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Conventional Fusion FAQ Glossary Part 6/26 (F)

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Archive-name: fusion-faq/glossary/f
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
it to anyone who might be interested (and willing to help!!!).

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Glossary Part 6:  Terms beginning with "F"


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).


# F:  Variable typically used for force; sometimes flux.

& F:  Chemical symbol for the element fluorine.

* FLiBe or flibe or FLIBE:  Fluorine-LIthium-BEryllium; see
entry "Flibe" below.

@ FEDC:  Fusion Engineering Design Center; see entry

@ FLR:  Finite Larmor Radius; see entry

@ FPD:  Fusion Power Demonstration facility; see DEMO.

@ FRC:  Field-Reversed Configuration; see entry

@ FY:  Fiscal Year; see entry

& Fabry-Perot Interferometer:  A type of interferometer with
two parallel mirrors (with a variable separation of a
few centimeters) arranged so that incoming light is reflected
between them multiple times before ultimately being transmitted.
Useful in spectroscopy because it gives very good frequency
resolution without losing too much of the incident signal.

* Faraday Rotator:  A device which rotates the plane of
polarization of an optical-light pulse, typically by
using a glass disc suitably doped with a magnetic ion
and placed in a magnetic field.  These devices are used
to isolate (protect) a laser amplifier chain against
back-reflection from the laser target; the Faraday rotator
in this case gives a 90-degree phase change on the round
trip, so that the returning light is rejected by a
polarizer which transmits the outgoing light.

* Fast neutron:  Neutron with energy greater than roughly
100,000 electron volts (100 keV).  Distinguished from slow or
thermal neutrons.  (See appropriation entries.)

* Fertile Material:  In nuclear physics, this refers to a nuclide
which converts to fissile material (see entry) upon neutron
capture and subsequent radioactive decay.  Examples include
Uranium-238 and Thorium-232.

& Field:  In physics, any macroscopic quantity which exists
(and typically varies) througout a region of space.  Standard
examples include Electric and Magnetic fields, velocity flow fields,
gravitational fields, etc.

& Field Lines:  Lines in space along which a field is either
changing or not changing (depends on the field) but which help
to create diagrams which characterize the behavior and effects
of the field.  For instance, electric field lines run in the
direction that the electric field will push charged particles;
the strength of the field is proportional to the density of
the field lines.  On the other hand, the magnetic force pushes
particles in a direction perpendicular to both the particle's
velocity and the direction of the magnetic field line.

> Field-Reversed Configuration:  A compact torus produced in a
theta pinch and having (in principle) no toroidal field.  The
potential advantages for a fusion reactor include a simple (linear)
machine geometry, an average plasma pressure close to the confining
field pressure, and physical separation of formation and burn
chambers.  The are predicted to be violently unstable to tilting, but
this is rarely observed.  See also: compact torus, theta pinch.
(Arthur Carlson,

* Field Shaping Coils:  Type of poloidal field coils (in a tokamak)
which create magnetic fields which shape and control the plasma.
Used to constrain horizontal and vertical displacements of the plasma,
as well as (in some configurations) produce non-circular plasma
cross sections (poloidal cross-section) and/or create one or more
divertor separatrices.  (See relevant entries.)

* Finite Larmor Radius:  In many plasma theories the size of the
Larmor radius (or gyroradius - see entries) is assumed to be
negligibly small, or infinitesimal.  Different effects occur when
the size of the Larmor radius is finite and needs to be considered.
(Anyone out there with a succint, but more detailed explanation?)

* First Wall:  The first physical boundary that surrounds a plasma.

* Fiscal Year (FY):  Year used to open and close accounting records;
not necessarily the same as the calendar year.  (For instance, the
U.S. government's Fiscal Year begins Oct 1 and ends Sept 30.)

* Fishbones:  Oscillations in soft x-ray emissions which occur
during intense neutral-beam heating; associated with a recurring
m=1 internal kink mode.  Mode was given its name from its
characteristic signal (looked like the bones of a fish, of course).
Fishbones are associated with loss of fast ions from the plasma
and are triggered by exceeding the upper limit on plasma beta.
(see relevant entries)

* Fissile Material:  Material containing a large number of
easily fissionable nuclei which give off multiple neutrons in
the fission process.  Usual meaning is that if a sufficiently
large amount of fissile material is put together, a fission
chain reaction can occur.  Sometimes used synonymously with
"fissionable material," i.e., material that *can* be fissioned
(though often under restricted circumstances, such as only with
thermal (slow) neutrons).  A more restricted meaning use of
fissile material limits the concept to those materials which can
be fissioned by neutrons of all energies (fast & slow).
Examples include Uranium-235 and Plutonium-239.

