound speed in ordinary
fluids. Named for the Swedish Nobel Laureate, Hannes Alfven.
Alfve'n waves. Waves of a much lower frequency than the ion
cyclotron frequency, occurring in a plasma or in a conducting fluid
immersed in a magnetic field and characterized by a transverse
motion of the lines of force together with the plasma.
Alpha particle. A positively charged particle made up of two neu-
trons and two protons bound together; the nucleus of a helium atom.
Alpha particles are produced by the deuterium-tritium fusion reac-
tion and serve to sustain the plasma temperature in an ignited fusion
reactor.
Ambipolar diffusion. Diffusion process in which the electrons and
ions escape to the walls at exactly the same rate.
Ambipolar potential. An electrostatic potential generated by a
plasma to maintain equal loss rates for electrons and ions.
Anisotropy. The condition of having different properties in different
directions, for example, in the velocity distribution of particles.
Anomalous transport. Processes of energy and particle transport in
plasmas that generally exceed classical rates and are associated with
plasma instabilities and fluctuations.
ANTARES. A CO2-laser facility in operation at the Los Alamo s
National Laboratory.
Antenna. A device for coupling radio-frequency power to electro-
magnetic waves that may be launched into a plasma.
Aspect ratio. The ratio of the major radius of a torus to the plasma,
. .
Or minor, radius.
Aurora, auroras, aurora borealis, auroral zone. Also known as the
northern (and southern) lights, the term aurora refers to light
emissions that originate at 100-120 km altitudes above the surface of
the Earth. The aurora are typically most frequent between 60° and
70° north and south geomagnetic latitudes. Auroral activity moves
equatorward during magnetic storms. Aurora are created when
electrons and protons, which can be accelerated to 10 keV or more
OCR for page 285
GO OSSAR Y 285
in energy at altitudes of 5000-10,000 km, collide with upper-
atmospheric neutral atoms.
Auxiliary heating. Any form of plasma heating other than the intrinsic
(ohmic) heating by the plasma currents themselves. Neutral-beam
injection and radio-frequency heating are the most common types of
auxiliary heating.
Ballooning instability. A mode that is localized in regions of unfavor-
able magnetic-field curvature and that becomes unstable when the
plasma pressure gradient exceeds the local magnetic stress.
Banana orbit. An orbit with a banana shape that charged particles
can follow in a toroidal magnetic field.
Barn. Unit of area used in expressing the cross sections of atoms,
nuclei, electrons, and other particles. One barn is equal to 10-24
square centimeter. See also Cross section.
Beam-plasma interactions. A beam of energetic particles injected
into a plasma usually loses its energy not by colliding with the plasma
particles directly but by exciting collective modes of oscillation of
the plasma, which then damp into the energy of plasma particles.
The general study of such processes is given the name beam-particle
interactions.
Beta value. Ratio of the outward pressure exerted by the plasma to
the inward pressure that the magnetic confining field is capable of
exerting. Equivalent to the ratio of particle energy density to
magnetic-field energy density.
Black holes. The general theory of relativity predicts the existence of
a state of gravitational collapse in which the infalling matter ap-
proaches infinite density. Because of the time dilation that occurs in
general relativity, the collapse appears to an exterior observer to be
nearly frozen. No light can escape to the exterior world from the
interior of the object thus the name black hole. Black holes of
stellar mass, thought to be responsible for accreting galactic x-ray
sources, have a radius of about 10 km. Black holes in active galactic
nuclei have radii of about 109 km, smaller than the solar system and
very much smaller than the surrounding galaxy.
Blanket. Region surrounding a fusion reactor core, within which
fusion neutrons are slowed down, heat is transferred to a primary
coolant, and tritium is bred from lithium.
Breakeven. Condition that the fusion power produced in a plasma
exceed the power needed to maintain the plasma temperature. For
tritium plasmas heated by deuterium beams, the requirements for
breakeven are relaxed relative to the Lawson criterion for thermal
plasmas. See also Lawson criterion.
OCR for page 286
286 GLOSSAR Y
Bremsstrahlung. Radiation emitted as a result of deflection (e.g.,
through near collisions) of rapidly moving charged particles.
Brillouin instability. Decay of an intense laser light wave into a
scattered light wave and an ion acoustic wave. Occurs in the
low-density plasma surrounding an irradiated target in inertial-
confinement fusion.
Brillouin scattering.
acoustic-like plasma wave.
Reflection of electromagnetic radiation from an
Bumpy torus. A toroidal configuration created from many canted
simple-mirror sectors connected together to form a torus. Stability in
the regions of unfavorable magnetic curvature is generally provided
by hot mirror-confined electron rings. See also Elmo Bumpy Torus.
Carbon dioxide laser. A laser that produces intense light pulses in the
far-infrared portion of the spectrum, specifically with wavelength of
about 10 micrometers. See also Laser.
Central cell. The main, cylindrical confinement region of a tandem
magnetic mirror.
Cerenkov radiation. A distinct type of electromagnetic radiation
caused by electrons or protons traveling through or near matter.
Charge exchange. Process in which there is a transfer of an electron
from a neutral atom to a singly charged positive ion, the latter
becoming neutral and the former charged.
