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Conventional Fusion FAQ Glossary Part 9/26 (I)

Archive-name: fusion-faq/glossary/i
Last-modified: 4-Feb-1995
Posting-frequency: More-or-less-quarterly
Disclaimer: While this section is still evolving, it should
be useful to many people, and I encourage you to distribute

Glossary Part 9:  Terms beginning with "I"


Edited by Robert F. Heeter, rfheeter@pppl.gov

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).


# I:  variable used to indicate total current through a conductor.

@ IAEA:  International Atomic Energy Agency; see entry
@ IBHP:  Integrated Biological Hazard Potential; see entry

@ ICE: Ion Cyclotron Emission; see entry

@ ICF:  Inertial Confinement Fusion; see entry

@ ICH:  Ion Cyclotron Heating - see ICRH

@ ICRF:  Ion Cyclotron Range of Frequencies

@ ICRH:  Ion Cyclotron Resonance Heating; see entry

@ IEEE:  Institute of Electrical and Electronic Engineers; see entry

@ INEL:  Idaho National Engineering Laboratory; see entry

@ IPP:  Max Planck Institute for Plasma Physics; see entry

@ IR:  Infrared (region of the electromagnetic spectrum)

@ ITER:  International Thermonuclear Experimental Reactor; see entry

% Idaho National Engineering Laboratory:  U.S. Department of energy
laboratory involved in engineering studies for fusion and fission
reactors, among other things.  Not surprisingly, located in Idaho.

* Ignition:  In fusion, as in an ordinary (chemical) fire,
ignition is the point where the temperature and confinement
of heat in the fuel (plasma in the case of fusion) are 
such that energy released from ongoing reactions is sufficient
to maintain the temperature of the system, and no external
heating is needed.  An ignited fusion plasma produces so 
much energy from fusion reactions that the plasma is fully
heated by fusion reaction products (alpha particles in the
case of D-T fusion), and the plasma no longer needs any 
external source of power to maintain its temperature.
(The plasma may, however, still need something to maintain 
its confinement; this gives us control over the fusion 
reaction and helps prevent fusion reactors from having 
"meltdown" problems like fission reactors.)

* Ignition Temperature:  For given values of density and
energy confinement, the temperature at which ignition occurs.
(see ignition above)

> Impact Fusion:  Fusion approach where a "fuel" projectile
is acclerated and impacted into either a stationary target or
another projectile.  (Valuable for scientific purposes but
not a candidate for a fusion energy source because the
likelihood of fusion occurring in a single collision is 
too low.  Multiple accelerated pellets colliding with spherical
symmetry might be a viable inertial confinement approach, though.)

* Impact fusion drivers:  macroparticle/projectile accelerator
which could be used in inertial confinement fusion.

* Impurities: atoms of unwanted elements in the plasma, 
which tend to degrade plasma performance, and in the case of 
fusion plasmas tends to inhibit fusion ("poisoning the reactor").
See also poisoning.

* Impurity Control:  Processes which reduce or control the level
of impurities in a plasma, and thereby improve its quality;
see also wall conditioning.

* Inboard side:  portion of a tokamak (or other toroidal device)
closest to the central axis.  (As distinguished from "outboard side.")

* Incoherent scattering:  Type of scattering in which the scattering
elements act independently, so that no definite phase relationships
exist among the different parts of the scattered 
beam (particles or photons).

& Index of Refraction:  For a given wavelength, this is the ratio 
of the velocity of light in vacuum (c) to the velocity of light
in a refractive material (e.g., glass, plasma, etc.).

& Inductance:  Characteristic relating the magnetic flux generated
through a loop of wires to the current in the wires; Phi=LI.

& Induction:  A changing magnetic flux through a current loop will
induce an electric field which will drive a current through the loop.
This is the principle behind an AC transformer, where an oscillating
electric voltage in one loop of the transformer creates a current
which generates an oscillating magnetic field, which then induces
a different voltage and current in a second loop.

