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Conventional Fusion FAQ Glossary Part 4/26 (D)

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


Edited by Robert F. Heeter,

Guide to Categories:

* = vocabulary specific to plasma/fusion/energy research
& = basic/general 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).


@ D: nuclear/chemical symbol for deuterium/deuteron

@ DC, dc:  Direct Current; see entry

@ DCLC:  Drift Cyclotron Loss Cone instabilities; see entry.

@ DT:  Deuterium-Tritium; see entry labeled DT Fuel

@ DIII-D:  not an acronym (anymore); see entry

@ DOE:  Department of Energy (United States); see entry

@ dpa:  Displacements per atom; see entry

@ DPP:  Division of Plasma Physics; see APS-DPP

* D-shaped plasma:  A toroidal plasma whose cross section
(poloidal plane) is a D (instead of a circle).  A D-shape
has a higher beta limit (see entry) than a circular shape.

* Debye Length: The characteristic distance over which charges are
shielded in a plasma.  See also: Debye shielding.
lambda_D = ( epsilon_0 k_B T_e / (n_e e^2) )^(1/2)
lambda_D[m] = (7.434*10^3)*(_e[eV])^(1/2)*n[m^(-3)]^(-1/2)
(Arthur Carlson,

! Debye, Peter Joseph:  Physical chemist, studied behavior of
conductive solutions (plasmas have some similar behaviors).

* Debye Radius:  See Debye Length.

* Debye Sheath:  The region of strong electric field in front of
a material surface in contact with a plasma.  Its characteristic
thickness is the Debye length, and it is caused by Debye shielding
of the negative surface charge resulting from electrons flowing to
the surface much faster (initially) than the ions.  The lost
electrons leave behind a region of net positive charge which
gradually diminishes the strength of the electric field
over the debye length.  See also: Debye Length, Debye Shielding.
(Arthur Carlson,, with modifications by
John Cobb,

* Debye Shielding:  If a positive (or negative) charge is inserted
into a plasma, it will change the local charge distribution by
attracting (repelling) electrons.  The net result is an additional
negative (positive) charge density which cancels the effect of the
initial charge at distances large compared to the Debye length.
(There is a corresponding effect of shielding by the ions, which,
for various and subtle reasons, usually is less important.)
See also: Debye Length.
(Arthur Carlson,

* Debye Sphere:  Sphere around a charged test particle whose
radius is equal to the Debye length.

& Decay, Radioactive: See radioactive decay.

* Decay Modes:  Different pathways for decay of radioactive nuclei.
The decay modes for a given unstable state can include beta
emission (negative = electron, positive = positron), electron
capture, alpha emission, fission, and gamma emission.
(Did I miss any?) See entries for each mode for more information.

* Dee-Shaped:  see D-shaped plasma above.

* Degenerate Configuration:  Magnetic field configuration in
which the magnetic lines of force close exactly on themselves
after passing around the configuration a finite number of times.

* Dense Plasma Focus:  See Plasma Focus.  (Densities of up
to 10^26 particles/m^3 have been reported.)

& Density:  amount per unit of volume, or per unit surface area, or
per unit length. (Usually specified or clear from context which
of these is meant).  Several types:
 Charge density   - amount of charge per unit (volume, area, length)
        Current density  - current flow per unit transverse surface area.
        Energy density   - amount of energy per unit volume.
        Flux density     - flux per unit of transverse surface area.
        Mass density     - mass per unit volume.
        Number density   - number of particles per unit volume.
        Particle density - same as number density.

% Department of Energy:  (DOE) Department within the
executive branch of the U.S. government (at the cabinet
level) which has managed and overseen federally-sponsored
energy research.  The DOE was formed in 1977 from ERDA,
the Energy Research and Development Administration,
and (I think) the Atomic Enegy Commission (AEC).

& Deuterium: A heavy isotope of hydrogen whose nucleus
contains both a neutron and a proton.

* Deuteron: A deuterium ion; nucleus consisting of a proton
and a neutron.

* Diagnostics:  (from Herman) Procedures for determining
(diagnosing) the state of a plasma during an experiment;
also refers to the instruments used for diagnosing.

* Diamagnetic Effects:  Application of a magnetic field to a plasma
will tend to create circulating current within the plasma that will
reduce the strength of the magnetic field.

* Diffusion:  The interpenetration of one substance into another
as a result of thermal / random motion of the individual particles.
(e.g., the diffusion of a plasma across a magnetic field as a
result of collisions which cause particles to move along new
field lines.)  See also classical diffusion, neoclassical diffusion,
anomalous diffusion, transport.

* Direct Conversion:  The generation of electricity by direct
recovery of the kinetic energy of the charged fusion reaction

& Direct Current:  Electric current which is unchanging in
time, or at least not oscillating.  Opposite of Alternating Current.

* Direct Drive:  An approach to inertial-confinement fusion
in which the energy of the driver (laser or particle beam)
is directly incident on the (usually spherical) target,
causing compression heating via ablation of the target surface.

* Dispersion Relation:  For a given wave, the dispersion relation
relates the temporal frequency of a wave (w, or omega) to its
wavenumber k and other physical quantities characteristic of
the system.  Dispersion relations can be quite simple
(e.g., w = k * c for light; c being the speed of light),
and they can also be quite complex, with interesting
mathematical structure.  The dispersion relation and its
mathematical structure provide important information
about the wave, including the phase and group velocities.
(See relevant entries.)  Note that the meaning of "dispersion
relation" is different in plasma physics than in other fields.