Charge neutrality. Refers to the strong tendency for a plasma to be
everywhere free of net electrical charge.
Classical confinement. Best possible limiting case for plasma con-
tainment. Only rare collisions between particles are considered as
the agent that can lead to cross-field losses. It is also referred to as
classical diffusion.
Coherent radiation. Monochromatic radiation in which all elements
of the wave radiate in unison.
Collective accelerator. A device in which charged particles are
accelerated in the electric and magnetic fields of neighboring
charges.
Collective effects. Simultaneous interaction of many charged parti-
cles, usually through their Coulomb force.
Collision. Close approach of two or more particles, atoms or nuclei,
during which such quantities as energy, momentum, and charge may
be exchanged.
Collisionless plasma. A plasma in which the density is so low, or
temperature so high, that close binary collisions have practically no
significance, because the time scales of interest are smaller than the
collision time.
OCR for page 287
GLOSSAR Y 287
Collisionless shock waves. A supersonic airplane creates a shock
wave, which causes the familiar sonic boom. The dissipation for
ordinary gas-dynamic shocks is due to collisions between gas
particles. High-temperature plasmas are collisionless. The dissipa-
tion for shock waves in collisionless plasmas is created by micro-
scopic collective plasma modes excited in the shock front.
Compact toroid. A toroidal confinement configuration utilizing
poloidal magnetic fields but no externally produced toroidal field. Its
compactness arises from the absence of toroidal field coils linking the
doughnut-shaped plasma.
Confinement parameter. The product of number density and energy
confinement time of a plasma. See also Lawson criterion.
Controlled thermonuclear fusion. Process In which very light nuclei,
heated to a high temperature in a confined region, undergo fusion
reactions under controlled conditions, with the associated release of
energy that may be harnessed for useful purposes.
Corona. In inertial confinement fusion, refers to the low-density
plasma surrounding the irradiated target. In the Sun, it refers to the
outermost layer composed of hot, tenuous plasma. See also Solar
_ . . . ~ . ~. .
corona.
Cosmic rays. Cosmic rays are energetic electrons, protons,
positrons, neutrons, neutrinos, atomic nuclei, and other particles
accelerated by cosmic processes. Some cosmic rays, typically below
109 eV energy, are accelerated at the Sun and in the solar system;
those above 109 eV energy are largely accelerated in our galaxy;
those above about 1017 eV energy are accelerated outside our galaxy.
Cosmic rays, elemental and isotopal abundances.
Elemental abun-
dance refers to the abundances of different atomic nuclei in the
cosmic radiation. Nuclear isotopes refer to atomic nuclei with the
same number of protons but differing numbers of neutrons. Studies
of these abundances reveal much about the material from which the
cosmic rays were accelerated, about how they were accelerated, and
the density of galactic material through which they have traveled.
Cross section. A measure of the probability that a reaction (nuclear
or other) will occur. Usually measured in barns, it is the apparent (or
effective) area presented by a target nucleus (or particle) to an
oncoming particle.
Current drive. The process by which the required toroidal currents
are created and maintained in a tokamak. In a conventional tokamak,
these currents are maintained for a finite pulse duration by inductive
transformer action but then decay. Currents can also be driven
OCR for page 288
288 G f OSSAR Y
noninductively by radio-frequency waves, in which case they may be
maintained indefinitely.
Cyclotron frequency. The natural oscillation frequency of a charged
particle immersed in an external magnetic field. One mode of
oscillation is associated with electron gyrations, the other with ion
gyrations.
Cyclotron radiation. Radiation emitted at the cyclotron frequency by
charged particles in a magnetic field as a result of their natural
gyration in that field. Sometimes called synchrotron radiation,
especially for very fast particles in which case the-radiation is
emitted at high harmonics of the cyclotron frequency.
Cyclotron resonance. Resonance absorption of energy from an alter-
nating electric field by electrons or ions in a magnetic field when the
frequency of the electric field equals the cyclotron frequency.
Debye shielding. Departure from the inverse-square law of interaction
between point charges caused by the presence of neighboring
charges.
Deuterium. An isotope of the hydrogen atom with one proton and one
neutron in its nucleus and a single orbital electron.
Deuteron. The nucleus of a deuterium atom.
Diagnostics. Procedure for determining (diagnosis), by one means or
another, exactly what is happening inside a plasma during an
experiment; also refers to the instruments used for diagnosing.
Direct conversion. Generation of electricity by direct recovery of the
kinetic energy of the charged fusion reaction products.
Direct illumination. Refers to an approach to inertial-confinement
fusion in which the target is directly irradiated by laser light.
Disruption. A sudden loss of plasma confinement in a tokamak,
resulting from inadequate control of kinklike instabilities.
Divertor. Component of a toroidal fusion device that provides a
magnetic field to divert charged particles in the outer shell of the
discharge into a separate chamber where they strike a barrier,
become neutralized, and are pumped away. In this way, energetic
particles in the outer shell are prevented from striking the walls of
the main discharge chamber and releasing secondary particles that
would cool the discharge.
