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Conventional Fusion FAQ Glossary Part 12/26 (L)


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Archive-name: fusion-faq/glossary/l
Last-modified: 20-Feb-1995
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===============================================================
Glossary Part 12:  Terms beginning with "L"

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).
==================================================================

LLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLL

# L: variable typically used to indicate self-inductance;
see inductance.

# Li: chemical symbol for the element lithium; see entry.

@ L-mode: see low mode.

@ LAMPF: Los Alamos Meson Physics Facility; see entry

@ LANL: Los Alamos National Laboratory; see entry

@ Laser: Light Amplification by Stimulated Emission of Radiation.
     see entry.

@ LBL: Lawrence Berkeley Laboratory; see entry

@ LCFS: Last Closed Flux Surface; see entry

@ LLE: Laboratory for Laser Energetics; see entry

@ LLNL: Lawrence Livermore National Laboratory; see entry

@ LMFBR: Liquid-Metal Fast-Breeder Reactor; see entry

@ LMR: Liquid-Metal Reactor; see entry

@ LN2: Liquid (diatomic) Nitrogen (N2)

@ LOCA: Loss-of-Coolant Accident; see entry

@ LWR: Light-Water Reactor; see entry

% Laboratory for Laser Energetics:  Second-largest (?) inertial
confinement research facility in the United States; located at
the University of Rochester in New York state.  Home of Omega;
future home of Improved-Omega.

& Lagrangian:  The difference between the kinetic energy and the
potential energy of a system of particles, expressed as a
function of generalized coordinates and velocities.  Equations
of motion can be derived from the Lagrangian.  (see an intermediate
or advanced mechanics text for more information.)

* Lagrangian coordinates:  coordinates which follow fluid motion.
(As distinct from Eulerian coordinates; see entry).

* Landau Damping:  Damping of a wave propagating in a hot plasma,
due to the interaction of the wave with particles whose velocity
is close to the phase velocity of the wave.  Depends on the shape
of the velocity-space distribution function at the phase velocity
of the wave.  More info from John Cobb, with modifications:  

The phenomenon is very similar to surfing on water waves at the 
beach.  If a particle's speed is just slightly lower than the wave, 
then the particle can "catch the wave" and surf along at the wave 
speed.  In so doing, the particle will gain some energy, which will 
be at the expense of the wave.  This is called Landau Damping, since 
the loss of energy tends to damp the wave.  At the same time, if a 
particle moves just slightly faster than the wave, then it will also 
be caught on the wave.  However, in this case, it will slow down, 
giving the wave some extra energy.  In this case particles transfer 
energy to the wave; this is called inverse Landau damping.  Which 
effect dominates depends on whether there are more particles moving 
faster than the wave or more particles moving slower.  Thus it 
depends on the derivative of the distribution function with respect 
to velocity, evaluated at the wave's phase velocity.  Landau dmaping 
can lead to the decay of waves.  Inverse Landau damping can be a 
mechanism for some kinetic instabilities.

! Langmuir, Irving (1881-1957): American chemist, won Nobel Prize in 
chemistry in 1932, developed the theory of Langmuir probes (see 
entry).  Numerous inventions for General Electric (lighting).

* Langmuir frequency:  See plasma frequency.

* Langmuir oscillation:  See electrostatic waves.

* Langmuir probe: a small conductive electrode used to measure the
density, temperature, and electric potential (voltage) of a plasma.
Plasma parameters are deduced from the probe's "Characteristic"
current-drawn vs. voltage-applied curve.

& Larmor radius: the radius of the path of a charged particle 
moving in a magnetic field (and transverse to the field lines).
Also known as gyroradius and cyclotron radius.
 
& Laser:  An optical device that amplifies and concentrates light
waves, emitting them in a narrow, intense beam.  Laser light 
radiation is notable for its brightness and to some extent 
for its monochromaticity and spatial and temporal coherence.

> Laser Fusion:  Form of inertial confinement fusion where
laser beams are used to compress and heat the fuel pellet.

* Laser interferometer:  an interferometer which uses a laser
as a light source (see entries).  Because of the monochromatic
nature and high brightness of laser light, laser interferometers
can operate with much longer beam paths and path differences
than conventional interferometers.

* Laser scattering device: See Thomson scattering device.

* Last Closed Flux Surface (LCFS):  [from Art Carlson]  The boundary 
between the interior region of a tokamak (or other device), where the 
field lines close back on themselves, and the scrape-off layer (see 
entry), where the run into a material wall. (See also separatrix.)

% Lawrence Berkeley Laboratory:  Located in Berkeley, CA; Another
large U.S. science laboratory; minor (?) U.S. fusion research center.  