* Displacements Per Atom: (dpa)  This is a measure of the
amount of radiation damage in neutron-irradiated materials;
e.g., 10 dpa means each atom in the material has been
displaced from its structural lattice site and average of
10 times (due to interactions between the atoms and the
energetic neutrons irradiating the material.)

* Disruption:  Plasma instabilities (usually oscillatory modes)
sometimes grow and cause disruptions of the carefully-engineered
plasma conditions in the reactor.  Major disruptions can cause
an abrupt temperature drop and the termination of the plasma.
Stored energy in the plasma is rapidly dumped into the rest
of the plasma system (vacuum vessel walls, magnet coils, etc.)
and can cause significant damage if precautions are not taken.

* Disruptive Instability:  Instability which causes a disruption;
see entry for disruption.

* Dissociative Recombination:  The combination of an electron
with a positive molecular ion, followed by dissociation of
the molecule in which the resulting atoms/molecules carry
off the excess energy released in the recombination.

& Distribution Function:  Function characterizing the density of
particles located at a given point in phase space (a combination
of either velocity or position coordinates) at a given time.
The velocity-space distribution function gives the number of
particles with a particular velocity; the position-space
distribution function is synonymous with the particle density
in position-space.  Different combinations of position and
spatial coordinates are useful in different problems.

* Divertor: Component of a toroidal fusion device that diverts
charged particles on the outer edge of the plasma into
a separate chamber where they strike a barrier and become
neutralized.  In a reactor, the divertor would incorporate a
system for pumping out the neutralized particles as exhaust
from the machine.  A divertor, like a limiter, prevents the
particles from striking and degrading the chamber walls
and dislodging secondary particles that would cool and
contaminate the plasma.  Whereas a limiter is a material
object used to limit the shape of the plasma, a divertor is a
magnetic-field construction.  The advantage of the divertor
is that it allows the neutralization region to be
removed from the main plasma.  See also: limiter.

* Doppler Broadening:  Frequency spreading which causes
broadening of single-frequency radiation (e.g., spectral
lines) when the radiating bodies (atoms, molecules, etc.)
have different velocities.  Radiation from each individual
radiating body has a different Doppler shift, and
the collection of radiations at different frequencies
broadens the peak of the line in an intensity-vs-frequency

& Doppler Effect:  Variation in the frequency of a
wave (as measured by an observer) due to relative motion
between the observer and the source of the wave.
(The observed frequency increases if the source is moving
towards the observer.)

& Doppler Shift:  The amount of change in the observed
frequency of a wave due to the Doppler effect; sometimes
called the Doppler frequency.

> DIII-D:  Latest in a series of tokamaks designed
by General Atomics (formerly GA Technologies) in San Diego
making plasmas with noncircular cross sections, including
kidney shapes and D-shapes.

* Doublet Device / Doublet Plasma:  Tokamak-type devices
where the plasma cross-section is kidney-shaped, with
a deep indentation in the middle so that the plasma has
two major rings of current (on top and bottom).

* Drift Cyclotron Loss Cone Instabilities:  (DCLC)
This is an electrostatic microinstability (frequencies
at harmonics of the ion cyclotron frequency) which is of
major concern in small mirror devices.  Mode is driven
by radial gradients in the electron density, and causes
loss of ions due to non-conservation of magnetic moment
(see adiabatic invariant) as they interact with the mode,
and are dispersed in velocity space into the loss cone.
Stabilization is accomplished by increasing the plasma
size and by partially filling the loss cone with a
continuous extermal warm plasma stream.

* Drift Motion:  Ordinarily particles placed in a magnetic
field will simply orbit in circles, but if the magnetic field
is not uniform, or curves, or there is an electrical field
perpendicular to the magnetic field, or another force is applied
perpendicular to the magnetic field, then the "guiding centers"
of the particle orbits will drift (generally perpendicular to
the magnetic field and to the applied force).  There are several
sorts of drifts; refer to a plasma physics text for more
information (see Section 11: Bibliography).  For a good
introduction at the undergraduate physics level, see Chen.

* Drift Pumping:  A process that removes ions trapped in
a thermal barrier using radial transport induced by an
exterally-applied radiofrequency field tuned to resonate
with the azimuthal drift frequency.

* Drift Surface:  Surface on which the guiding center of
a particle is constrained to move, due to the effects of
the laws of adiabatic invariance on its drift motion.

* Drift Velocity:  Characteristic velocity at which the center
of a particle's orbit ("guiding center") drifts when drift motion
(see above) occurs.

* Drift Waves:  Oscillations in a magnetically-confined plasma
arising in the presence of density gradients (such as at the
plasma's surface).  These resemble the waves that propagate
at the interface of two fluids with different density in
a gravitational field.

* DT Fuel:  (Deuterium-Tritium) Easiest fuel mixture to use
in achieving fusion; unless otherwise specified, probably refers
to a 50-50 (by numbers or by moles) mix of deuterium and tritium.

* Duty Factor:  Ratio of the duration of time when a system is
actually operating to the total time for a complete cycle
of the system.  e.g., if a tokamak experiment runs for 5 seconds
and then sits for 500 seconds while the power supplies are
recharged, then the duty factor is 1%.  Similar to capacity
factor for powerplants.

* Dye laser:  A type laser in which the active material
(the material which emits the laser light) is a dye.
These lasers are tunable when the dye has very large
molecules (such as acridine red or esculin) and the laser
action takes place between the first excited and ground
electronic states, because each of these states contains
a broad continuum band of vibrational-rotational levels.

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