H
Glossary


Advanced Reactor Innovation Evaluation Study (ARIES):

A comprehensive study of tokamak fusion power plants undertaken by a collaboration of U.S. fusion laboratories in the early 1990s. Four designs were studied: ARIES-I, a device based on modest extrapolations from the tokamak physics database; ARIES-II and ARIES-IV, two second stability devices, which differed in their fusion power core composition; and ARIES-III, which, unlike the others, utilized the deuterium-helium-3 fusion reaction instead of the deuterium-tritium reaction. Other more advanced configurations have been studied as well; ARIES-RS used a reversed-shear (RS) tokamak while ARIES-AT studied an advanced tokamak (AT).

Advanced tokamak (AT):

A tokamak that would operate continuously, with the current driven by a combination of noninductive external drive and the natural pressure-driven currents that occur in plasmas. ATs require careful optimization of pressure and confinement. The continuous operation is highly desirable for fusion power production.

Alfvén wave:

A fundamental plasma phenomenon that is primarily magnetohydrodynamic in character, involving oscillation of the magnetic field and, in some cases, plasma pressure. In tokamaks, these waves are typically strongly damped (they would spontaneously decay if externally excited).

Alpha particles:

He2+, a positively charged particle consisting of two protons and two neutrons; denoted by the Greek letter alpha (α); a helium-4 nucleus. An alpha particle is a typical product of fusion reactions.



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Burning Plasma: Bringing a Star to Earth H Glossary Advanced Reactor Innovation Evaluation Study (ARIES): A comprehensive study of tokamak fusion power plants undertaken by a collaboration of U.S. fusion laboratories in the early 1990s. Four designs were studied: ARIES-I, a device based on modest extrapolations from the tokamak physics database; ARIES-II and ARIES-IV, two second stability devices, which differed in their fusion power core composition; and ARIES-III, which, unlike the others, utilized the deuterium-helium-3 fusion reaction instead of the deuterium-tritium reaction. Other more advanced configurations have been studied as well; ARIES-RS used a reversed-shear (RS) tokamak while ARIES-AT studied an advanced tokamak (AT). Advanced tokamak (AT): A tokamak that would operate continuously, with the current driven by a combination of noninductive external drive and the natural pressure-driven currents that occur in plasmas. ATs require careful optimization of pressure and confinement. The continuous operation is highly desirable for fusion power production. Alfvén wave: A fundamental plasma phenomenon that is primarily magnetohydrodynamic in character, involving oscillation of the magnetic field and, in some cases, plasma pressure. In tokamaks, these waves are typically strongly damped (they would spontaneously decay if externally excited). Alpha particles: He2+, a positively charged particle consisting of two protons and two neutrons; denoted by the Greek letter alpha (α); a helium-4 nucleus. An alpha particle is a typical product of fusion reactions.

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Burning Plasma: Bringing a Star to Earth Auxiliary heating: Power applied to tokamaks to raise the internal temperature when the contribution from ohmic heating is relatively small. Auxiliary heating usually uses neutral beams or radio-frequency waves. Beta: β = p/(B2/2μo). The ratio of plasma gas pressure (p) to magnetic field pressure (B2/2μo) in a tokamak; p is the gas pressure in pascals (newtons per square meter), B is the magnetic field strength in teslas, and μo = 4π × 10–7 henrys per meter. Beta limit: Maximum beta attainable, usually resulting from a deterioration in the confinement. Blanket: The physical system surrounding the hot plasma. It provides shielding and absorbs fast neutrons, converts the energy into heat, and produces tritium. Blanket technology for the practical application of harnessing fusion energy is still under development. The ultimate design may include a liquid metal such as molten lithium, which produces tritium when it captures neutrons. Bootstrap current: In 1970, theorists predicted that a toroidal electric current will flow in a tokamak that is fueled by energy and particle sources that replace diffusive losses. This diffusion-driven bootstrap current, which is proportional to beta and flows even in the absence of an applied voltage, could be used to provide the confining magnetic field: hence the concept of a bootstrap tokamak, which has no toroidal voltage. A bootstrap current consistent with theory was observed many years later on the Joint European Torus and the Tokamak Fusion Test Reactor; it now plays a role in the design of experiments and power plants (especially advanced tokamaks). Burning plasma: A fusion plasma in which alpha particles from the fusion reactions provide the dominant heating of the plasma. Confinement: The containment of plasma particles and energy within a container for some extended period of time. A fusion reactor must confine the fuel plasma long enough at high enough density and temperature in order to be economically feasible. Confinement, magnetic: A method of containing a plasma or charged particles in a finite region using magnetic fields. Charged particles travel in helical paths around the magnetic field lines, which confine their motion to the local vicinity of the magnetic field. A properly shaped magnetic field prevents particles from escaping the confining field. A tokamak is one example of a magnetic-confinement device.

