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Conventional Fusion FAQ Glossary Part 1/26 (A)

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Archive-name: fusion-faq/glossary/a
Last-modified: 4-Feb-1995
Posting-frequency: More-or-less-monthly
Disclaimer: While this section is still evolving, it should
be useful to many people, and I encourage you to distribute
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Glossary Part 1:  Terms beginning with "A"


Edited by Robert F. Heeter,

Guide to Categories:

* = vocabulary specific to plasma/fusion/energy research
& = basic/general physics vocabulary
> = device type or machine name
# = name of a constant or variable
! = scientists
@ = acronym
% = labs & political organizations
$ = unit of measurement

The list of Acknowledgements is in Part 0 (intro).


# A: symbol used to indicate either area or magnetic
vector potential.

$ A: abbreviation for Amperes; see entry.

@ AAPT: American Assocation of Physics Teachers; see entry

@ AC: Alternating Current; see entry.

@ ACT-I: Advanced Concepts Torus I; see entry.

@ AEC: (US) Atomic Energy Commission; see entry

@ AIC: Alfven Ion Cyclotron Instability; see entry

@ AIP:  American Institute of Physics; see entry

@ AJP:  American Journal of Physics; see entry

@ ALEX:  (see entry "ALEX" below)

@ AMBAL:  (see entry "AMBAL" below)

@ ANL: Argonne National Laboratory; see entry

@ ANS:  American Nuclear Society; see entry

@ APS:  American Physical Society; see entry

@ APS-DPP:  American Physical Society - Division of Plasma
Physics; see entry.

# Ar:  Chemical symbol for the element Argon

@ ARIES: Advanced Reactor Innovative Engineering Study (?)
           See Entry under ARIES

@ ASDEX: Axially Symmetric Divertor EXperiment; see entry

@ ASDEX-U: ASDEX-Upgrade; see entry for ASDEX.

@ ASME:  American Society of Mechanical Engineers

@ ATF:  Advanced Toroidal Facility; see entry.

* Absolute Instabilities: A class of plasma instabilities
growing exponentially with time at a point in space, in
contrast to convective instabilities (see entry).

* Absorption: In plasma physics, the loss of (electromagnetic)
energy to a medium.  For instance, an electromagnetic wave
which propagates through a plasma will set the electrons
into motion.  If the electrons make collisions with other
particles, they will absorb net energy from the wave.

* Absorption Coefficient: Measures the degree of wave
absorption (see Absorption above); defined as the fraction
of wave energy lost as the wave travels a unit distance.

& Activation: Activation occurs when a particle interacts
with an atomic nucleus, shifting the nucleus into an
unstable state, and causing it to become radioactive.
In fusion research, where deuterium-tritium is a common
fuel mixture, the neutron released when (D + T) combine
to form (4He + n) can activate the reactor structure.
Sometimes called "radioactivation."  See also activation
product, activation analysis.

& Activation Analysis: Method for identifying and measuring
chemical elements in a sample of material.  Sample is first
made radioactive by bombardment with neutrons, charged
particles, or gamma rays.  Newly formed radioactive atoms
in the sample then give off characteristic radiations
(such as gamma rays) that tell what kinds of atoms are
present, and how many.

* Activation Product: The unstable nucleus formed when
activation occurs.  (See activation above.)

& Adiabatic:  Not involving an exchange of heat between the
system said to be adiabatic and the rest of the universe.

& Adiabatic Compression:  Compression (of a gas, plasma, etc.)
not accompanied by gain or loss of heat from outside the system.
For a plasma in a magnetic field, a compression slow enough that
the magnetic moment (and other adiabatic invariants - see entry)
of the plasma particles may be taken as constant.

* Adiabatic Invariant:  Characteristic parameters which do not
change as a physical system slowly evolves; the most commonly
used one in plasma physics is the magnetic moment of a charged
particle spiraling around a magnetic field line.

* Aftercooling:  Cooling of a reactor after it has been
shut down.

* Afterglow:  Recombination radiation emitted from a cooling
plasma when the source of ionization (heating, etc) is removed.
(See entry for recombination radiation.)

* Advanced Fuels:  There are several elements/isotopes which
could be fused together, besides the DT fuel mixture.  Many such
fuel combinations would have various advantages over DT, but
it is generally more difficult to achieve fusion with these
advanced fuels than with the DT mix.  See fuels section of FAQ
for discussion.

> Advanced Concepts Torus I: (ACT-I)  A steady-state toroidal
device built primarily for studies of RF heating (see entry)
and RF current drive (see entry).  Operated at PPPL but shut
down several years ago.