Double layers. Current-carrying plasmas can create thin layers of
strong electric field when the current is sufficiently intense. Since
these layers contain sheets of positive and negative charge in close
proximity, they have been called double layers. It has been sug-
gested that double layers accelerate the particles responsible for the
aurora.
OCR for page 289
GLOSSAR Y 289
Drift waves. Oscillations in a magnetically confined plasma that arise
in the presence of density gradients.
Driver. A powerful laser or particle beam used in inertial confinement
fusion, to impart energy to a pellet, thereby causing it to implode.
Dynamo processes. A term used to describe the process by which the
energy in turbulent motions is converted to magnetic-field energy in
conducting fluids and plasmas.
Earth's magnetic tail. The interaction of the solar wind with the
Earth's magnetic field creates a long magnetic tail that extends more
than 1000 earth radii downstream of the Earth. The north lobe of the
tail contains a magnetic field that is directed toward the Earth, and
the south lobe contains flux directed away from the Earth. Jupiter's
magnetic tail is at least 5 AU long.
EBT. See Elmo Bumpy Torus.
ECRH. See Electron cyclotron resonance heating.
Electromagnetic radiation. Radiation consisting of electric and mag-
netic waves that travel at the speed of light and can be transmitted
through a vacuum.
Electron. Elementary particle with a unit negative electrical charge
and a mass very much smaller than that of the proton.
Electron bunching. Formation of tight electron clumps in space and
time.
Electron cyclotron resonance heating. Mode of heating of a plasma
by resonant absorption of energy based on waves induced in the
plasma at the cyclotron frequency of the electrons or at harmonics of
the cyclotron frequency.
Electron density. The number of electrons in a unit volume.
Electron-positron pair plasmas. Plasmas with very-high-energy den-
sities can contain significant numbers of positrons. Those plasmas
that contain only electrons and positrons are called pair plasmas.
Pulsars, for example, are thought to generate pair plasmas.
Electron temperature.
The temperature at which ideal gas molecules
would have an average kinetic energy equal to that of electrons in a
plasma under consideration.
Electron volt (eVJ. Unit of energy equal to the energy acquired by a
singly charged particle in passing through a potential difference of 1
volt. 1 eV = 1 .6 x JO- 19 joule. A plasma in which the particles have
an average energy of 1 eV has a "temperature" of about 10,000
degrees Celsius.
Electrostatic plugging. The use of a positive electrostatic potential in
the end cells of a tandem mirror to achieve axial confinement of ions
in the central cell.
OCR for page 290
290 GLOSSAR Y
Electrostatic potential. Refers to the ability of a point or region in a
plasma to attract or repel charged particles. For example, a region of
positive electrostatic potential will attract electrons and repel posi-
tively charged ions.
Elm o Bumpy Torus. A magnetic fusion concept in which high-beta
electron rings produced by microwave heating stabilize a bumpy
torus.
End cells. See End plugs.
End plugs. Minimum-B mirror cells at the ends of a tandem mirror,
within which mirror action provides the dominant mechanism for
confining beam-injected energetic ions, thereby providing a positive
electrostatic potential for the confinement of thermal ions in the
central cell.
Energy-confinement time.
Time required for a plasma to lose (via
radiation or other loss mechanisms) an amount of energy equal to its
average kinetic energy.
Fertile material. Nuclide that will convert to fissile material on
neutron capture and radioactive decay (e.g., uranium-238 or
thorium-232.
Field-reversed configuration. A confinement configuration of the
compact toroid class that utilizes poloidal magnetic fields only. The
configuration is generally formed on rapid, dynamic time scales.
Fissile material. Any material fissionable by neutrons of all energies,
especially including thermal (slow) neutrons as well as fast neutrons
(e.g., uranium-235 and plutonium-239.
Fluctuations. Refers to small-scale oscillations in a plasma, usually
caused by weak instabilities.
Flute instability. See Interchange instability.
Free-electron laser. A laser that uses free (as opposed to bound)
electrons as its active medium.
Free-electron radiation source. A source that uses free electrons
(from an electron gun, for example) to generate electromagnetic
radiation. See also Free-electron laser and Gyrotron.
Fusion. Merging of two light atomic nuclei into a heavier nucleus,
accompanied in general by the release of energy.
Fusion-fission hybrid. Reactor in which energy is produced by both
fusion and fission reactions. A fusion neutron source is typically
surrounded by a subcritical blanket containing fissile material. If
fertile material is also contained in the blanket, the reactor will
produce additional fissile material.
Gamma rays. Gamma rays are highly energetic photons with energies
exceeding about 50,000 electron volts.
OCR for page 291
GLOSSAR Y 291
Geometrical optics. A calculational technique for following the prop-
agation of electromagnetic waves by tracing the trajectories of rays
through a refracting medium, e.g., a plasma. The technique is valid
if the wavelength is much shorter than the scale size of the plasma.
Guiding center. The average center of rotation of a charged particle
in motion in a magnetic field.
Gyroirequency. See Cyclotron frequency.
Gyroradius. See Radius of gyration.
Gyrotron. A device for producing microwave energy that utilizes a
strong axial magnetic field in a cavity resonator to produce azimuthal
bunching of an electron beam. Also called an electron cyclotron
maser.
