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Archive-name: fusion-faq/glossary/s
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Glossary Part 19: Terms beginning with "S"
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).
==================================================================
SSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSS
@ (?) SHIVA: see entry under Shiva, below.
@ SI: Systeme Internationale; see SI Units
@ SM: Symmetric Mirror
@ SN: Single Null; see entry for Divertor
@ SNL: Sandia National Laboratories; see entry
@ SOL: Scrape-Off Layer; see entry
@ STM: Symmetric Tandem Mirror experiment.
$ Sv: Sievert; see entry
* Safety Factor: (q) The number of times a field line goes around a
torus "the long way" for each time around "the short way". In a
tokamak, this number is typically near unity in the center of the
plasma and between two and 6 or 8 at the edge. So-called because it
helps to determine the degree of stability the plasma has against
certain instabilities. The safety factor is the inverse of the
rotational transform, and can be expressed mathematically
as q = (r * Bt )/(R * Bp), where r and R are the minor and major
radii of the torus, and Bt and Bp are the toroidal and poloidal
magnetic fields.
! Sakharov, Andrei: Russian physicist; among other achievements, he
is credited with the initial design of the tokamak.
% Sandia National Laboratories: Located in Albuquerque, New Mexico.
Another large DOE laboratory; has PBFA-II (Particle Beam Fusion
Accelerator, an ICF device) and some pinch devices. Some divisions
located in Livermore (Sandia-Livermore).
* Sawtooth: When a tokamak runs with enough current to achieve
q < 1 on the magnetic axis, the plasma parameters (n, T, B)
oscillate with a "sawtooth" waveform. The oscillation is
localized to a region roughly within the q=1 surface, and
arises from internal MHD effects. Confinement is degraded
within the sawtooth region.
* Scaling Laws: These are mathematical rules explaining how
variation in one quantity affects variations in other quantities.
For instance, in a tokamak reactor it's generally believed that
energy confinement depends on the size of the device and the strength
of the magnetic field, but the precise nature of the dependence is
not fully understood, so empirical "scaling laws" are tested to
see what the dependence is. Scaling laws are useful for extrapolating
from parameter regimes where the mathematical relationships
between the various quantities are known, into unexplored regimes.
& Scattering: The deflection of one particle as a result of
collisions with other particles or with waves. See also Elastic.
* Schlieren Method: An optical technique that detects density
gradients occuring in a fluid flow. In its simplest form, light
from a slit is collimated by a lens and focused onto a knife
edge by a second lens; the flow pattern is placed between the
two lenses, and the resulting diffraction pattern is observed on
a screen or photographic film placed behind the knife edge.
* Scientific Feasibility: Fusion will be considered scientifically
feasible when (a) experiments are done which reach scientific
breakeven-type plasma conditions (see entry on breakeven), and
(b) the experimental results suggest that the approach can be
"scaled up" into a power-producing system. Tokamak fusion
reactors are closing in on (a), and tokamak researchers think
(b) holds as well, so they are designing a power-producing
machine (ITER) to demonstrate net energy production from tokamak
fusion. Inertial confinement is also approaching this point.
* Scrape-Off Layer (SOL): [from Art Carlson] Outer layer of a
plasma which is affected ("scraped off") by a divertor or limiter.
That is, the outer layer of a magnetically confined plasma (ca. 2 cm
thick) where the field lines penetrate a material surface (limiter or
divertor plate) rather than close upon themselves. This region
defines the outer limit of the plasma because any plasma crossing
into the SOL is rapidly lost since transport along the field is much
faster than that across the field. That is, particles follow these
field lines into the material surface and are lost from the plasma.
* Screw Pinch: A variant on the theta pinch, in which axial
currents (as in a z pinch, but less intense) produce a poloidal
magnetic field (in addition to the usual longitudinal field),
thus making a corkscrew field configuration. See also theta pinch,
z pinch, pinch device.
* Second-stability Region: A high pressure region where the plasma
becomes stable to the pressure-gradient-driven ballooning
ballooning instability. The plasma is stable in the limit of
small pressure gradients, becomes unstable at some intermediate
pressure, and then becomes stable again at still higher pressures.
Tokamaks operating in the second-stability region would be more
attractive because the higher pressures (beta) would provide more
fusion reactivity per unit volume of plasma, allowing smaller
reactors to be built.