* Inductive Current Drive:  Method to drive current in a toroidal
plasma by using the torus of conducting plasma as the second coil
in a transformer.  The primary coil usually runs down the center
of the torus; changes in the current driven through the primary
coil create changing magnetic fields which drive current in
the plasma.  The current thus driven can be used to heat the plasma
as well (see also ohmic heating; induction).

* Inertial Confinement Fusion:  Approach to fusion where the plasma
is imploded so quickly that the inertia of the converging particles
is so high that many fuse before they disperse.  This is the method
used in a hydrogen bomb; ICF schemes for power production usually
use small pellets of fuel in an attempt to make "miniature"
h-bomb type explosions.  Methods for imploding the pellet include
bombardment from all sides with high-powered laser and particle
beams, and of course implosion in a fission bomb.  Parts of ICF
fusion research remain classified due to their military 
implications and applications, though much ICF research was recently

* Instability:  A state of a plasma (or any other physical system) 
in which a small perturbation amplifies itself to a considerable 
alteration of the state of the system.  In plasmas instabilities
sometimes leads to disruptions (see entry).  Most instabilities are 
associated with waves and other natural modes of oscillation in the plasma, 
which can sometimes grow.  There are (unfortunately!) 
many kinds.   See also:  Flute instability, MHD instability, 
Interchange instability, microinstability, kink instability, 
resistive instability, trapped particle instability, 
two-stream instability, universal instability, and 
velocity-space instability.

% Institute of Electrical and Electronic Engineers:  Professional
society for this branch of engineering.

* Integrated Biological Hazard Potential (IBHP):  Total
biological hazard potential of a collection of radioactive
materials summed over their decay lifetimes.  See also BHP.
One measure of the IBHP is the amount of water one would need
to use to dilute the materials to the point where the water
would be safe to drink.

* Integrated neutron flux:  Sum (integral) of the neutron
flux (neutrons per unit time per unit area, see flux)
over all time; total number of neutrons which passed through
a unit area.  Important figure-of-merit in testing effects of
neutron radiation on materials, and in assessing how long
such materials can survive exposure to neutron sources
(such as fission reactor cores and D-T fusion plasmas).

* Intensity:  This term has different meanings in different
contexts.  Can refer to the amount of power (energy per unit
time) incident on a unit surface area, or flowing through a
unit volume.  Can refer to the number of particles or photons
incident, per unit time, on a unit area, or flowing through 
a unit volume.  Also, for an amount of a radioactive material,
intensity can refer to the number of radioactive disintegrations
per unit time. 

* Interchange Instability:  In the simplest form, if you
place a high-density fluid on top of a low density fluid,
gravity will pull the high density fluid downwards so that
the low-density fluid ends up on top.  The two fluids
therefore interchange places.  More generally, an interchange
instability occurs when two types of fluid are situated with
an external force such that the potential energy is not
a minimum; the two fluids will then interchange locations to
bring the potential energy to a minimum.  In plasmas with 
magnetic fields, the plasma may interchange position with
the magnetic field.  A prime example is the flute instability 
in mirror machines.  (See MHD, instability, flute, mirror.)

* Interference:  When two waves propagate through the same
region of space, they interfere with each other.  Neither
wave is altered, but the amplitudes of the waves add (or
cancel, if they're of opposite sign) to give the total 
effect to the medium at that point.  

* Interferometer:  Device which measures changes in a medium
by looking at effects on the interference of two waves which
are passed through that medium.  See interferometry,
laser interferometer, optical inteferometer, Fabry-Perot 
interferometer, microwave interferometer.

* Interferometry:  Method of gathering information about a
medium by using an interferometer or similar technique.
        Optical - Uses light as the wave to be interfered.
        Microwave - Uses microwaves instead.  Microwave interferometry
   is especially useful in plasma physics for measuring plasma 

> Internal ring devices:  Toroidal configurations in which 
current-carrying rings are suspended (either mechanically
or magnetically) inside the plasma chamber.

% International Atomic Energy Agency: (from Herman)  An
autonomous intergovernmental organization established in 1956
with the purpose of advancing peaceful uses of atomic energy,
with headquarters in Vienna.

