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=============================================================== Glossary Part 4: Terms beginning with "D" FREQUENTLY USED TERMS IN CONVENTIONAL FUSION RESEARCH AND PLASMA PHYSICS Edited by Robert F. Heeter, firstname.lastname@example.org 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). ================================================================== DDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDD @ D: nuclear/chemical symbol for deuterium/deuteron @ DC, dc: Direct Current; see entry @ DCLC: Drift Cyclotron Loss Cone instabilities; see entry. @ DT: Deuterium-Tritium; see entry labeled DT Fuel @ DIII-D: not an acronym (anymore); see entry @ DOE: Department of Energy (United States); see entry @ dpa: Displacements per atom; see entry @ DPP: Division of Plasma Physics; see APS-DPP * D-shaped plasma: A toroidal plasma whose cross section (poloidal plane) is a D (instead of a circle). A D-shape has a higher beta limit (see entry) than a circular shape. * Debye Length: The characteristic distance over which charges are shielded in a plasma. See also: Debye shielding. lambda_D = ( epsilon_0 k_B T_e / (n_e e^2) )^(1/2) lambda_D[m] = (7.434*10^3)*(_e[eV])^(1/2)*n[m^(-3)]^(-1/2) (Arthur Carlson, email@example.com) ! Debye, Peter Joseph: Physical chemist, studied behavior of conductive solutions (plasmas have some similar behaviors). * Debye Radius: See Debye Length. * Debye Sheath: The region of strong electric field in front of a material surface in contact with a plasma. Its characteristic thickness is the Debye length, and it is caused by Debye shielding of the negative surface charge resulting from electrons flowing to the surface much faster (initially) than the ions. The lost electrons leave behind a region of net positive charge which gradually diminishes the strength of the electric field over the debye length. See also: Debye Length, Debye Shielding. (Arthur Carlson, firstname.lastname@example.org, with modifications by John Cobb, email@example.com) * Debye Shielding: If a positive (or negative) charge is inserted into a plasma, it will change the local charge distribution by attracting (repelling) electrons. The net result is an additional negative (positive) charge density which cancels the effect of the initial charge at distances large compared to the Debye length. (There is a corresponding effect of shielding by the ions, which, for various and subtle reasons, usually is less important.) See also: Debye Length. (Arthur Carlson, firstname.lastname@example.org) * Debye Sphere: Sphere around a charged test particle whose radius is equal to the Debye length. & Decay, Radioactive: See radioactive decay. * Decay Modes: Different pathways for decay of radioactive nuclei. The decay modes for a given unstable state can include beta emission (negative = electron, positive = positron), electron capture, alpha emission, fission, and gamma emission. (Did I miss any?) See entries for each mode for more information. * Dee-Shaped: see D-shaped plasma above. * Degenerate Configuration: Magnetic field configuration in which the magnetic lines of force close exactly on themselves after passing around the configuration a finite number of times. * Dense Plasma Focus: See Plasma Focus. (Densities of up to 10^26 particles/m^3 have been reported.) & Density: amount per unit of volume, or per unit surface area, or per unit length. (Usually specified or clear from context which of these is meant). Several types: Charge density - amount of charge per unit (volume, area, length) Current density - current flow per unit transverse surface area. Energy density - amount of energy per unit volume. Flux density - flux per unit of transverse surface area. Mass density - mass per unit volume. Number density - number of particles per unit volume. Particle density - same as number density. % Department of Energy: (DOE) Department within the executive branch of the U.S. government (at the cabinet level) which has managed and overseen federally-sponsored energy research. The DOE was formed in 1977 from ERDA, the Energy Research and Development Administration, and (I think) the Atomic Enegy Commission (AEC). & Deuterium: A heavy isotope of hydrogen whose nucleus contains both a neutron and a proton. * Deuteron: A deuterium ion; nucleus consisting of a proton and a neutron. * Diagnostics: (from Herman) Procedures for determining (diagnosing) the state of a plasma during an experiment; also refers to the instruments used for diagnosing. * Diamagnetic Effects: Application of a magnetic field to a plasma will tend to create circulating current within the plasma that will reduce the strength of the magnetic field. * Diffusion: The interpenetration of one substance into another as a result of thermal / random motion of the individual particles. (e.g., the diffusion of a plasma across a magnetic field as a result of collisions which cause particles to move along new field lines.) See also classical diffusion, neoclassical diffusion, anomalous diffusion, transport. * Direct Conversion: The generation of electricity by direct recovery of the kinetic energy of the charged fusion reaction products. & Direct Current: Electric current which is unchanging in time, or at least not oscillating. Opposite of Alternating Current. * Direct Drive: An approach to inertial-confinement fusion in which the energy of the driver (laser or particle beam) is directly incident on the (usually spherical) target, causing compression heating via ablation of the target surface. * Dispersion Relation: For a given wave, the dispersion relation relates the temporal frequency of a wave (w, or omega) to its wavenumber k and other physical quantities characteristic of the system. Dispersion relations can be quite simple (e.g., w = k * c for light; c being the speed of light), and they can also be quite complex, with interesting mathematical structure. The dispersion relation and its mathematical structure provide important information about the wave, including the phase and group velocities. (See relevant entries.) Note that the meaning of "dispersion relation" is different in plasma physics than in other fields. * Displacements Per Atom: (dpa) This is a measure of the amount of radiation