Heavy-ion beams. Beams of ions of heavy elements, such as ura-
nium, of gigavolt energies that could be produced in conventional
high-energy accelerators and might be used as drivers in inertial-
confinement fusion.
Helical. Spiraling. Usually refers to the trajectory of a field line or
charged particle in a configuration with both toroidal and poloidal
magnetic fields.
Hertz. Unit of frequency equivalent to one cycle per second.
Hohlraur';. A hollow chamber containing electromagnetic radiation in
thermodynamic equilibrium with the hot chamber walls. In its
application to inertial-confinement fusion, the laser light is shone into
the hohlraum and converted into x rays that serve as the pellet
driver.
Hybrid reactors. See Fusion-fission hybrid.
Hydrodynamic efficiency. In inertial-confinement fusion, refers to the
fraction of the absorbed driver energy that is delivered in kinetic
energy to the fuel.
Hydrodynamic instability. A process in inertial-confinement fusion in
which nonuniformities in target irradiation may be unstably amplified
by hydrodynamic motions of the target surface. Analogous to the
instability that develops when a heavier fluid is supported against
gravity by a lighter fluid.
Hydromagnetic instability. Instability arising from macroscopic mo-
tions of a conducting fluid as a result of its interaction with a
magnetic field.
Hydromagnetics. See Magnetohydrodynamics.
ICRH. See Ion cyclotron resonance heating.
Ideal MHD. Magnetohydrodynamic model for an assumed infinitely
conducting plasma.
Ignition. The high-temperature conditions at which the energy depos
OCR for page 292
292 GLOSSAR Y
ited in a plasma through the fusion process just equals the energy
losses.
Implosion. A violent inward compression.
Impurities. Ions in a fusion plasma of elements other than those of the
reacting hydrogenic fuel.
Inertial confinement. A dynamic, nonmagnetic plasma confinement
scheme that uses compressional inertial forces, e.g., laser radiation
compressing a D-T pellet.
Interchange instability. The type of hydromagnetic instability in
which the plasma interchanges position with the magnetic field; also
called a "cute instability," since it would be expected that the
interface between the plasma and the magnetic field would become
fluted.
Inverse bremsstrahlung. A process, inverse to that of bremsstrahlung
radiation, in which electromagnetic waves (especially laser light in
inertial confinement fusion) are absorbed by collisional defections of
electrons vibrating in the wave fields. Also called bremsstrahlung
absorption.
Ion. Atom that has become charged as a result of gaining or losing
one or more orbital electrons. A completely ionized atom is one
stripped of all of its electrons.
Ion acoustic wave. A longitudinal compression wave affecting the ion
density of a plasma.
Ion cyclotron resonance heating. Mode of heating a plasma by
resonant absorption of energy based on waves induced in the plasma
at the cyclotron frequency of the ions or at harmonics of the
cyclotron frequency.
Ion temperature. The temperature at which ideal gas molecules
would have an average kinetic energy equal to that of the ions in a
plasma under consideration.
Ionization. Process of removing an electron from a neutral atom,
thereby creating an ion.
Ionosphere (terrestrial, planetaryJ. Ionosphere refers to a layer of
plasma in the upper atmosphere that is maintained by photoioniza-
tion by solar ultraviolet radiation and by collisions with energetic
particles of magnetospheric and solar-system origin. In contrast to
magnetospheric plasmas, ionospheric plasmas are collision domi-
nated, since they interact with neutral atmospheric gases.
Irradiation. Exposure to radiation.
Isotope. One of several species of the same element, possessing
different numbers of neutrons (but the same number of protons) in
their nuclei.
OCR for page 293
GLOSSAR Y 293
JET. Joint European Torus, a large tokamak that is owned in
common by the European communities. It has been built at the
Culham Laboratory and is generally comparable with the TFTR.
JT-60. Large Japanese tokamak, currently under construction. It is
generally comparable with JET and TFTR but will not use tritium
plasmas.
Kilometric radiation. Beam-plasma interactions in the auroral accel-
eration region excite radio waves with wavelengths of a few kilom-
eters thus the term kilometric radiation.
Kink instability. Hydromagnetic instability that sometimes develops
in a plasma column carrying a strong axial current. The column
becomes unstable and undergoes a gross lateral displacement toward
the walls of the discharge vessel.
Klystron. An evacuated electron-beam tube in which an initial veloc-
ity imparted to electrons in the beam results subsequently in density
modulation of the beam; used as an amplifier or oscillator for
microwave radiation.
Krypton-fluoride laser.
A laser under development with wavelength
of about 0.25 micrometer. See also Laser.
Langmair wave. Same as a plasma wave.
Larmor radius. See Radius of gyration.
Laser. A device that utilizes the natural oscillations of atoms or
molecules between energy levels for generating intense, coherent
electromagnetic radiation in the ultraviolet, visible, or infrared
regions of the spectrum.
Laser fusion. Nuclear fusion process that occurs when a small pellet
of fuel material is compressed by a burst of laser light. See also
Inertial confinement.
Laser pumping.
A , ~ Use of electron beams or the radiation from intense
light sources, for example, to invert the population of lasing mate-
rials.