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Burning Plasma: Bringing a Star to Earth Confinement time: The amount of time it takes for energy or particles to leave the plasma. Current distribution: The variation of plasma current density within the plasma, usually expressed as a function of the distance from the magnetic axis. Current drive: Any of a number of means to maintain or increase electrical current in a plasma by using external devices such as neutral-beam or radio-frequency power generators. Deuterium: Isotope of hydrogen having one proton and one neutron in its nucleus and an atomic mass of 2. Deuterium behaves like hydrogen in chemical reactions, but behaves much differently in nuclear reactions. Deuterium-tritium (D-T) reaction: The fusion of a deuteron and a triton, leading to the release of energy and the production of a helium-4 nucleus (alpha particle) and a neutron. The reaction reaches its maximum cross section at fairly low energy (≈40 to 50 keV). Accordingly, it will be the preferred fuel in fusion power plants. The reaction is D + T → 4He + n + energy. Deuteron: Nucleus of a deuterium atom. DIII-D: The third-generation tokamak developed by General Atomics in San Diego, California, the largest operational tokamak in the United States. Its principal parameters are these: major radius, 1.7 m; minor radius, 0.7 m; toroidal field, 2.1 T; plasma current, 2 MA. Disruption, disruptive instability: A complex phenomenon involving magnetohydrodynamic instability, which results in rapid heat loss and termination of a discharge. Plasma control may be lost, triggering a vertical displacement event whereby the whole plasma moves up (or down) away from its equilibrium position. This phenomenon places a limit on the maximum density, pressure, and current in a tokamak. Divertor: A magnetic field configuration affecting the edge of the confinement region, designed to divert impurities and helium ash to a target chamber. Edge-localized mode (ELM): An instability that occurs in short, periodic bursts during the high-confinement regime in divertor tokamaks. It causes transient heat and particle loss into the divertor, which can be damaging. FESAC: Fusion Energy Sciences Advisory Committee of the U.S. Department of Energy.