* Separatrix: [from Art Carlson] In a divertor tokamak (and some
other configurations), the last closed flux surface (see entry) is
formed not by inserting an object (limiter) but by manipulating the
magnetic field, so that some field lines take a topologically
different route (through the divertor, rather than simply around the
central plasma). The boundary between the two types of field lines is
called the separatrix.
* Sheared Flow: Fluid flow where the magnitude of the fluid velocity
changes along a direction perpedicular to the direction of the fluid
flow. (Freeway traffic often exhibits sheared flow in that traffic
in the "fast lane" moves more rapidly than traffic in the slow lane
with the exits...) Sheared flow typically correlates with reduced
transport and enhanced confinement. (This definition is rather
informal and may not be fully technically correct - R.F. Heeter)
* Shear Fields: As used in plasma physics, this refers to magnetic
fields having a rotational transform (or, alternatively, safety
factor) that changes with radius (e.g., in the stellarator concept,
magnetic fields that increase in pitch with distance from the
magnetic axis.)
* Sheath: See Debye Sheath
> Shiva: 20-beam Nd-glass fusion laser facility at LLNL. Was
completed in 1977 and used for target irradiation experiments
until mid-1981. Succeeded by Nova.
* Shock Heating: Heating produced by the impact of a shock wave.
* Shock Tube: A gas-filled tube used in plasma physics to
quickly ionize a gas. A capacitor bank charged to a high voltage
is discharged into the gas at one tube end to ionize and heat
the gas, producing a shock wave that may be studied as it
travels down the tube.
* Shock Wave: Wave produced (e.g., in a gas or plasma) as a
result of a sudden violent disturbance. To produce a shock
wave in a given region, the disturbance must take place
in a shorter time than the time required for sound waves to
traverse the region.
$ Sievert: International unit for radiation dose. One Sievert
equals 100 rem (see entry for rem); average per-capita exposure
is about 0.3 Sv, primarily from natural background (see entry)
and medical x-rays.
* Shock Heating: The heating produced by the impact of a shock wave.
* Shock Wave: Wave produced as a result of a sudden, violent
disturbance which occurs in a particular region faster than sound
waves can traverse the region.
* Shot: Fusion jargon for the production of a (short-lived) plasma.
In the early days, plasmas were produced by the "discharge" of
capacitor banks, which (frequently) made a BANG. A modern tokamak
produces a few dozen "shots" per day, each lasting a few seconds and,
if nothing goes wrong, inaudible. See also: capacitor, tokamak
(Arthur Carlson, awc@ipp-garching.mpg.de)
$ SI Units: (also known as MKS, MKSA Units) System of measurement
in which the fundamental units are meters, kilometers, seconds, and
the ampere.
$ Sievert: Unit of absorbed radiation dose equivalent to 100 rem.
(see also rem, rad, Gray) The sievert is based on the Gray in the
same way that the rem is based on the rad, I believe.
& Solenoid: Cylindrical coil of wire which, when current
flows through it, acts as an electromagnet. For long solenoids
with many turns, the magnetic field inside the center is
nearly uniform.
* Solid Breeder: Solid lithium-bearing compounds, usually
ceramics such as Li2O and LiAlO2, which might be used in the
blanket of a D-T fusion reactor to produce ("breed") additional
tritium fuel from the n + Li => He + T (+n) reactions.
* Solid State Laser: A laser using a transparent substance
(crystalline or glass) as the active medium, doped to provide
the energy states necessary for lasing. The pumping mechanism
is the radiation from a powerful light source, such as a
flashlamp. The ruby, Nd-YAG, and Nd:glass lasers are
solid-state lasers.
* Solitons: Stable, shape-preserving and localized solutions
of nonlinear classical field equations. Of recent interest
as possible models of extended elementary particles.
* Sound Waves: See entries on compression waves, waves.
* Space Frame or Spaceframe: Three-dimensional "optical bench"
that holds laser components stable from vibrational and
thermal excursions.
* Spallation: See radiation damage, surface.
* Spatial Filter: Device consisting of a lens pair and a pinhole
aperture stop. Intensity fluctuations over the spatial extent
of a laser beam are removed by passing the focused beam through
the aperture stop. The pinhole must be placed in a vacuum to
prevent air breakdown by the focused beam. These filters are
used to counter the effects caused by self-focusing.
> Spherator: Single-ring multipole device with an additional
current-carrying rod perpendicular to the ring axis.