> International Thermonuclear Experimental Reactor (ITER):
Huge fusion reactor being planned by the EC, US, Japan,
and Russia (former USSR?).  Should generate far more
energy than it consumes.  Research goals include engineering
studies of reactor materials, component designs for steady-state
devices, and testing/proving commercial feasibility.  Discussed
in sections 5 and 9.

* Ioffe Bars:  Special configuration of conductors which, when
added to a conventional magnetic mirror, generate a "magnetic
well" which stabilizes the mirror against MHD instabilities.

& Ion:  An atom (or molecule) which has become charged as a 
result of gaining or losing one or more orbiting electrons.  
A completely ionized atom is one stripped of all its electrons.

* Ion acoustic wave:  a longitudinal compression wave in the
ion density of a plasma, which can occur at high electron
temperatures and low frequencies, caused by a 

* Ion Cyclotron Emission (ICE):  As ions gyrate around in a magnetic
field (see also larmor radius or cyclotron radius), they radiate 
radio-frequency electromagnetic waves.  This is known as ion 
cyclotron emission, and can be measured to help diagnose a plasma.

* Ion Cyclotron Resonance Heating:  Like Electron Cyclotron 
Heating, but heats ions using waves near the ion cyclotron 
frequency.  See Electron Cyclotron Heating.

* Ion diode:  Device for producing and accelerating ion beams
for light ion drivers for inertial confinement fusion.  Ions 
are produced in an anode plasma, extracted as space-charge-limited
ion flow, and accelerated to the cathode, composed of a confined
electron swarm, by an applied electric field.  Millions of 
amperes of current at millions of volts have been produced this way.

* Ion Temperature: the temperature corresponding to the
mean kinetic energy of the ions in a plasma.

& Ionization:  Process by which a neutral atom is converted to an ion 
(or one ion is converted to another of a different type), by 
removal or addition of electrons.

& Ionization Energy:  Generally refers to the amount of energy 
required to strip a particular electron from an atom.  The 
first-ionization-energy is a commonly used quantity in many fields 
of physics and chemistry.  Typically measured in electron-volts.
Equivalent to the atomic binding energy of the electron.

& Ionization Potential:  See ionization energy.

* Ionizing radiation:  Any high-energy radiation which can 
displace electrons from atoms or molecules, thereby producing ions.
Examples:  alpha-particle radiation; beta radiation; x-rays, 
gamma, and hard ultraviolet light; and accelerated ions.
Ionizing radiation in large quantities may cause severe skin 
and tissue damage and adverse effects.  (On the other hand, 
but not to belittle the hazards of radiation, we are 
continuously exposed to a "natural background" of ionizing
radiation too.) 

* Ionosphere:  Ionized region of the upper earth atmosphere, which
behaves like a plasma, including reflection of AM radio waves and
generation of auroral glows.

* Irradiation:  Process of exposure to radiation.

* Isomer, Nuclear:  two nuclei with the same nuclear mass (total
number of protons and neutrons) but different nuclear compostions.
(e.g.: T & 3He are isomers: T has 1p, 2n; 3He has 2p, 1n)

& Isotope: One of several species of the same element, 
possessing different numbers of neutrons but the same number 
of protons in their nuclei.  Most elements have several 
stable isotopes, and also several possible unstable and 
semi-stable isotopes.  The chemical and physical properties
of the different isotopes are generally the same (except for the
slight mass difference and the possibility of radioactivity).
Examples include the hydrogen isotopes protium (ordinary
hydrogen), deuterium, and tritium (two neutrons, one proton); 
also uranium 238, 233, and 235.  The chemistry of an element 
depends only on the number of protons (nuclear charge) and 
is therefore the same for all isotopes of an element, but 
the nuclear properties of different isotopes will be 
different.  There are roughly 300 known stable isotopes,
and over 1000 unstable ones.

& Isotropic:  adjective which describes a medium whose 
physical properties are independent of the direction in
which they are measured.

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