> Advanced Toroidal Facility: (ATF) A large stellarator device
developed at Oak Ridge National Lab (ORNL), but recently mothballed.
See Section 5 for more information.

> Alcator: Name given to a set of tokamaks designed and built at MIT;
these machines are distinguished by high magnetic fields with
relatively small diameters.  The high magnetic field helps create
plasmas with relatively high current and particle densities.
The current incarnation is Alcator C-mod, and is described further
in Section 5.  Alcator C was donated to LLNL for use as the
Microwave Tokamak eXperiment (MTX), now shut down.

> Alcator A:  First of the Alcator series of tokamaks at MIT;
was operational from 1969 to 1982.

> Alcator C:  Commissioned in 1978; used extensively to study
plasma confinement under strong ohmic heating (see entries).
Also studied high-density plasmas and used frozen fuel pellet
injection.  Set record values of Lawson product (density
times confinement time; see entries) of 8 x 10^19 m^3-seconds.
Was donated to Livermore (LLNL; see entry) for use as the
Microwave Tokamak eXperiment (MTX: see entry), now shut down.

> Alcator C-mod:  Successor to Alcator C; actually a completely
new device.  Currently operational; described in more detail in
Section 5.

> Alcator DCT:  Proposed fully-superconducting extension of
the Alcator series; never built.

* Alcator Scaling:  A proposed empirical law in which the
energy confinement time is proportional to the product
of the average density and the square of the plasma radius
(see relevant entries).

> ALEX:  A single-cell, minimum-B magnetic mirror system
(see entries) in which the magnetic field was generated by a
baseball coil (see entry) wound on a 60 cm sphere.  Formerly
operated at the Rensselaer Polytechnic Institute, Troy, New York.

* Alfven Ion Cyclotron instability: (AIC)  An electromagnetic
microinstability near the ion cyclotron frequency; driven by
the ion loss cone in a mirror device.  (See relevant entries.)

* Alfven velocity:  Phase velocity of the Alfven wave;
equal to the speed of light divided by the square root
of (1 plus the ratio of the plasma frequency to the cyclotron
frequency for a given species).  i.e.,

     Va = c / SQRT(1 + plasma freq. / cyclotron freq.)

(As defined in Stix, _Waves in Plasmas_, 2nd ed. 1992, p. 31)

* Alfven waves: Transverse electromagnetic waves that are
propagated along lines of magnetic force in a plasma.  The waves
have frequency significantly less than the ion cyclotron frequency,
and are characterized by the fact that the field lines oscillate
(wiggle) with the plasma.  The propagation velocity depends on the
particle density and the strength of the magnetic field.  "[Relatively]
Low frequency ion oscillation in the presence of an equilibrium
magnetic field.  Also called the transverse hydromagnetic wave along Bo.
The torsional Alfven wave in cylindrical geometry was first measured
in liquid mercury by B. Lehnert.  Alfven waves were first generated
and detected in plasma by Allen, Baker, Pyle, and Wilcox in Berkeley
and by Jephcott in England in 1959."  (quoting from Chen's book;
see bibliography) - Albert Chou

! Alfven, Hannes Olof:  Nobel Prize-Winning Plasma Physicist
and Astronomer who first suggested the possibility of MHD waves
in 1942.

* Alpha Channeling:  Term for an idea (so far theoretical)
in magnetic confinement fusion; the idea is that plasma
waves can be used to control the alpha particles produced
in a fusion reactor, to transfer their energy directly to fuel
ions, and to help push them out of the plasma.  This could
potentially help to substantially improve the power output
capabilities of fusion plasmas.

* Alpha emission:  Form of nuclear decay where the nucleus
emits an alpha particle (see entry below).

* Alpha particle:  The nucleus of a Helium-4 atom; is a
typical product of fusion reactions; also released
in various nuclear decay processes.  Alpha particles readily
grab electrons from other sources, becoming neutral helium;
even energetic alpha particles are easily stopped by thin
barriers (sheets of paper, dead layers of skin, etc.), so that
as a radiological hazard alpha particles are only dangerous if
they are generated inside one's body (where the skin cannot
protect tissue from damage).  Alpha particles are common
products in fusion reactions between light elements.

& Alternating Current: (AC) Electrical Current (see entry) which
alternates in direction with time.  (For instance, household
electric current is AC alternating at 60 oscillations/sec
(60 Hertz) in the United States, and 50 Hertz in many other

> AMBAL:  An ambipolar trap (tandem mirror) located at
Novosibirsk in Russia.  (Any additional information would
be welcome.)

