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=============================================================== Glossary Part 8: Terms beginning with "H" FREQUENTLY USED TERMS IN CONVENTIONAL FUSION RESEARCH AND PLASMA PHYSICS Edited by Robert F. Heeter, email@example.com 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). ================================================================== HHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHH # H: chemical symbol for the element hydrogen; see entry # He: chemical symbol for the element helium; see entry. @ HIREX: High-REsolution X-ray spectroscopy @ H-mode: see high-mode @ HTO: (Hydrogen-Tritium-Oxygen) Water with a tritium atom replacing a hydrogen. See entry for tritium. * Half-life: For a given quantity of a radioactive isotope, there is a time period in which half the nuclei will decay to a different state; this period is called the half-life. Measured half-lives range from less than millionths of a second (for very short-lived isotopes) to billions of years (for isotopes which are almost stable, but not quite). The time in which half the atoms of a particular radioactive isotope disintegrate to another nuclear form. By analogy, "half-life" can also be used to describe similar time-periods for other sorts of exponential decay phenomena. * Hall Effect: Transverse electric field which develops in a conductor (as a result of the Lorentz Force acting on the charge carriers) when current is driven across a magnetic field. * Halo: The cold, dense plasma formed outside the last closed flux surface during a vertical displacement event. The large currents which flow through this plasma stop the displacement and transfer the force to the vacuum vessel. If care is not taken in design, the halo currents can be large enough to threaten the structural integrity of the vacuum vessel or in-vessel components. Whereas the center of a tokamak plasma is too hot for material probes to survive, probes (such as magnetic-field coils) can sometimes be placed in the halo, and can measure things such as the halo current (see below). See also entry for vertical instability. * Halo Current: Currents in the halo region of a plasma discharge. See entry for halo above. * Hamada coordinates: A particular magnetic-flux coordinate system useful for MHD calculations. In this system the current density and magnetic field lines are straight and the Jacobian of the coordinate transformation equals one. & Hamiltonian Function: Function arising from the Hamiltonian approach to mechanics which characterizes the total energy of a system as a function of generalized coordinates and momenta and can be used to obtain the dynamical equations of motion of the system. (Consult an intermediate or advanced mechanics text for more info.) > Hard-core pinch device: plasma pinch-discharge device using a solid central conductor ("hard-core"). The discharge then occurs in an annular region about the central conductor. & Hartree-Fock approximation: a refinement of the Hartree method (see entry) in which one uses determinants of single-particle wave functions rather than products, thereby introducing exchange terms into the Hamiltonian. & Hartree method: An iterative, variational method of finding an approximate quantum-mechanical wavefunction for a system of many electrons, in which one attempts to find a product of single-particle wave functions, each of which is a solution of the Schrodinger equation with the field deduced from the charge density distribution due to all the other electrons; also known as the self-consistent field method. & Heat exchanger: device that transfers heat from one fluid (liquid or gas) to another (or to an external environment). * Heavy Hydrogen: somewhat informal alternative name for deuterium. (see entry for deuterium). * Heavy Water: (D2O) Water with enriched content of deuterium relative to hydrogen (greater than the natural abundance of 1 D per 6500 H). Heavy water is used as a moderator in some fission reactors (see CANDU entry) because it slows down neutrons effectively but also has a low collision cross-section for absorption of neutrons. > Heliac: A confinement configuration which superimposes an l=1 stellarator-type field upon a tokamak-like poloidal field. The resulting plasma configuration is a helix bent around into a loop. * Helicity: (from John Cobb) A measurement of the topological "tangledness" of magnetic field lines. It is formally defined as the scalar product of the magnetic vector potential with the magnetic field, K = A dot B. If the plasma is perfectly conducting, then helicity is a conserved quantity. (Without resistance, field lines cannot reconnect, and magnetic topology is conserved, so helicity is conserved). (See frozen-in flow). If the plasma has a small amount of resistivity, then Helicity is not exactly conserved. However, the total helicity inside of a given flux surface is often conserved to a good approximation. In that case, the dynamics of a plasma can be analyzed as an evolution toward a minimum energy state subject to the constraint of a conserved total helicity (See Taylor State, J.B. Taylor). This is often used in analyzing the equilibrium and relaxation of RFP's and other toroidal devices. > Helios Facility: Los Alamos laser inertial fusion facility. & Helium: Element whose nuclei all contain two protons. Stable isotopes are 3He and 4He. 3He is rare on earth (only 1.3 ppm of naturally-occuring He), can be generated from decaying tritium (half life of about 12 years), and is relatively abundant in the crust of the moon. Helium is the second most abundant element in the universe and in the sun, and occurs at about (I believe) 1 part per million in earth's atmosphere. Helium is also found in significant quantities in natural gas deposits. The nucleus of the He atom is also known as an alpha particle. Helium is chemically inert, behaves nearly as an ideal gas under a wide range of pressures and temperatures, and can only be liquefied at 4 Kelvin (at atmospheric pressures). One mole of He weighs 4 grams. ! Hertz, Heinrich: 19th-century German physicist; first (?) observed low-frequency electromagnetic waves. $ Hertz: Unit of frequency equal to one complete oscillation (cycle) per second. Common abbreviation is Hz. * High-beta plasma: A plasma in which the beta value (see entry) is typically 0.1 to 1. * High-mode or H-mode: A regime of operation most easily attained during auxiliary heating of diverted tokamak plasmas when the injected power is sufficiently high. A sudden improvement in particle confinement time leads to increased density and temperature, distinguishing this mode from the normal "low mode." However, H-mode has been achieved without divertors, auxiliary heating, or a tokamak. (H-modes have been observed in stellarators.) & Holography: A technique for recording and later reconstructing the amplitude and phase distribution of a wave disturbance. & Homopolar generator: A direct-current generator in which the poles presented to the armature are all of the same polarity, so that the voltage generated in the active conductors has the same polarity at all times. A pure direct current is thus produced without commutation. * Hot cells: Heavily radiation-shielded enclosure in which radioactive materials can be handled by persons using remote manipulators and viewing the materials through shielded windows or periscopes. * Hybrid diode: An ion diode that uses a field coil in series with the ion diode's accelerating gap to generate sufficient magnetic flux in the diode for electron control. The diode is a combination of the Applied-B diode's ion source and the Ampfion diode's field coil. * Hybrid reactor: see fusion-fission hybrid. * Hybrid resonance: A resonance in a magnetized plasma which involves aspects of both bunching of lighter species parallel to the magnetic field, characterized by the plasma frequency; and perpendicular particle motions (heavier species) characterized by the cyclotron frequency. & Hydrogen: (H) Element whose nuclei all contain only one proton. Isotopes are protium (p, no neutrons) deuterium (D or d, one neutron), and tritium (T or t, two neutrons). Hydrogen is the lightest and the single most abundant element in the universe, and in the sun. Hydrogen is a major element in organic compounds, water (H2O), and many other substances. Hydrogen is ordinarily a gas, but can be liquefied at low temperatures, and even solidified at low temperature and high pressure. Hydrogen gas can burn explosively in the presence of oxygen. * Hydrogen bomb or H-bomb: (from Herman) An extremely powerful type of atomic bomb based on nuclear fusion. The atoms of heavy isotopes of hydrogen (deuterium and tritium) undergo fusion when subjected to the immense heat and pressure generated by the explosion of a nuclear fission unit in the bomb. * Hydrogen embrittlement: A decrease in the fracture strength of metals (embrittlement) due to the incorporation of hydrogen within the metal lattice. * Hydromagnetic Instability: See MHD Instability * Hydromagnetics: see magnetohydrodynamics (MHD)