> Spheromak: [from Art Carlson] A compact torus with comparable
toroidal and poloidal magnetic fields. The plasma is roughly
spherical and is usually surrounded by a close-fitting conducting
shell or cage. Both the poloidal and toroidal magnetic fields
are generated by plasma currents. There are no toroidal field
coils "linking" the plasma through the central plasma axis.
External force is supplied by poloidal field coils outside
the plasma separatrix. The resulting configuration is approximately
a force-free magnetic field. The spheromak machine geometry can
be simpler than a tokamak, but the close-fitting wall is a source
of impurities and the current cannot be inductively driven. After
early experiments failed to achieve a reasonable beta, interest
has ebbed. The spheromak can also be considered as the
low-aspect-ratio limit of the tokamak. See also: compact torus.
* Spin-Polarized Fusion: A method to enhance nuclear fusion
reaction rates in some fusion fuels by polarizing the nuclear
spins.
! Spitzer, Lyman: Early Princeton Fusion Scientist;
astrophysicist who first proposed orbiting space telescope;
inventor of the stellarator.
& Spontaneous Emission: Radiation randomly emitted by excited
atoms or ions. Contrast with stimulated emission.
* Sputtering: Process by which atoms are ejected from a solid
surface by bombardment with plasma particles. See entry
for "Radiation Damage, Surface."
* Stability: characteristic of some types of equilibrium states;
see equilibrium.
> Starfire Tokamak: A conceptual design study of a modular
tokamak reactor that operates in a steady-state condition
while using conventional power-generating systems.
& Stark Effect: The effect an electric field has on the
spectral lines emitted from excited atoms. The effect may
arise from externally-applied electric fields, from internal
fields due to the presence of neighboring ions or atoms (pressure),
or from the electric field associated with the Lorentz
(v cross B) force (motional stark effect). Spectroscopic
measurements of plasmas using the pressure-based and motional
Stark effects are useful for diagnostic purposes.
> Stellarator: (adapted from Herman) Device invented by Lyman Spitzer
for the containment of a plasma inside a racetrack-shaped
(sometimes a figure-8) tube. The plasma is contained by a magnetic
field created by helical windings around the tube. More generally,
a toroidal sort of device that attempts to average out particle
drifts that would otherwise take plasma to the walls of the vacuum
vessel by imposing a given amount of helicity to the toroidal field
lines. "A toroidal plasma configuration, which, unlike a tokamak,
is not axially symmetric. The poloidal fields necessary for
confinement are produced by external coils (rather than a current
in the plasma), either helical coils in addition to plane toroidal
field coils, or out-of-plane toroidal field coils (pioneered in
Germany on Wendelstein 7-AS). The stellarator is generally
considered to be the most serious alternative to the tokamak. Since
the concept is inherently steady state, it would not have the
tokamak's problems with thermal and mechanical cycling, current
drive, and disruptions."
-- Arthur Carlson, awc@ipp-garching.mpg.de
& Stimulated Emission: Radiation coherently emitted by excited
ions when driven by a passing light wave and the appropriate
transition wavelength. "Laser" means Light Amplification by
Stimulated Emission of Radiation; it occurs when there is a
population inversion between the upper and lower energy states
of the transition, such that stimulated emission can dominate
excitation. Stimulated emission is coherent and codirectional
with the stimulating wave, and the rate of stimulated emission
is proportional to the intensity of the stimulating wave.
* Strong (Nuclear) Force:
* Sturm-Liouville Problem: The general problem of solving a
linear differential equation of order 2n, together with
2n boundary conditions; also known as the eigenvalue problem.
* Superconductor: A type of electrical conductor that permits
a current to flow with zero resistance. Without superconducting
coils, a toroidal magnetic-confinement fusion reactor would not
be possible, because too much energy would be required to
maintain the magnetic fields against resistive energy losses
in the coil conductors.
* Suydam Stability Criterion: A criterion for the stability
of modes localized arbitrarily close to a mode-rational surface
(see rational surface) in a circular cylindrical geometry.
& Synchrotron Radiation: (Also known as cyclotron radiation.)
Electromagnetic energy radiated from a charged particle moving
in a curved orbit (typically in a magnetic field), due to the
acceleration required to change the direction of the particle's
velocity. See also bremsstrahlung.
* Symmetry axis: [from Art Carlson] The straight line (usually
vertical) through the center of a configuration, when the
configuration is symmetric to all (axisymmetric, like the tokamak)
or some (periodic, like the stellarator) rotations about this
line. Usually the z-axis.
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