* Ambipolar Diffusion:  Diffusion process in which buildup
of spatial charge creates electric fields which cause
electrons and ions to leave the plasma at the same rate.
(Such electric fields are self-generated by the plasma
and act to preserve charge neutrality.)

% American Association for the Advancement of Science: (AAAS)
Organization dedicated to promoting science research and
education in the United States.  Publishers of _Science_.

% American Association of Physics Teachers: (AAPT)  Professional
society of physics teachers in the United States.  Organizes
conferences on physics education.  Publishers of _American
Journal of Physics_ (AJP)

% American Institute of Physics: (AIP)  Organization dedicated
to promoting physics research and the dissemination of physics
knowledge; publishers of many physics books.

% American Nuclear Society: (ANS)  Professional society of nuclear
scientists in the United States.

% American Physical Society: (APS)  Professional society of physicists
in the United States.  Organizes major conferences and publishes
many peer-reviewed journals.

% American Physical Society - Division of Plasma Physics: (APS-DPP)
Branch of the APS for plasma physicists, including fusion scientists.
The Annual Meeting of the APS-DPP is the largest plasma physics
conference in the United States.

$ Ampere, kiloampere, megampere:  (from Herman) The standard
unit for measuring the strength of an electric current
representing a flow of one coulomb of electricity per second.
1 kiloampere = 1000 amperes; 1 megampere = 1,000,000 amperes.
Common abbreviations:  A, amps, kiloamps, megamps, kA, MA

! Ampere, Andre-Marie (1775-1836):  French physicist responsible
for much of what is known about the fundamentals of electromagnetism.

& Ampere's Law:  General equation in electromagnetism relating
the magnetic field and the currents generating it.

* Aneutronic Fuels:  Advanced fusion fuels which would not
produce fusion neutrons.  See fuels section of FAQ for discussion.

$ Angstrom:  A unit of distance equal to 10^-10 meters or 10^-8 cm.

& Angular Momentum:  Momentum involved in the rotation of a body
about an axis; conserved like ordinary momentum (see momentum).
Angular momentum is defined as the cross product of ordinary momentum
with the position vector running from the axis of rotation to the
body whose angular momentum is being determined.  Torque is the
rate of change of angular momentum with time.  (see also torque)

& Anisotropy:  Term used to describe a medium whose characteristic
properties vary in with direction of travel through the medium.
(e.g., velocity of light transmission, conductivity of heat or
electric current, compressibility, etc.)

* Anomalous Diffusion:  Diffusion in most plasma devices,
particularly tokamaks, is higher than what one would predict from
understood causes.  The observed, "typical" diffusion is referred to
as "anomalous" because it has not yet been explained.  Anomalous
diffusion includes all diffusion which is not due to collisions
and geometric effects.  While such effects were not understood
when the term was coined, and most still aren't, diffusion due
to well-understood wave phenomena is still 'anomalous'.  "Classical"
diffusion and "Neo-classical" diffusion are the two well-understood
diffusion theories, neither is adequate to fully explain the observed
"anomalous" diffusion.  See also:  entries for classical and
neoclassical diffusion.  (Acknowledgements to Philip Snyder)

* Antares:  Laser-target irradiation system (i.e., laser fusion
research device) at Los Alamos National Lab; was operational in
1982.  (The author would welcome current information.)

& Aperture:  The opening in an optical system which restricts the
size of the bundle of rays incident on a given surface.  (Usually
circular and specified by diameter.)

* Applied-B Diode:  An ion diode with an applied magnetic field
to prevent electrons flowing from cathode to anode.  The applied
magnetic field also regularizes the electron swarm to reduce
beam divergence.

* Arc:  A type of electrical discharge between two electrodes;
characterized by high current density.  Similar in meaning
to "spark" in common language.

% Argonne National Laboratory:  One of the U.S. Department of Energy
basic-research Laboratories, located in Illinois... (need more info!)

> Argus:  Two-beam, 5-terawatt Nd-glass laser system used at Livermore
(LLNL) for inertial-confinement fusion research from 1976 to 1981.

* ARIES: Set of four fusion reactor design studies which investigated
the safety, economic, and environmental implications of various
advances in fusion reactor science and technology.

* Ash:  Fusion reaction products trapped in a plasma.  Ash is
bad because (a) it generally radiates more strongly than the fuel
ions, and thus reduces energy confinement, and (b) it creates
additional plasma pressure and/or reduces pressure available for fuel
ions. (due to beta limits, see beta)  Controlling ash is a major
area of fusion research.  Ideally one would be able to extract
the ash ions after diverting an appropriate fraction of their
energy to heating the fuel ions, and then convert the remaining ash
energy to electricity.  Current research involves using RF waves to
transfer energy from ash ions to fuel ions, and to push the ash into
the scrape-off layer, where it can be collected via divertors.
(See also scrape-off layer, divertors)

* Ash control - see ash, divertors.

