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Conventional Fusion FAQ Glossary Part 8/26 (H)

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Archive-name: fusion-faq/glossary/h
Last-modified: 4-Feb-1995
Posting-frequency: More-or-less-quarterly
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Glossary Part 8:  Terms beginning with "H"


Edited by Robert F. Heeter,

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).


# 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)

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