% Lawrence Livermore National Laboratory:  Located in Livermore, CA,
about an hour east of SF in the Bay Area.  Home of the Nova laser 
inertial confinement fusion program; Nova is the largest
laser in the world.  Home of the former mirror projects MFTF 
(Mirror Fusion Test Facility, shut down on the day it became
operational, or thereabouts, due to budget cutting), 
TMX-U (Tandem Mirror eXperiment Upgrade), and the recently 
shut down Microwave Tokamak eXperiment (MTX).  Some notable 
older fusion experiments at Livermore included Table Top, Toy Top,
Baseball (and Baseball-II) and TMX (predecessor to TMX-U).
Livermore is also the site of the Rotating Target Neutron Sources 
(I and II) for testing materials samples in high-intensity 14 MeV
neutron fluxes and the High Field Test Stand for testing neutral
beams.  Workplace of Albert Chou and several other 
sci.physics.fusion participants. :)  

* Lawson Criterion:  Scientific breakeven criterion based on the 
product of energy confinement time and particle density.  Together
with plasma temperature, the Lawson value of a plasma indicates
how close it is to self-sustained (ignited) fusion; see also 
ignition.  

& Lenz's Law:  Electromagnetism law which states that whenever
there is an induced electromotive force (emf) in a conductor,
it is always in such a direction that the current it would induce
would act in opposition to the change which caused the 
induced emf.

> Levitron:  Single-ring multipole device with an additional
current-carrying rod perpendicular to the ring axis.

* Light-ion fusion:  Light-Ion-Beam-Driven Inertial Confinement 
fusion, using beams of light ions driven at implosion targets.  
Pulsed-power driven accelerators are relatively efficient and 
cost-effective, but beam-focusing is a technical hurdle for 
this approach.

> Light-Water Reactor:  Class of fission reactors using ordinary
"light" water as a coolant, rather than liquid metal or heavy
water (water with deuterium instead of hydrogen).

* Limiters:  Structures placed in contact with the edge of 
a confined plasma which are used to define the shape of 
the outermost magnetic surface.  See also: divertor.

* Line-tying:  Connection of field lines from the end of 
an open-ended device (such as a mirror system) to a conducting
plate.  The rigidity of field lines trapped in the plate can
be transferred to the high-field region of the mirror by using
a cold, moderately-dense plasma in between.  Line-tying helps
to stabilize against interchange instabilities (see entry).

* Liquid Metal:  Metal which has been heated past its melting point
and can be used as a working fluid for pumping heat out from a 
powerplant.  Liquid metal used as coolant in a system where 
significant magnetic fields exist, it  behaves differently due 
to MHD effects; these cause pressure which resists fluid 
circulation, suppression of turbulence, and altered flow 
patterns compared to non-magnetic liquid metal systems.

> Liquid-Metal Reactor:  (Fission) reactor which uses liquid metal
as the reactor coolant.

> Liquid-Metal Fast-Breeder Reactor:  (LMFBR) Fission breeder 
reactor concept (see entry for breeder reactor) using 
liquid-metal coolant and breeding additional fuel off fast 
neutrons.

& Lithium: (Li)  Third element in the periodic table, so all isotopes
contain 3 protons.  Pure lithium at room temperature is a soft
silver-white material, the lightest of all metals.  It is 
chemically very reactive, making it hazardous.  Lithium liquefies at
355 degrees Fahrenheit, making it viable as a liquid-metal
coolant.  Lithium nuclei have two stable isotopes: 
Li-6 (7.5% abundance) and Li-7 (92.5%).  Lithium is a candidate 
for breeding tritium (for D-T fusion) from neutrons, via the 
reactions: 

      n + 6Li -> 4He + T + 4.8 MeV 
      n + 7Li -> 4He + T + n - 2.5 MeV.

* Longitudinal Waves:  (by John Cobb, with editing) Waves where the 
variation of the field is partially or totally in the direction of 
propagation (parallel to wavennumber, k [a vector]).  Examples 
include sound waves and Langmuir waves.  Contrasted with transverse 
waves, where the variation is perpendicular to the direction of 
propagation, such as light waves.

* Lorentz dissociation:  dissociation of molecular ions by Lorentz
ionization (see entry).

& Lorentz Force:  Total electromagnetic force on a charged particle
moving in electric & magnetic fields.  F = q(E + (v/c)xB).  See
also force, cross product, charge, velocity, and relevant 
variable symbols.

* Lorentz Gas:  Plasma model in which the electrons are assumed
not to interact with each other, but only with ions (Z -> infinity)
and where the ions are assumed to remain at rest/fixed (M-i -> 
infinity).  Also known as "electron gas."