Lawson criterion. Condition that the product of number density and
confinement time of a plasma (confinement parameter) must equal
approximately 10'4 cm3-s at a temperature of about 70,000,000
degrees to produce net power in a fusion reactor.
Light-ion beams. Beams of protons, lithium, or carbon ions of a few
megavolt energies, generated by pulsed power accelerators and to be
used as drivers in inertial-confinement fusion.
Light-ion fusion. Inertial fusion concept using light ions (e.g., pro-
tons).
Limiter. A mechanical structure placed in contact with the edge of a
OCR for page 296
296 GLOSSAR Y
Microinstabilities. Small-scale plasma instabilities leading to fluctua-
tions and anomalous transport.
Micrometer. A unit of length used to measure very short distances,
such as the wavelength of laser light. One micrometer equals 10-6
meter.
Microscopic instability. Short-wavelength, high-frequency instability
causing fine-grained plasma perturbations and turbulence.
Microwaves. Electromagnetic radiation with a wavelength of a few
centimeters or less.
Minimum-B configuration. Name given to a magnetic configuration
that increases everywhere in strength with increasing distance from
the plasma that it is confining. In such a configuration, the plasma
finds itself in a region of minimum magnetic potential and is highly
stable.
Minimum-energy state. Refers to the condition in which a dynamical
system, such as a plasma, is in a configuration of minimum potential
energy and is therefore highly stable.
Mirror. See Magnetic mirror.
Mirror ratio. In a magnetic-mirror configuration, the ratio of the
strength of the magnetic field at the strongest point on its axis to the
weakest field strength between the two magnetic mirrors.
Monte Carlo method. A statistical technique for computing the
motion of a large number of individual particles.
Negative-ion beams. An advanced form of neutral injection in which
negative hydrogenic ions (i.e., with excess electrons) are used. The
neutralization efficiency, after acceleration to very high energies, is
superior to that of positive ions.
Neoclassical. Term used to characterize classical-like collisional
diffusion in finite toroidal geometries, where geometrical effects
enhance the transport relative to an infinitely long straight system.
Neodymium-glass lasers. A laser that produces intense light pulses in
the near-infrared portion of the spectrum, specifically with a wave-
length of about 1 micrometer. See also Laser.
Neutral-beam heating. See Neutral injection.
Neutral injection. A technique for heating plasmas in which
hydrogenic ions are accelerated to high energies, neutralized, in-
jected across the magnetic field of a confinement device, and
subsequently ionized by the plasma inside the magnetic container.
Neutron. Uncharged elementary particle with mass about the same
as that of the proton and found in the nucleus of every atom heavier
than hydrogen. The energy from fusion reactions appears mainly in
the form of energetic neutrons.
OCR for page 297
G~OSSAR Y 297
Neutron star. A condensed star that is stabilized against gravitational
collapse by the pressure of degenerate nucleons, which are as tightly
packed as quantum-mechanical law allows. The centers of large
neutron stars are composed primarily of neutrons, with a slight
admixture of electrons and protons, whereas the outer layers are an
exotic metal composed of neutron-enriched nuclei. Neutron stars are
believed to have superstrong magnetic fields. All pulsars are neutron
stars, and some galactic x-ray sources are due to mass accretion onto
a neutron star from a binary companion star.
Nonlinear. Refers to waves and instabilities that have reached am-
plitudes at which the disturbances are no longer a small perturbation
of the equilibrium.
Nonlinear wave. Large electromagnetic perturbation about the
plasma equilibrium state. r · 1 _~ _ __~ ~
Nonneutral plasma. Plasma composed of a single charged species
that is not electrically neutralized by a second species.
NOVA. A neodymium-glass laser facility under construction at the
Lawrence Livermore National Laboratory. Also, the result of ex-
plosion of a star with a mass of about one solar mass.
Nucleosynthesis. Nucleosynthesis refers to the processes by which
all elements of the periodic table, except hydrogen and helium, are
created by nuclear burning in stellar interiors.
Numerical models (fluid, MHD, hybrid, kinetic). Numerical models of
plasma dynamics can be created at both the fluid and kinetic levels
of description. Fluid models solve the equations of fluid dynamics,
and MHD models solve the corresponding magnetohydrodynamic
equations for magnetized, conducting fluids. Kinetic models follow
the motions of the individual electrons and ions in the self-
consistently calculated electric and magnetic fields of the plasma.
Hybrid models treat the ions kinetically and the electrons as a fluid.
Ohmic heating. Heating resulting from the resistance a medium offers
to the flow of electric current. In plasmas subjected to ohmic heating,
ions are heated almost entirely by transfer of energy from the hotter
electrons. Also called Joule heating.
Open system. See Magnetic mirror.
PBFA II. Particle Beam Fusion Accelerator II, a light-ion beam
accelerator to be used in inertial-confinement fusion, under construc-
tion at Sandia National Laboratories.
Parametric decay. Decay of one wave into two other waves. For
example, decay of an intense laser light wave into a plasma wave and
an ion acoustic wave.
Parametric instability.
Instability that occurs in a system whose
OCR for page 298
298 G. f OSSAR Y
equilibrium is weakly modulated in time or space. The modulation
produces a coupling of the linear modes of the unmodulated system
and can lead to destabilization. Interaction between three waves,
one of which (the pump) feeds energy to the other two.
