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# Conventional Fusion FAQ Glossary Part 7/26 (G)

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Archive-name: fusion-faq/glossary/g
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 7:  Terms beginning with "G"

FREQUENTLY USED TERMS IN CONVENTIONAL FUSION RESEARCH
AND PLASMA PHYSICS

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
$= unit of measurement The list of Acknowledgements is in Part 0 (intro). ================================================================== GGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGG$ G:  abbreviation for Gauss; see entry

@ GA:  General Atomic; see entry.

@ GDC:  Glow Discharge Cleaning; see entry.

@ GN2:  Gaseous diatomic Nitrogen (N2)

@ GNP:  Gross National Product; see entry.

@ GW:  Gigawatt; see entry

@ GWe:  Gigawatt - electrical energy; see gigawatt

@ GWt:  Gigawatt - thermal energy; see gigawatt

& Gain:  (Amplification)  Increase in a signal transmitted
from one point to another through an amplifier.

# gamma:  Third letter in the Greek alphabet; variable used for
a number of things in phyics; in plasma physics gamma is often
used as the variable for growth rates of instabilities.

* Gamma Emission:  Nuclear decay process whereby the nucleus goes
from an excited state to a more stable state by emitting a gamma
ray.  (See entry for gamma ray.)

* Gamma Rays:  Electromagnetic radiation (photons) with energies
greater than (roughly) 100 keV (that is, 100,000 electron volts).
Gamma radiation frequently accompanies alpha and beta decays,
and always accompanies fission.  Gamma rays are highly penetrating
and are best shielded against using dense materials, such as
lead or depleted uranium.  (Gamma rays are similar to X-rays, but
are generally higher in energy and nuclear in origin.)  See

% Garching:  A town in Germany just north of Munich, where the Max
Planck Institute for Plasma Physics (see entry) is located.  "Garching"
in plasma physics frequently refers to the Max Planck Institute.

* Gas Blanket:  A cold, dense volume of gas surrounding a hot
plasma and used to protect a material wall from bombardment
by hot ions (with subsequent sputtering and impurity production).

! Gauss, Carl Friedrich: (1777-1855) German mathmetician, astronomer
and physicist.

$Gauss - unit of magnetic field strength (CGS units) 10,000 gauss = 1 tesla (see also Tesla) & Gaussian Units - See CGS Units % General Atomic: U.S. corporation involved in fusion research; operates the DIII-D device in San Diego; see also Doublet III-D. (Officially known as GA Technologies, I believe; or is that the name of the parent company???) & Getters: Materials which absorb ("get") atoms and can be used in purifying near-vacuum atmospheres.$ Gigawatt:  Unit of power equal to 10^9 watts, 1000 megawatts,
or 1 million kilowatts.  See entry for watt.  1 gigawatt is a typical
size for a nuclear fission reactor, and is expected to be the typical
size of a fusion reactor.

* Glow Discharge:  Low-density, low-temperature plasma discharge
(such as in a fluorescent light) which, well, glows.  Sputtering
in glow discharges is useful in plasma processing of materials.
The voltage applied to the plasma must be greater than the
ionization potential of the gas used; most of the plasma voltage
drop is near the cathode, where the majority of ionization occurs.
Discharge is sustained by secondary electrons emitted when ions
or recombination radiation impact on the cathode; electrons are
accelerated away from the cathode and ionize neutral gas in the
discharge.

* Glow Discharge Cleaning:  Cleaning in which impurities are
removed by sputtering in a glow discharge. (?)

* Grad-Shafranov Equation:  Reduced MHD-equilibrium equation for
an axisymmetric, toroidal plasma.  (Similar reduced equations
can be derived for the cases of helical symmetry and for
the straight cylinder.)  Analytic and numerical studies of these
equations are important in exploring potential plasma
configurations.

-> Additional info Contributed by James Crotinger, with minor
revisions:

The lowest order force balance in the plasma is simply that
the Lorentz force must be balanced by the pressure force.
This balance, combined with Maxwell's equations, determines
the equilibrium configuration of the magnetic field.  When
the toroidal configuration is axisymmetric, and the equilibrium
plasma flow is zero, the magnetic field may be written in
terms of a stream function \psi that satisfies the

\Delta*\psi = - \mu_0 R^2 p'(\psi) - FF'(\psi).
Here p is the plasma pressure and F = R B_\phi.
(R is the radial distance from the axis of the machine)

(Alternatively, leaving out the equation):
In an axisymmetric torus, in the absence of equilibrium plasma
fluid flows, the magnetic field may be written in
terms of a scalar potential. When the plasma is in equilibrium
(forces balance and the plasma is stationary), this scalar
potential obeys a non-linear elliptic equation known as the

& Gradient:  Mathematical term for the operator which determines
the magnitude and direction of the greatest rate-of-change of a
given function with position.  Similarly used to describe such
a rate-of-change.  For instance, at a given point on a hill, the
slope of the hill in the steepest uphill direction is the gradient
of the altitude funtion for the hill.

& Gravitational Force:  Force which attracts two bodies together
based on the product of their masses and the reciprocal of
the square of their distances.  "Gravity" is the force field
created by one massive body (like the earth) which another body
(like you) will experience.

* Gross National Product:  Total value of goods and services
produced in a country; measure of economic strength of a nation.

\$ Gray:  A unit of absorbed dose of radiation, equal to one
joule of energy per kilogram of mass.   1 Gray = 100 rads.
Defined relative to the material into which such radiation passed,
which should therefore be specified.

& Group Velocity:  This is derived from the dispersion relation
(see entry) as Vgroup = dw/dk; the group velocity is the rate
at which modulations or information within a wave travel through
a given medium.

* Guiding Center:  Particles placed in a magnetic field will
gyrate in circles around the magnetic field lines, and drift in
various directions.  The guiding center represents the
instantaneous center of the circular motion.  The idea is that
you can think of the guiding center as drifting, and the particle
as orbiting the guiding center.

* Gyrofrequency:  See cyclotron frequency.

* Gyromagnetic Ratio:  Ratio of the magnetic moment to the
angular momentum of a particle.  (see magnetic moment, angular
momentum)

* Gyrotron:  A device for producing microwave energy that
utilizes a strong axial magnetic field in a cavity resonator
to produce azimuthal bunching of an electron beam.