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=============================================================== Glossary Part 18: Terms beginning with "R" FREQUENTLY USED TERMS IN CONVENTIONAL FUSION RESEARCH AND PLASMA PHYSICS Edited by Robert F. Heeter, firstname.lastname@example.org Guide to Categories: * = plasma/fusion/energy vocabulary & = basic 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). ================================================================== RRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRR @ Rad: Radiation Absorbed Dose; see entry "rad" @ Rem: Raditation (or Roentgen) Equivalent for Man; see entry "rem" @ R&D: Research and Development. @ RD&D: Research, Development, and Demonstration. @ RF: Radio Frequency; see entry @ RF Current Drive: Radio Frequency Current Drive; see entry @ RF Heating: Radio Frequency Heating; see entry ! R.F. Heeter: Plasma physics graduate student at PPPL; the editor of the sci.physics.fusion FAQ, bibliography, and glossary. :) (note similarity to RF Heating.) @ RF Plugging: See entry for radiofrequency plugging @ RF Trapping: See entry for radiofrequency trapping @ RFC: Reversed-Field Configuration: see Field-Reversed Configuration. @ RFP: Reversed-Field Pinch; see entry @ RFX: Reversed-Field eXperiment; see entry @ RGA: Residual Gas Analyzer $ Rad: radiation absorbed dose. A unit used to measure the amount of radiation energy absorbed per gram of a given substance, that is the dose. One rad means absorption of 100 ergs of energy per gram. See also gray, rem, sievert. * Radial Ponderomotive Force Stabilization: In magnetic mirror devices, use of rf waves in the neighborhood of the ion cyclotron frequency to stabilize interchange modes. The radial ponderomotive force produced by a radial gradient in the applied rf electric field opposes the destabilizing centrifugal force resulting from bad magnetic field curvature. The net particle current is in the direction that would result from field lines with good curvature, eliminating the drive for the interchange instability. & Radiation: (Sense 1) Process of emission of energy from a body in the form of light or heat waves, or energetic particles such as alpha particles, electrons, or neutrons. (Sense 2) Radiation also refers to *what is emitted* when an object radiates (but not *what does the emitting*). A nucleus which does the emitting is said to be radioactive. Electrons in atoms can also emit radiation in the form of ordinary visible light; such atoms are not said to be radioactive. * Radiation Damage, Bulk: General term describing changes in chemical and/or metallurgical properties of structure components of fusion reactor caused by atomic displacement and nuclear transmutation events occuring as a result of exposure to a radiation environment (such as the neutrons emitted from a fission or D-T fusion reactor). * Radiation Damage, Surface: General term describing damage to the surface of the containment structure which directly interfaces with the thermonuclear plasma; includes such phenomena as radiation blistering, charged-particle (or neutron) sputtering, and spallation or exfoliation of layers of the surface. & Radioactive Decay: Spontaneous transformation of one nuclide into a different nuclide or into a different energy state of the same nuclide. This process results in a decrease, with time, in the number of originally radioactive atoms in a sample. See Decay Modes for a listing of the different mechanisms by which radioactive decay can occur. & Radioactive waste: Equipment and materials from nuclear operations which are radioactive and for which there is no further anticipated use. Wastes are generally classified as high-level (having radioactivity concentrations of hundreds to thousands of curies per gallon or cubic foot), low-level (in the range of 1 microcurie per gallon or cubic foot), or intermediate (between high and low). See also curie. & Radioactivity: Characteristic property of unstable nuclei which decay to other nuclei by emission of radiation. A list of common decay / transmutation modes is given under "decay modes". * Radio Frequency or radiofrequency: Term used to describe electromagnetic radiation with frequencies less than infrared, but greater than "audio frequencies," i.e., greater than about 15,000 Hz. Wavelengths are therefore less than about 20,000 km and greater than about 0.01 mm. (These numbers are not precise.) * Radio Frequency Current Drive: Plasma waves in the radio-frequency range can be used to push plasma particles in such a way that current forms in the plasma; this is a method of non-inductive current drive (see entry) which would allow for steady-state fusion reactors to operate. * Radio Frequency Heating: Process for heating the plasma by transferring energy to ions or electrons using waves generated by an external oscillator at an appropriate frequency. (This is similar to how a microwave oven heats food.) There are various types: see also ECRH, ICRH, and Lower Hybrid... * Radiofrequency Plugging: Use of axial ponderomotive force to plug an open-ended device. First demonstrated on RFC-XX, Institute of Plasma Physics, Nagoya University, Nagoya, Japan, and later demonstrated in the Phaedrus device at the University of Wisconsin. * Radiofrequency Trapping: Use of RF waves to pitch angle scatter particles flowing axially into a magnetic mirror cell. The particles are scattered out of the loss cone and are trapped. (See entries for pitch angle scattering, magnetic mirror, loss cone.) * Railgun Accelerator: Projectile accelerator which accelerates the particle using electromagnetic forces which arise when the particle completes an electrical circuit between two conducting rails connected to a source of high current. * Raman Effect: A phenomenon observed in the scattering of light as it passes through a transparent medium; the light undergoes a change in frequency and a random alteration in phase due to a change in rotational or vibrational energy of the scattering molecules. * Ramsauer Effect: Term for a quantum-mechanical effect allowing free electrons within