Particle ring. A configuration of the compact toroid class that utilizes
energetic ion or electron rings to create a field-reversed configura-
tion.
Photons. The quanta of electromagnetic radiation.
Pinch elect. Constriction of a plasma column carrying a large
current, caused by the interaction of that current with its own
encircling magnetic field.
Pitch angle. The angle that a particle's velocity vector makes with
the direction of the magnetic field. See also Loss cone and Pitch-
angle scattering.
Pitch-angle scattering. A charged particle has a helical orbit in a
uniform magnetic field. The pitch of the helix is called the particle's
pitch angle, which is determined by the ratio of the components of
the particle's velocity parallel and perpendicular to the direction of
the magnetic field. When two particles collide, or one particle
interacts with a plasma wave, the pitch angles are changed or
scattered. Pitch-angle scattering leads to losses of mirror-confined
particles in the Earth's radiation belts and in fusion devices.
Plasma. Ionized gaseous system, composed of an electrically equiv-
alent number of positive ions and free electrons, irrespective of the
presence of neutral particles; in view of its prevalence throughout
the universe, sometimes called the fourth state of matter.
Plasma confinement. Operation intended to prevent, in an effective
and sufficiently prolonged manner, the particles of a plasma from
striking the walls of the container in which the plasma is produced.
Plasma cross section. Refers to the shape of the cross section formed
by cutting a doughnut-shaped toroidal plasma.
Plasma equilibrium. Plasma system in which there is an overall
quasi-steady balance of forces.
Plasma frequency. Natural frequency of oscillation of a plasma,
caused by the collective motion of the electrons acting under the
restoring force of their space-charge attraction to the relatively
stationary ions.
Plasma instability. State of a plasma in which a small perturbation
amplifies, resulting in an alteration of the equilibrium of the system.
Plasma radiation. Electromagnetic radiation emitted from a plasma,
primarily by free electrons undergoing transitions to other free states
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or to bound states of atoms and ions, but also by bound electrons as
they undergo transitions to other bound states.
Plasma wave. A disturbance of a plasma involving oscillation of its
constituent electrons at the plasma frequency. The term plasma
wave is often used more generally to denote collective modes of
oscillation in a plasma.
Polarization. All electromagnetic radiation can be characterized by
its frequency and the direction in which the electric field of the
electromagnetic waves oscillates. When the electric field averaged
over time takes on all possible directions (relative to the direction of
propagation of the radiation), the radiation is said to be unpolarized.
When the electric field has a preferred direction, or when its
direction rotates coherently, the radiation is polarized. The synchro-
tron process, for example, generates polarized radiation.
Poloidal diverter. A diverter that takes out poloidal magnetic field
lines, forming a separatrix in the poloidal field. See Divertor.
Poloidal field. A magnetic field that encircles a toroidal plasma the
short way around. See also Toroidal field.
Ponderomotive force. Radiation pressure exerted by an electromag-
netic wave on charged particles.
Positron. The laws of quantum electrodynamics allow the existence
of a particle conjugate to the electron, which has the same mass as
the electron but the opposite (positive) charge.
Proton. Elementary particle with a single positive electrical charge
and a mass more than 1800 times larger than that of the electron; the
nucleus of an ordinary hydrogen atom.
Pulsars. Rapidly spinning magnetized neutron stars that generate
beams of electromagnetic radiation, usually radio emissions, in their
rotating magnetospheres. These emissions are detected as periodi-
cally spaced pulses that repeat at the spin frequency of the parent
neutron star.
Pulse power system. High-current, high-voltage accelerator that pro-
duces short energy bursts by pulse compression.
Pumped limiter. An advanced form of mechanical limiter, containing
channels through which neutral gas formed by plasma recombination
can be pumped away. See also Limiter.
Quasars. Quasars appear as unresolved starlike images on astronom-
ical photographic plates. However, the spectrum of the radiation that
they emit is strongly red shifted, indicating that they are enormously
powerful objects located at great distances from our galaxy. Many
theoreticians believe that quasars derive their huge luminosity from
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accretion onto a massive black hole at the center of an otherwise
normal galaxy.
REP. See Reversed-field pinch.
Radar backscatter. The velocity distributions of the particles that
make up a plasma, and the collective fluctuations in the plasma, may
be diagnosed by directing a beam of electromagnetic radiation at the
plasma and analyzing the backscattered signal. Radar beams
backscattered from the ionosphere have provided valuable informa-
tion about ionospheric plasmas.
Radiation. Emission and propagation of energy by means of electro-
magnetic disturbances that display wavelike behavior. See also
Plasma radiation.
Radio-frequency heating. A technique for heating plasmas by the
absorption of the energy contained in electromagnetic waves
launched by an antenna or waveguide into the containment vessel.
Various types of radio-frequency oscillators can provide the power
sources for such techniques.
Radio galaxies. That class of active galaxies that radiate significant
quantities of energy in the form of radio emissions. A typical radio
galaxy has twin emission lobes aligned along a line that passes
through the center of the parent galaxy.