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Burning Plasma: Bringing a Star to Earth Fusion: A nuclear reaction in which two light atomic nuclei combine to form another element with the release of energy. The production of all elements up to nickel (Ni) happens via the fusion process (nucleosynthesis). Neutron bombardment of medium-sized nuclei heavier than nickel produces heavier nuclei. These processes occur in stars and are responsible for the presence of essentially all of the elements heavier than helium in the universe. Greenwald limit: The Greenwald normalized density is given by n20πa2/Ip, where n20 is the electron density expressed in units of 1020 m–3, a is the plasma minor radius in meters, and Ip is the plasma current in megamperes. In many tokamaks this value does not exceed 1, so the Greenwald density is a measure of the density limit for a tokamak. Helium ash: Fusion reactions in a deuterium-tritium plasma produce energetic alpha particles (helium nuclei), which heat the plasma as they slow down. Once this heating has happened, the alpha particles have no further use: They constitute helium ash, whose removal and replacement by deuterium-tritium fuel are required to prevent dilution of the plasma. H-mode: A high-confinement regime that has been observed in tokamak plasmas. It develops when a tokamak plasma is heated above a characteristic power threshold, which increases with density, magnetic field, and machine size. It is characterized by a sharp temperature gradient near the edge (resulting in an edge “temperature pedestal”), edge-localized modes, and about a 100 percent increase in energy confinement time compared with that of the normal low-confinement regime, or L-mode. Ion cyclotron heating: Auxiliary heating method using radio-frequency waves at frequencies (about 20 to 50 MHz) matching the frequency at which ions gyrate around the magnetic field lines. ITER: International Thermonuclear Experimental Reactor. The ITER experiment will be a burning plasma experiment based on the tokamak concept—the leading magnetic-confinement fusion concept, named after the Russian word for a toroidally (or doughnut) shaped magnetic field. ITER is expected to be larger than existing tokamaks, with a major radius of 5 to 8 m, and is expected to use superconducting magnets to confine the hot plasma. The negotiations to start the ITER project are being attended by the European Union, Russia, Japan, China, South Korea, Canada, and the United States (which rejoined the negotiations in January 2003).

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Burning Plasma: Bringing a Star to Earth Joint European Torus (JET): The largest tokamak in the world with a major radius of 2.96 m. It is sited at Culham in the United Kingdom. JT-60U: The flagship tokamak of the Japanese magnetic-confinement fusion program, similar in size to JET. L-mode: The “normal” low-confinement regime, opposite to the high-confinement regime, or H-mode, of additionally heated tokamak operation. Magnetohydrodynamics (MHD): A mathematical description of the plasma and magnetic field, which treats the plasma as an electrically conducting fluid. Often used to describe the bulk, relatively large-scale properties of a plasma. Major radius: The radius from the center line of the torus to the axis that is the center of the small cross section. MFE: Magnetic fusion energy; the use of magnetic-confinement configurations for fusion plasmas to generate electrical energy. Minor radius: The radius of the small cross section of a torus. Mode: Wave or oscillation in a plasma. Neoclassical tearing mode: The plasma state that occurs when the magnetic island produced by a tearing mode perturbs the bootstrap current, which further amplifies the island and degrades confinement or leads to a disruption. Neoclassical theory: Classical collisional plasma transport theory, corrected for toroidal effects. The neoclassical theory predicts the existence of the bootstrap current. Neutral beam: An energetic beam of neutral particles. It is typically produced by accelerating charged particles, or ions, which are subsequently neutralized in an electron exchange process. Neutral-beam heating: In magnetic fusion, neutral beams use isotopes of hydrogen and are primarily used to heat the plasma. Ohmic heating: Inductive heating created by using a transformer to drive a current in the plasma. This heating is necessarily pulsed. Pedestal, temperature: In the high-confinement regime, the temperature at the top of the steep temperature gradient region at the plasma edge.

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Burning Plasma: Bringing a Star to Earth Plasma: A state of matter characterized by unbound negative electrons and positive ions that may conduct electrical current. Plasma is often called the fourth state of matter, along with the other three: solids, liquids, and gases. It is estimated that more than 99 percent of matter in the universe exists as plasma; examples include stars, nebulae, and interstellar particles. The temperature of a typical plasma may be 100,000 K or more, and plasmas vary in particle density from about 106 per cubic meter (solar wind) to 1030 per cubic meter (the core of a star). Plasmas are relatively rare natural occurrences on Earth, but many applications of plasma discharges have been found. Examples of plasma can be found in lightning, the aurora borealis, fluorescent and neon-type lights, arc welding, and machines built to study nuclear fusion. Plasma pressure: Proportional to the product of plasma density and temperature. In magnetic-confinement devices, this outward pressure is counterbalanced by magnetic forces. Plasma rotation: Bulk rotation of the plasma in the toroidal or poloidal direction. Neutral-beam injection can cause plasma rotation in the toroidal direction at velocities of typically 100 km/s. Poloidal field: The component of the magnetic field parallel to the minor circumference. The poloidal field is essential for confinement and, in a tokamak, is generated by the plasma current (cf. Stellarator); this is in contrast to the larger toroidal field, which is generated externally. Reconnection, magnetic: Involves the breaking and reconnecting of oppositely directed magnetic field lines in a plasma. In the process, magnetic field energy is converted to plasma kinetic and thermal energy. Reversed-field pinch: A toroidal magnetic-confinement device in which the poloidal and toroidal fields are of comparable magnitude. To maintain stability, the toroidal field reverses close to the edge of the plasma when a critical plasma current is exceeded. RF: Radio frequency—electromagnetic energy having a frequency from 104 to 1012 Hz. Scaling laws: Empirical or theoretical expressions for how various plasma phenomena (e.g., confinement, power threshold, and so on) vary with the tokamak conditions using a range of free parameters to be fixed by “best fits” of the scaling law to tokamak data. They are particularly useful for predicting the performance of future tokamaks.