& Fission (Nuclear): Nuclear decay process whereby a large
nucleus splits into two smaller (typically comparably-sized)
nuclei (which are thus nuclei of lighter elements), with or
without emission of other particles such as neutrons.
When it occurs, fission typically results in a large energy
release.  Fission can occur spontaneously in some nuclei, but
is usually caused by nuclear absorption of gamma rays,
neutrons, or other particles.  See also spallation, radioactivity.

* Fission Bomb:  see atomic bomb, A-bomb.

* Fission Reactor:  (from Herman) A device that can initiate
and control a self-sustaining series of nuclear fissions.

* Flat-top:  Stable period in the middle of a tokamak
discharge, characterized by a flat, stable peak in a plot
of plasma (current, temperature) vs. time.

* Flibe:  Molten salt of Fluorine, Lithium, and Beryllium;
candidate blanket/coolant/breeder material for fusion reactors.

* Flute Instability:  Term used to describe an interchange
instability in which the perturbation is uniform parallel
to the magnetic field. In cyclindrical geometry, the structure
resembles a fluted column (as in classical architecture).
Occurs in some mirror machines.

& Flux:  The total amount of a quantity passing through a given
surface per unit time.  Typical "quantities" include field lines,
particles, heat, energy, mass of fluid, etc.  Common usage in
plasma physics is for "flux" by itself to mean "magnetic field

& Flux Density:  Total amount of a quantity passing through a
unit surface area in unit time.  See also flux, above.

* Flux freezing:  See frozen-in law.

* Flux surfaces:  See magnetic flux surfaces.

* Flux trapping:  See frozen-in law.

& F-number:  In optics, denotes the ratio of the equivalent focal
length of an objective lens to the diameter of its entrance pupil.

* Fokker-Planck Equation:  An equation that describes the time rate
of change of a particle's velocity as a result of small-angle
collisional deflections.  Applicable when the cumulative effect of
many small-angle collisions is greater than the effect of rarer
large-angle deflections.

& Force:  Rate of change of momentum with time.  Forces are said
to cause accelerations via F = ma (Newton's law).  There are four
primary forces known presently:  the gravitational, electromagnetic,
weak nuclear, and strong nuclear forces.  The gravitational and
electromagnetic forces are long-range (dropping as 1/distance^2),
while the nuclear forces are short range (effective only within
nuclei; distances on the order of 10^-15 meters).  The
electromagnetic force is much stronger than the gravitational force,
but is generally cancelled over large distances because of the
balance of positive and negative charges.  Refer to entries for each
force for more information.  See also momentum.

* Free Electron:  An electron not bound to an atom, molecule, or
other particle via electric forces.

* Free Wave:  A wave (e.g., electromagnetic) travelling in a
homogeneous infinite medium (no boundary conditions).

* Frozen-in Flow Law:  In a perfect conductor, the total magnetic
flux through any surface is a constant.  In a plasma which is nearly
perfectly conducting, the relevant surfaces move with the plasma;
the result is that the plasma is tied to the magnetic field, and
the field is tied to the plasma.  Motion of the plasma thus
deforms the magnetic field, and vice versa.

* Fusion (Nuclear): a nuclear reaction in which light atomic
nuclei combine to form heavier nuclei, typically accompanied
by the release of energy.  (See also Controlled
Thermonuclear Fusion)

% Fusion Engineering Design Center:  Facility managed by
ORNL and staffed mainly by industrial personnel; undertakes
detailed engineering design of planned fusion facilities.
(Is it still in operation?  The reference I have is out of date.)

* Fusion Reactor: Device which creates energy in a controlled
manner through fusion reactions.

> Fusion-Fission Hybrid: Proposed nuclear reactor relying
on both fusion and fission reactions.  A central fusion
chamber would produce neutrons to provoke fission in a
surrounding blanket of fissionable material.
The neutron source could also be used to convert other
materials into additional fissile fuels (breeder hybrid).
Safer than a plain fission reactor because the fission fuel
relies on the fusion neutrons, and therefore won't spontaneously
melt down.  On the other hand, hybrids are more complex because
of the fusion power core, and still generate fission's radioactive
byproducts.  But could be more economical and have easier technical
requirements than a straight fusion reactor.

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