damage in neutron-irradiated materials; e.g., 10 dpa means each atom in the material has been displaced from its structural lattice site and average of 10 times (due to interactions between the atoms and the energetic neutrons irradiating the material.) * Disruption: Plasma instabilities (usually oscillatory modes) sometimes grow and cause disruptions of the carefully-engineered plasma conditions in the reactor. Major disruptions can cause an abrupt temperature drop and the termination of the plasma. Stored energy in the plasma is rapidly dumped into the rest of the plasma system (vacuum vessel walls, magnet coils, etc.) and can cause significant damage if precautions are not taken. * Disruptive Instability: Instability which causes a disruption; see entry for disruption. * Dissociative Recombination: The combination of an electron with a positive molecular ion, followed by dissociation of the molecule in which the resulting atoms/molecules carry off the excess energy released in the recombination. & Distribution Function: Function characterizing the density of particles located at a given point in phase space (a combination of either velocity or position coordinates) at a given time. The velocity-space distribution function gives the number of particles with a particular velocity; the position-space distribution function is synonymous with the particle density in position-space. Different combinations of position and spatial coordinates are useful in different problems. * Divertor: Component of a toroidal fusion device that diverts charged particles on the outer edge of the plasma into a separate chamber where they strike a barrier and become neutralized. In a reactor, the divertor would incorporate a system for pumping out the neutralized particles as exhaust from the machine. A divertor, like a limiter, prevents the particles from striking and degrading the chamber walls and dislodging secondary particles that would cool and contaminate the plasma. Whereas a limiter is a material object used to limit the shape of the plasma, a divertor is a magnetic-field construction. The advantage of the divertor is that it allows the neutralization region to be removed from the main plasma. See also: limiter. * Doppler Broadening: Frequency spreading which causes broadening of single-frequency radiation (e.g., spectral lines) when the radiating bodies (atoms, molecules, etc.) have different velocities. Radiation from each individual radiating body has a different Doppler shift, and the collection of radiations at different frequencies broadens the peak of the line in an intensity-vs-frequency plot. & Doppler Effect: Variation in the frequency of a wave (as measured by an observer) due to relative motion between the observer and the source of the wave. (The observed frequency increases if the source is moving towards the observer.) & Doppler Shift: The amount of change in the observed frequency of a wave due to the Doppler effect; sometimes called the Doppler frequency. > DIII-D: Latest in a series of tokamaks designed by General Atomics (formerly GA Technologies) in San Diego making plasmas with noncircular cross sections, including kidney shapes and D-shapes. * Doublet Device / Doublet Plasma: Tokamak-type devices where the plasma cross-section is kidney-shaped, with a deep indentation in the middle so that the plasma has two major rings of current (on top and bottom). * Drift Cyclotron Loss Cone Instabilities: (DCLC) This is an electrostatic microinstability (frequencies at harmonics of the ion cyclotron frequency) which is of major concern in small mirror devices. Mode is driven by radial gradients in the electron density, and causes loss of ions due to non-conservation of magnetic moment (see adiabatic invariant) as they interact with the mode, and are dispersed in velocity space into the loss cone. Stabilization is accomplished by increasing the plasma size and by partially filling the loss cone with a continuous extermal warm plasma stream. * Drift Motion: Ordinarily particles placed in a magnetic field will simply orbit in circles, but if the magnetic field is not uniform, or curves, or there is an electrical field perpendicular to the magnetic field, or another force is applied perpendicular to the magnetic field, then the "guiding centers" of the particle orbits will drift (generally perpendicular to the magnetic field and to the applied force). There are several sorts of drifts; refer to a plasma physics text for more information (see Section 11: Bibliography). For a good introduction at the undergraduate physics level, see Chen. * Drift Pumping: A process that removes ions trapped in a thermal barrier using radial transport induced by an exterally-applied radiofrequency field tuned to resonate with the azimuthal drift frequency. * Drift Surface: Surface on which the guiding center of a particle is constrained to move, due to the effects of the laws of adiabatic invariance on its drift motion. * Drift Velocity: Characteristic velocity at which the center of a particle's orbit ("guiding center") drifts when drift motion (see above) occurs. * Drift Waves: Oscillations in a magnetically-confined plasma arising in the presence of density gradients (such as at the plasma's surface). These resemble the waves that propagate at the interface of two fluids with different density in a gravitational field. * DT Fuel: (Deuterium-Tritium) Easiest fuel mixture to use in achieving fusion; unless otherwise specified, probably refers to a 50-50 (by numbers or by moles) mix of deuterium and tritium. * Duty Factor: Ratio of the duration of time when a system is actually operating to the total time for a complete cycle of the system. e.g., if a tokamak experiment runs for 5 seconds and then sits for 500 seconds while the power supplies are recharged, then the duty factor is 1%. Similar to capacity factor for powerplants. * Dye laser: A type laser in which the active material (the material which emits the laser light) is a dye. These lasers are tunable when the dye has very large molecules (such as acridine red or esculin) and the laser action takes place between the first excited and ground electronic states, because each of these states contains a broad continuum band of vibrational-rotational levels.