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Subject: Conventional Fusion FAQ Glossary Part 1/26 (A)

This article was archived around: 11 Nov 1999 12:24:46 GMT

All FAQs in Directory: fusion-faq/glossary
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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 it to anyone who might be interested (and willing to help!!!).
=============================================================== Glossary Part 1: Terms beginning with "A" FREQUENTLY USED TERMS IN CONVENTIONAL FUSION RESEARCH AND PLASMA PHYSICS Edited by Robert F. Heeter, rfheeter@pppl.gov 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). ================================================================== AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA # 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 countries.) > 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.