* Lorentz ionization:  Ionization of neutral atoms (taken generally
at a highly-excited state) obtained by launching them at high
velocity across a strong magnetic field.  The neutral atoms feel
an electric field proportional to their perpendicular velocity
times the magnetic field strength, and if this electric field
is strong enough ionization can occur.

* Lorentz Model - see Lorentz Gas

% Los Alamos Meson Physics Facility (LAMPF):  Physics research
facility at Los Alamos National Lab; major site for U.S. 
muon-catalyzed fusion research in the 1980s.  May be shut down soon.

% Los Alamos National Laboratory (LANL):  Major DOE research 
facility, located in Los Alamos, New Mexico, about an hour west of 
Santa Fe.  Former home of a frozen-deuterium-fiber Z-pinch device,
which was dismantled.  Home to an active theory division, including
the Numerical Tokamak Grand Challenge (being performed on the CM-5
massively-parallel supercomputer).

Also home to former alternative-concepts experimental devices like 
Scyllac, FRX-A, FRX-B, FRX-C/LSM, ZT40, and the aborted CPRF which 
was killed in 1991 when it was almost complete (budget cuts).

Currently there are some small in-house experiments, including one on 
electrostatic confinement as a possible fusion device, and/or a 
compact neutron source. They also do theory and experimental 
collaboration with other labs worldwide.
 
(Information provided by John Cobb and Ed Chao)


* Loss Cone:  (from John Cobb, with modifications and additions) 
In a magnetic mirror machine, particles with a large velocity 
parallel to the magneitc field and a small velocity perpendicular 
to the field will be able to escape past the magnetic mirror 
(see magnetic mirror). In that case the velocity distribution 
function (see distribution function) will be almost zero in the 
region of velocity space that allows particles to escape. The 
shape of that region (in a velocity space diagram with parallel 
velocity and perpendicular velocity as the axes) is a cone. When a 
particle undergoes a collision, its velocity gets somewhat 
randomized. Particles that are scattered into that cone are lost very 
quickly (in one mirror bounce time). Thus it is called a loss cone. 
Because of the loss cone, the theoretical maximum particle 
confinement time of a magnetic mirror machine can be only a few times 
the particle collision time; this is generally seen as a showstopper 
for mirror-based fusion research.

* Loss of Coolant Accident (LOCA):  Powerplant accident where 
the supply of coolant to the hot power-producing core is 
interrupted, or where the coolant drains out for some reason.
Can lead to meldown of a fission reactor core in extreme cases, 
or to small nuclear explosions (e.g., Chernobyl).  Fusion 
reactors are expected to be less vulnerable to LOCAs, but these 
must still be designed for.

* Low-activation materials:  In fission reactors, one is forced
to deal with the radioactive byproducts of the fission process,
but in fusion reactors one generally has a choice of what materials
to expose to neutrons produced by the fusion process.  A major 
problem for fusion reactors is developing materials (such as for
the reactor vacuum vessel structure) which can be exposed to 
high levels of neutron bombardment without becoming permanently
radioactive.  Candidate structural materials which have 
relatively low induced radiactivation (generally relative to 
stainless steel) are known as low-activation materials; these 
include titanium, vanadium, and silicon-carbide.

* Low Aspect Ratio:  (entry from John Cobb, slightly edited)
An aspect ratio for a torus that is small (minor radius is almost as
big as major radius).  There are many fusion devices which are 
designed to have a low aspect ratio.  Such devices look more like 
tractor tires than bicycle tires, as toruses go.  There are reasons
to believe that low aspect ratio devices will offer some advantages 
for a fusion reactor.  Usually, ease of theoretical and/or numerical 
analysis is not one of these advantages :>.

* Low-beta plasma:  a plasma in which the beta value (see entry)
is typically 0 to 0.01.

* Low mode or L-Mode:  (from Herman) The "normal" behavior of 
a tokamak plasma, characterized by poor confinement and a particular
scaling of decreasing confinement with increasing temperature.

* Lower hybrid frequency:  

* Lower Hybrid Heating:  form of RF heating using Lower Hybrid Waves.

* Lower Hybrid Waves:  "Electrostatic ion oscillations at a frequency
intermediate to the electron extraordinary wave (high frequency) and 
the magnetosonic wave (low frequency).  Not waves, strictly speaking,
because they do not propagate (I think)." 
        - Albert Chou, albert@seas.ucla.edu

* Luminescence:  Light emission that cannot be attributed merely
to the temperature of the emitting body, but results from such
causes as chemical reactions at ordinary temperatures, electron
bombardment, electromagnetic radiation, and electric fields.





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