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Conventional Fusion FAQ Glossary Part 18/26 (R)

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Archive-name: fusion-faq/glossary/r
Last-modified: 25-Feb-1995
Posting-frequency: More-or-less-quarterly
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Glossary Part 18:  Terms beginning with "R"


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).


@ Rad:  Radiation Absorbed Dose; see entry "rad"

@ Rem:  Raditation (or Roentgen) Equivalent for Man; see entry "rem"

@ R&D:  Research and Development.

@ RD&D:  Research, Development, and Demonstration.

@ RF:  Radio Frequency; see entry

@ RF Current Drive:  Radio Frequency Current Drive; see entry

@ RF Heating:  Radio Frequency Heating; see entry

! R.F. Heeter:  Plasma physics graduate student at PPPL; the editor 
of the sci.physics.fusion FAQ, bibliography, and glossary. :)
(note similarity to RF Heating.)

@ RF Plugging:  See entry for radiofrequency plugging

@ RF Trapping:  See entry for radiofrequency trapping

@ RFC: Reversed-Field Configuration: see Field-Reversed Configuration.

@ RFP: Reversed-Field Pinch; see entry

@ RFX: Reversed-Field eXperiment; see entry

@ RGA: Residual Gas Analyzer

$ Rad:  radiation absorbed dose.  A unit used to measure the
amount of radiation energy absorbed per gram of a given 
substance, that is the dose.  One rad means absorption of
100 ergs of energy per gram.  See also gray, rem, sievert.

* Radial Ponderomotive Force Stabilization:  In magnetic
mirror devices, use of rf waves in the neighborhood of
the ion cyclotron frequency to stabilize interchange
modes.  The radial ponderomotive force produced by
a radial gradient in the applied rf electric field
opposes the destabilizing centrifugal force resulting
from bad magnetic field curvature.  The net particle
current is in the direction that would result from
field lines with good curvature, eliminating the drive
for the interchange instability.

& Radiation:  (Sense 1) Process of emission of energy from a body 
in the form of light or heat waves, or energetic particles 
such as alpha particles, electrons, or neutrons.  (Sense 2) 
Radiation also refers to *what is emitted* when an object 
radiates (but not *what does the emitting*).  A nucleus
which does the emitting is said to be radioactive.  Electrons
in atoms can also emit radiation in the form of ordinary 
visible light; such atoms are not said to be radioactive.

* Radiation Damage, Bulk:  General term describing 
changes in chemical and/or metallurgical properties of
structure components of fusion reactor caused by atomic
displacement and nuclear transmutation events occuring
as a result of exposure to a radiation environment
(such as the neutrons emitted from a fission or D-T fusion

* Radiation Damage, Surface:  General term describing 
damage to the surface of the containment structure which
directly interfaces with the thermonuclear plasma;
includes such phenomena as radiation blistering, 
charged-particle (or neutron) sputtering, and spallation
or exfoliation of layers of the surface.

& Radioactive Decay:  Spontaneous transformation of one
nuclide into a different nuclide or into a different energy
state of the same nuclide.  This process results in a decrease,
with time, in the number of originally radioactive atoms
in a sample.  See Decay Modes for a listing of the different
mechanisms by which radioactive decay can occur.

& Radioactive waste: Equipment and materials from nuclear 
operations which are radioactive and for which there is 
no further anticipated use.  Wastes are generally classified
as high-level (having radioactivity concentrations of 
hundreds to thousands of curies per gallon or cubic foot),
low-level (in the range of 1 microcurie per gallon or
cubic foot), or intermediate (between high and low).
See also curie.

& Radioactivity:  Characteristic property of unstable nuclei
which decay to other nuclei by emission of radiation.  A list
of common decay / transmutation modes is given under "decay

* Radio Frequency or radiofrequency:  Term used to 
describe electromagnetic radiation with frequencies
less than infrared, but greater than "audio frequencies,"
i.e., greater than about 15,000 Hz.  Wavelengths are
therefore less than about 20,000 km and greater
than about 0.01 mm.  (These numbers are not precise.)

* Radio Frequency Current Drive:  Plasma waves in the 
radio-frequency range can be used to push plasma particles
in such a way that current forms in the plasma; this is a
method of non-inductive current drive (see entry) which
would allow for steady-state fusion reactors to operate.

* Radio Frequency Heating:  Process for heating the plasma by
transferring energy to ions or electrons using waves generated
by an external oscillator at an appropriate frequency.  (This is
similar to how a microwave oven heats food.)  There are various
types:  see also ECRH, ICRH, and Lower Hybrid...

* Radiofrequency Plugging:  Use of axial ponderomotive force
to plug an open-ended device.  First demonstrated on RFC-XX,
Institute of Plasma Physics, Nagoya University, Nagoya, Japan,
and later demonstrated in the Phaedrus device at the 
University of Wisconsin.

* Radiofrequency Trapping:  Use of RF waves to pitch angle
scatter particles flowing axially into a magnetic mirror cell.
The particles are scattered out of the loss cone and are
trapped.  (See entries for pitch angle scattering, magnetic 
mirror, loss cone.) 

* Railgun Accelerator:  Projectile accelerator which
accelerates the particle using electromagnetic forces which 
arise when the particle completes an electrical circuit between 
two conducting rails connected to a source of high current.

* Raman Effect:  A phenomenon observed in the scattering of
light as it passes through a transparent medium; the light
undergoes a change in frequency and a random alteration
in phase due to a change in rotational or vibrational
energy of the scattering molecules.