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Burning Plasma: Bringing a Star to Earth Solar corona: The Sun’s outer atmosphere, which displays a variety of features including streamers, plumes, and loops. Spherical torus, spherical tokamak: A very low aspect ratio torus approximating to a sphere (although topologically remaining a torus). Very low aspect ratio tokamaks are often called spherical tokamaks. Stellarator: A toroidal magnetic-confinement device whose poloidal field is generated by external helical coils (unlike the tokamak, in which it is generated by an internal current induced by transformer action). The absence of a plasma current gives stellarators significant potential advantages over tokamaks as fusion power plants (no disruptions, no current drive, and no stability control system). There are a number of different stellarator configurations: for example, the torsatron, heliotron, and helias. In general, stellarators have not been as successful as tokamaks, though a considerable level of research continues—notably in Germany, Spain, the United States, Russia, and Japan. Tearing mode: A class of resistive magnetohydrodynamic instability that has been predicted theoretically in tokamaks. Tokamak: The leading magnetic-confinement fusion concept, named after the Russian word for a toroidally (or doughnut) shaped magnetic field. The field the long way around the torus is the toroidal field; it is the main confining field for the particles. The toroidal field is produced from a set of poloidally constructed electromagnets. Tokamak Fusion Test Reactor (TFTR): Was the largest U.S. device, located at the Princeton Plasma Physics Laboratory, operating from 1982 to 1997. TFTR performed a major campaign using deuterium and tritium fuel between 1993 and 1997. It had a relatively high magnetic field of 5 T and a circular cross section. Tore Supra: A large tokamak with superconducting toroidal magnets and an actively cooled first wall. It is located in Cadarache, France. Toroidal: Having the specific geometrical shape of a torus. The toroidal direction is along the large circular axis of the torus. Torus: The shape of a simple doughnut. It is also the term used to describe the vacuum vessel used in tokamak fusion research. Transport: The processes by which particles and energy in the center of the plasma are lost to the edge of the plasma. Transport barrier: In certain operational scenarios (e.g., the high-confinement mode, or H-mode) a region of low transport exists, giving rise to a steep pressure gradient. Such a region is referred to as a transport barrier.

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Burning Plasma: Bringing a Star to Earth Tritium: Isotope of hydrogen having one proton and two neutrons in its nucleus. Tritium is radioactive, with a half-life of 12.3 years, and is essentially nonexistent in nature. Tritium can be produced by bombarding lithium with a neutron and inducing a fission reaction. 6Li + n → T + 4He + 4.8 MeV or 7Li + n → T + 4He + n – 2.5 MeV. Triton: Nucleus of a tritium atom. Turbulence: Randomly fluctuating, as opposed to coherent, wave action. For example, the turbulent surface of water beneath a waterfall can only be described in terms of its averaged properties, such as the scale and duration of fluctuations, whereas a more systematic description can be given to waves on the surface of a still pond.