Radius of gyration. For a charged particle moving transversely in a
uniform magnetic field, the radius of curvature of the projection of its
path on a plane perpendicular to the field. Also known as the Larmor
radius.
Raman instability. Decay of an intense laser light wave into a
scattered light wave and a plasma wave. Occurs in the low-density
plasma surrounding an irradiated target in inertial-confinement fu-
s~on.
Ray tracing. See Geometrical optics.
Rayleigh-Taylor instability. See Hydrodynamic instability.
Reconnection. That class of plasma processes by which magnetic-
field topologies which would be stable in MHD change owing to
collisional or collective dissipation. The simplest case is a plane
neutral sheet in which the magnetic field lines are oppositely directed
above and below the neutral layer, where the magnetic field is zero.
Reconnection causes the originally oppositely directed field lines to
pass through the neutral sheet and connect to one another. Recon-
nection converts magnetic energy into particle energy.
Refraction. Bending of oblique incident rays as they pass from a
medium having one refractive index into a medium having a different
refractive index.
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Relativistic particles. Classical Newtonian mechanics becomes in-
valid for particle velocities approaching the speed of light, where
Einstein's theory of relativity must be used. Particles whose veloc-
ities are close to the speed of light are said to be relativistic.
Resistive instability. Instability resulting from macroscopic motion of
a plasma with a finite electric conductivity. Resistive instabilities are
generally much weaker than hydromagnetic instabilities.
Resistive MHD. Magnetohydrodynamic model that allows for finite
plasma resistivity.
Reversed-field pinch. A toroidal configuration that utilizes the pinch
effect to confine a plasma carrying a large toroidal current and
provides stability against kink and interchange modes by introducing
a relatively weak, externally imposed toroidal magnetic field that
reverses its direction at the edge of the plasma.
Rotational transform. A toroidal magnetic-field configuration is said
to possess rotational transform if the lines of force, after one circuit
around the configuration, do not close exactly on themselves but are
rotated through some angle called the rotational transform angle.
Safety factor. The inverse of the rotational transform of a toroidal
magnetic-confinement system. In a tokamak, the value of the safety
factor must exceed unity to avoid kink instabilities.
Scattering. Deflection of one particle as a result of collisions with
another.
Second stability regime. Refers to a regime of toroidally confined
plasmas that is free from ballooning instabilities at arbitrarily large
beta values.
Separatrix. A magnetic surface that separates regions of closed field
lines from regions of open field lines.
Solar corona. The solar corona is the tenuous, outermost plasma
atmosphere of the Sun. It is heated to a temperature of about 1
million kelvins and extends to a distance of several solar radii from
the optical surface (photosphere) of the Sun. It ultimately blends into
the solar wind.
Solar coronal holes. Solar coronal holes are open magnetic-field
configurations in which the corona in the solar wind is generated.
Since the plasma inside the open configuration is cooler than in the
surrounding corona, it emits less x radiation. Thus, the open regions
appear as holes in generally bright x-ray photographs of the Sun.
Solar coronal loops.
Closed magnetic structures in the solar corona,
which take the characteristic form of loops emerging from the visible
surface of the Sun.
Solar pare. A solar flare is believed to be triggered by the rapid
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conversion of magnetic to particle energy in the solar corona. Strong
flares generate intense radio, optical, and x-ray emissions and launch
strong shock waves in the solar wind that propagate throughout the
solar system.
Solar photosphere. The outermost layer of the Sun, as observed in
optical light. It is the region from which most of the Sun's optical
photons escape into space.
Solar (stellarJ chromosphere. A thin layer between the photosphere
and corona, in which atomic spectral lines can be observed in
emission rather than absorption.
Solar-terrestrial physics. The study of the chain of processes, almost
all of which involve plasma physics, that links the generation of the
Sun's magnetic field deep inside the Sun, solar surface and coronal
magnetic activity, generation of the solar wind, the dynamics of the
Earth's magnetosphere, and the Earth's ionosphere and upper
atmosphere.
Solar wind. A supersonic, super-Alfv~nic plasma wind that is gener-
ated in open magnetic structures in the solar corona (solar coronal
holes) and streams throughout interplanetary space.
Soliton. Large-amplitude wave pulse that preserves its amplitude and
shape.
Space charge.
interact.
Spheromak. A confinement configuration of the compact toroid class
that utilizes toroidal and poloidal magnetic fields of comparable
magnitude but differs from the tokamak and reversed-field pinch in
that the toroidal field is produced entirely by currents within the
plasma. The configuration is generally formed by quasi-static induc-
tive techniques.
Stellarator. A toroidal confinement configuration that uses the com-
bination of a toroidal magnetic field and an additional field created by
helical windings. This magnetic configuration provides a rotational
transform in itself and permits containment in the absence of an axial
current in the plasma.
Strongly coupled plasma. Dense plasma in which the electrical energy
is comparable in magnitude with the particle kinetic energy.
Substorms. Rapid how reconfigurations of the Earth's magnet-
osphere, which are thought to be induced by changes in the direction
of the solar-wind magnetic field and triggered by reconnection in the
magnetic tail. Substorms greatly intensify the aurora in the upper
atmosphere.