* Ramsauer Effect:  Term for a quantum-mechanical effect 
allowing free electrons within a narrow range of energies 
to pass through a noble gas with very little elastic scattering.

* Rational Surface:  (related to q-factor, see entry)
Magnetic flux surface (see entry for this too) where the ratio
of toroidal to poloidal field strengths is a rational number;
this means that a particle travelling along this surface makes
an integer number of turns in each direction and then its orbit
closes in on itself.  The result is that the particle doesn't
sample the entire flux surface in its motion, which is important
for various technical reasons (which mostly result in reduced
confinement); see also magnetic island.

* Reactor:  See fission reactor, fusion reactor.

& Recombination Coefficient:  The rate of recombination of
positive ions with electrons (or negative ions) in a gas,
per unit volume, divided by the product of the particle 
densities of the two species (positive ions and 
electrons/negative ions) involved.

* Recombination Radiation: radiation produced when a 
free electron in a plasma is captured by an ion.

& Reflectivity:  Fraction of incident radiant energy which
is reflected by a given surface.  (The power which is not
reflected is either absorbed or transmitted.)

& Refraction:  Bending of waves as they pass from a medium
having one refractive index to a medium (or region within
a medium) having a different refractive index.

$ Rem:  Radiation (or Roentgen) Equivalent for Man.  Unit of 
absorbed radiation dose based on the definition rem = rad * quality.  
The quality factor depends on the type of radiation involved and 
is used to scale the radiation dose based on the relative 
harmfulness of different sorts of radiation, compared to
ordinary X-rays.  Annual US average dose is about 300 millirem 
(0.3 rem), of which more than 2/3 is natural (primarily radon
and cosmic rays), and the majority of the human-generated dose 
is due to medical uses (primarily X-rays).  See also radiation
dose, roentgen, sievert, rad, gray.

* Reserves:  Amount of a substance which can be extracted from
the earth with current technology at current prices.  Typically
much smaller than resources (see entry for resources).  

& Resistance (electrical):  Ability of a given object to
resist the flow of electrical current.  To drive a given 
current a voltage must be applied to overcome the resistance 
according to V = I * R (V = voltage, I = current, R = resistance).
Resistance is determined by resistivity and geometrical factors.

* Resistive Instability:  Instability resulting from macroscopic
equations used to model a plasma of finite conductivity / nonzero

& Resistivity:  Tendency for a material/substance
to resist the flow of electrical current and to dissipate its
energy.  Resistivity, when combined with certain geometry
factors (generally length and cross-sectional area for wires)
determines resistance. 

* Resource:  Total amount of a substance which exists in the
earth and could conceivably be extracted someday at some price
with some technology.  Typically much larger than reserves.  
(See also reserve.)

> Reversed-Field Pinch (RFP):  A toroidal magnetic confinement scheme
which could constitute an alternative to the Tokamak for building a
fusion reactor.  It is characterized by a magnetic field mostly
generated by the plasma itself, with toroidal and poloidal components 
of comparable intensities, in contrast with the Tokamak where most of
the field is toroidal and externally applied. The name of the
configuration is given by the fact that the toroidal component of the
magnetic field changes sign in the outer region of the plasma. The 
main attractivness of the Reversed Field Pinch is that, according to
presently established scalings, it could reach ignition without the 
need of auxiliary heating. 
(Emilio Martines,

> Reversed-Field eXperiment (RFX): It is the largest Reversed Field
Pinch device presently in operation.  Located in Padova (Italy) it 
is planned to reach a plasma current of 2 MA.
(Emilio Martines,

* Roentgen:  Unit of exposure to ionizing radiation.  The Roentgen
is that amount of gamma or X-rays (electromagnetic radiation)
required to produce ions carrying one electrostatic unit of
electrical charge (either positive or negative) in 1 cubic
centimeter of dry air under standard conditions.  Named after
Wilhelm Roentgen.  (Compare with curie, rad, gray, sievert.)

! Roentgen, Wilhelm:  German scientist who discovered X rays in 1895.

* Rogowski Loop or Coil:  A coiled wire loop which encircles a
current-carrying plasma.  Changes in total plasma current induce a
voltage in the loop; integrating (adding up) the voltage over time
gives the plasma current.

* Rotational Transform:  (labels: \iota = 2*PI/q)
Due to the combination of applied toroidal field and induced
poloidal field, the magnetic field lines wind helically around
the torus (and on most flux surfaces they fill the surface
ergodically).  The rotational transform is a measure of this
helicity, and is defined as the average angle the field line
shifts in the poloidal direction per complete circuit in the
toroidal direction. The quantity q = 2*\pi / \iota is known
as the ``safety factor'' because of its role in stability theory.
(Contributed by James Crotinger)

Entry from 1985 OSTI Glossary:  A magnetic field configuration
is said to posses rotational transform if the lines of force,
after one complete circuit around the configuration (e.g., a torus)
do not simply close exactly on themselves, but are instead
rotated through some angle about the magnetic axies.  
(See entries for flux surface, magnetic axis, toroidal and poloidal
field, helicity, safety factor.)

* Runaway Electrons:  Those electrons in a plasma that gain energy 
from an applied electrical field at a faster rate than they lose 
it through collisions with other particles.  These electrons tend 
to "run away" in *energy* (not position) from the cooler remainder 
of the background plasma, because the collision cross-section 
decreases as the particle's velocity increases, so that the faster 
the particle goes, the less likely it is to be stopped.  
See also:  collision cross-section.

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