a narrow range of energies to pass through a noble gas with very little elastic scattering. * Rational Surface: (related to q-factor, see entry) Magnetic flux surface (see entry for this too) where the ratio of toroidal to poloidal field strengths is a rational number; this means that a particle travelling along this surface makes an integer number of turns in each direction and then its orbit closes in on itself. The result is that the particle doesn't sample the entire flux surface in its motion, which is important for various technical reasons (which mostly result in reduced confinement); see also magnetic island. * Reactor: See fission reactor, fusion reactor. & Recombination Coefficient: The rate of recombination of positive ions with electrons (or negative ions) in a gas, per unit volume, divided by the product of the particle densities of the two species (positive ions and electrons/negative ions) involved. * Recombination Radiation: radiation produced when a free electron in a plasma is captured by an ion. & Reflectivity: Fraction of incident radiant energy which is reflected by a given surface. (The power which is not reflected is either absorbed or transmitted.) & Refraction: Bending of waves as they pass from a medium having one refractive index to a medium (or region within a medium) having a different refractive index. $ Rem: Radiation (or Roentgen) Equivalent for Man. Unit of absorbed radiation dose based on the definition rem = rad * quality. The quality factor depends on the type of radiation involved and is used to scale the radiation dose based on the relative harmfulness of different sorts of radiation, compared to ordinary X-rays. Annual US average dose is about 300 millirem (0.3 rem), of which more than 2/3 is natural (primarily radon and cosmic rays), and the majority of the human-generated dose is due to medical uses (primarily X-rays). See also radiation dose, roentgen, sievert, rad, gray. * Reserves: Amount of a substance which can be extracted from the earth with current technology at current prices. Typically much smaller than resources (see entry for resources). & Resistance (electrical): Ability of a given object to resist the flow of electrical current. To drive a given current a voltage must be applied to overcome the resistance according to V = I * R (V = voltage, I = current, R = resistance). Resistance is determined by resistivity and geometrical factors. * Resistive Instability: Instability resulting from macroscopic equations used to model a plasma of finite conductivity / nonzero resistivity. & Resistivity: Tendency for a material/substance to resist the flow of electrical current and to dissipate its energy. Resistivity, when combined with certain geometry factors (generally length and cross-sectional area for wires) determines resistance. * Resource: Total amount of a substance which exists in the earth and could conceivably be extracted someday at some price with some technology. Typically much larger than reserves. (See also reserve.) > Reversed-Field Pinch (RFP): A toroidal magnetic confinement scheme which could constitute an alternative to the Tokamak for building a fusion reactor. It is characterized by a magnetic field mostly generated by the plasma itself, with toroidal and poloidal components of comparable intensities, in contrast with the Tokamak where most of the field is toroidal and externally applied. The name of the configuration is given by the fact that the toroidal component of the magnetic field changes sign in the outer region of the plasma. The main attractivness of the Reversed Field Pinch is that, according to presently established scalings, it could reach ignition without the need of auxiliary heating. (Emilio Martines, martines%pdigi3.igi.pd.cnr.it) > Reversed-Field eXperiment (RFX): It is the largest Reversed Field Pinch device presently in operation. Located in Padova (Italy) it is planned to reach a plasma current of 2 MA. (Emilio Martines, martines%pdigi3.igi.pd.cnr.it) * Roentgen: Unit of exposure to ionizing radiation. The Roentgen is that amount of gamma or X-rays (electromagnetic radiation) required to produce ions carrying one electrostatic unit of electrical charge (either positive or negative) in 1 cubic centimeter of dry air under standard conditions. Named after Wilhelm Roentgen. (Compare with curie, rad, gray, sievert.) ! Roentgen, Wilhelm: German scientist who discovered X rays in 1895. * Rogowski Loop or Coil: A coiled wire loop which encircles a current-carrying plasma. Changes in total plasma current induce a voltage in the loop; integrating (adding up) the voltage over time gives the plasma current. * Rotational Transform: (labels: \iota = 2*PI/q) Due to the combination of applied toroidal field and induced poloidal field, the magnetic field lines wind helically around the torus (and on most flux surfaces they fill the surface ergodically). The rotational transform is a measure of this helicity, and is defined as the average angle the field line shifts in the poloidal direction per complete circuit in the toroidal direction. The quantity q = 2*\pi / \iota is known as the ``safety factor'' because of its role in stability theory. (Contributed by James Crotinger) Entry from 1985 OSTI Glossary: A magnetic field configuration is said to posses rotational transform if the lines of force, after one complete circuit around the configuration (e.g., a torus) do not simply close exactly on themselves, but are instead rotated through some angle about the magnetic axies. (See entries for flux surface, magnetic axis, toroidal and poloidal field, helicity, safety factor.) * Runaway Electrons: Those electrons in a plasma that gain energy from an applied electrical field at a faster rate than they lose it through collisions with other particles. These electrons tend to "run away" in *energy* (not position) from the cooler remainder of the background plasma, because the collision cross-section decreases as the particle's velocity increases, so that the faster the particle goes, the less likely it is to be stopped. See also: collision cross-section.