Sunspot. Cool dark regions observed optically in the solar
Local electric fields through which charged particles
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photosphere. Sunspots contain concentrations of strong magnetic
fields that connect to the corona above the photosphere and to the
solar convection zone below.
Super-Alfvenic.
Alfven wave.
A plasma flow speed that exceeds the speed of an
Superconductor. Type of conductor that permits an electrical current
to flow with zero resistance. Superconducting coils are expected to
be used as electromagnets in most types of fusion reactors.
Supernova. An explosion of an entire star.
Supernova remnant. The remains of a star's outer layers that are
observed to expand at high speeds outward into space following a
supernova explosion.
Superthermal electrons. Electrons in a plasma that have been accel-
erated by dc or fluctuating electric fields to energies much greater
than the average thermal energy, or temperature, of the plasma.
Synchrotron radiation. Relativistic particles in a magnetic field emit a
broadband continuous spectrum of synchrotron radiation, which was
first observed in high-energy particle accelerators, called synchro-
trons.
Tandem mirror. A magnetic-mirror configuration in which two
minimum-B mirror cells are used to plug the ends of a much larger,
cylindrical central mirror cell.
Target. In inertial-confinement fusion, refers to the pellet of D-T fuel
that is to be imploded by the laser light or particle beam.
Tearing instability. Resistive instability that grows at a rate slower
than the MHD rate but faster than the skin diffusion rate. The
instability tears poloidal field lines and reconnects them into a new
state of lower magnetic energy.
Terrella. Terrella means little Earth. The term is used to refer to
laboratory experiments designed to simulate the interaction of the
solar wind with the Earth's magnetosphere, in which a high-speed
Rowing plasma is directed toward a dipole magnetic field.
TFTR. Tokamak Fusion Test Reactor, a large toroidal device at
Princeton Plasma Physics Laboratory operating as a standard
tokamak capable of modest adiabatic compression. In addition to
operating as a hydrogen experiment, it will be capable of injecting
high-energy neutral deuterium into a tritium plasma, thereby produc-
ing a D-T plasma under breakeven reactor conditions.
Thermal barrier. Regions, located between the central cell and the
end plugs of a tandem mirror, in which the electrostatic potential is
driven negative by local electron heating and which serve to isolate
the electrons in the central cell from those in the end plugs, thereby
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increasing the efficiency with which the positive electrostatic plug-
ging potentials can be maintained.
Thermal conductivity (di;ffusivity>. Quantities that measure the rate at
which energy (heat) can be transported across (or along) the mag-
netic field in a plasma with a temperature gradient.
Thermonuclear burn. The process in which alpha particles from D-T
fusion reactions can sustain the plasma temperature, thereby pro-
longing the reacting conditions until much of the D-T fuel is
consumed.
Thermonuclear conditions. Achievement of an adequately confined
plasma having temperature and density sufficiently high to yield
significant release of energy from fusion reactions.
Tokamak. Name given to a specific magnetic-field geometry in con-
trolled fusion, involving confinement and heating of a plasma in a
toroidal configuration. A large current induced in the plasma pro-
vides the rotational transform necessary for confinement while
simultaneously heating the plasma.
Toroidal confinement. Name given to the general class of doughnut-
shaped magnetic fields in which lines of force close on themselves.
Stellarators, tokamaks, and reversed-field pinches are examples of
this class of devices.
Toroidal field. The main confining magnetic field, which encircles a
toroidal plasma the long way around. See also Poloidal field.
Toroidal-field coils. Coils in a toroidal system that provide the main
confining field. Each turn completely surrounds the minor axis of the
plasma.
Torsatron. A modification of the stellarator concept, in which both
toroidal and poloidal fields are generated by helical windings alone.
Transmission line. High-voltage coaxial system used to transmit
high-voltage pulsed currents from the accelerator to the plasma
target.
Trapped-particle instability. Slowly growing class of instabilities
driven by particles that cannot circulate freely in a toroidal system.
Trapped particles. Those particles in a toroidal configuration that are
unable to circulate freely around the torus but are reflected from
regions of relatively high field, as in a magnetic mirror. The term also
applies to particles trapped in the Earth's dipole field.
Tritium. An isotope of the hydrogen atom with one proton and two
neutrons in its nucleus and a single orbital electron.
Triton. The nucleus of a tritium atom.
Turbulence. Random mixing of large electromagnetic perturbations.
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Velocity space instability. A class of instabilities driven by particle
distributions that are not in thermal equilibrium
Waveguide. A device for transmitting relatively short-wavelength
electromagnetic waves into a plasma-containment vessel.
Wavelength. The length of a wave measured from one point to the
corresponding point on the next wave, usually measured from crest
to crest.
Wave-particle interactions. The interactions between particles and
collective modes of oscillation (waves) in a plasma are wave-particle
interactions. These lead to much more dissipation than do collisions,
when the plasma temperature exceeds about 10,000 kelvins.
X ray. A form of electromagnetic radiation emitted either when the
inner orbital electrons of an excited atom return to their normal state
or when a metal target is bombarded with high-speed electrons.
Z-pinch. Plasma that pinches because of high electrical currents and
self-magnetic fields.
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Representative terms from entire chapter:
electromagnetic radiation