* Ash removal - see ash, divertors.

* Aspect Ratio:  In toroidal geometry, the ratio of
the major diameter (total width of the torus) to the
minor diameter (width of a slice taken through one side
of the ring).  (This would be much better with a picture!)

In inertial-confinement fusion, aspect ratio refers to
the ratio of a fuel pellet's radius to its wall thickness.

& Atom:  (from Herman)  The smallest unit of an element that
retains the characteristics of that element.  At the center
of the atom is the nucleus, made up of neutrons and protons,
around which the electrons orbit.  Atoms of ordinary hydrogen,
the lightest element, consists of a nucleus of one proton
orbited by one electron.  (Note:  distinct from a molecule,
which is the smallest unit of a substance which retains the
characteristics of that substance.  It takes far less
energy to break apart a stable molecule into its constituent
atoms than to divide a stable atom into two smaller atoms.)
Note that in solids, atoms are typically two angstroms
(2 x 10^-10 meters) apart; in air the gas molecules are about
30 angstroms apart.  A drop of water has on the order
of 10^21 atoms in it.  Atoms are generally electrical neutral;
when an atom acquires an electrical charge (by gaining or
losing electrons) it is usually called an ion.

& Atomic Bomb, A-Bomb:  (from Herman) A weapon with a large
explosive power due to the sudden release of energy when the
nuclei of heavy atoms such as plutonium-239 or uranium-235
are split.  This fission is brought about by the bombardment
of the fuel with neutrons, setting off a chain reaction.
The bomb releases shock, blast, heat, light, and lethal
radiation.  The world's first atomic bomb was successfully
tested by the United States on July 16, 1945.

% Atomic Energy Commission: United States governmental
authority for atomic energy; split into ERDA and NRC in 1975.
(may not be 100% correct)

& Atomic Mass:  Mass of an atom relative to 1/12th the mass
of a carbon atom.  Approximately equal to the sum of the
number of protons and neutrons in the nucleus of the atom.

& Atomic Number (Z):  The number of protons in a nucleus; same
as the number of electrons in a neutral atom; determines the
position of an element in the periodic table, and hence its
chemical properties (see also isotope).

* Atomic Temperature:  The temperature corresponding to the mean
kinetic energy of the neutral atoms in a plasma.  (If there were
no ions or electrons, the atomic temperature would be what we
normally think of as the temperature of a gas, such as the air.)

* Auger effect:  Transition of an electron in an atom from a
discrete electronic level to an ionized continuous
level with the same energy; also known as autoionization.

& Avogadro's number: N = 6.02497 x 10^23.  Number of particles
in a mole of a substance.  Coefficient relating Boltzmann's
constant to the ideal gas constant. This is the number of
atoms per gram-atom.  See also: mole

> Axially Symmetric Divertor EXperiment (from Herman)
(ASDEX, Asdex: Garching, Germany)  A large tokamak designed
for the study of impurities and their control by a magnetic
divertor.  The H mode or high mode of operation with neutral
beam injection was first observed on ASDEX.

> Axially Symmetric Divertor EXperiment (ASDEX, Asdex):  "The original
ASDEX, located in Garching, Germany and decommisioned in 1990(?),
would qualify today as a medium-sized tokamak. It was designed for
the study of impurities and their control by a magnetic divertor.
The H mode or high mode of operation with neutral beam injection was
first observed on ASDEX.  Its successor ASDEX-Upgrade (a completely
new machine, not really an "upgrade") is larger and more flexible.
It is the first tokamak whose toroidal and poloidal field coils are
not linked, which will be a necessary design factor in a reactor.
It will achieve parameters at the edge which are very similar to
those needed for a power reactor." - Arthur Carlson

* Azimuth:  An angle measured clockwise relative to some
reference point on a circle (e.g., "south" or "north").

* Azimuthal: Generally an angle, measured "around" an object.
In spherical geometries, the angle which is *not* the "polar angle".
On the earth, one incarnation of the azimuthal angle is the longitude
of a location relative to the prime meridian through Greenwich,
England.  In toroidal geometries, the longitude idea still applies,
but the other angle is the "poloidal" angle, not the "polar" angle.
The azimuthal direction is the "long way" around a torus.
See also: poloidal.

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