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Archive-name: fusion-faq/glossary/b
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!!!). See reader questions & answers on this topic! - Help others by sharing your knowledge
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Glossary Part 2: Terms beginning with "B"
FREQUENTLY USED TERMS IN CONVENTIONAL FUSION RESEARCH
AND PLASMA PHYSICS
Edited by Robert F. Heeter, rfheeter@pppl.gov
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
Citations and Acknowledgements appear in Section 11 of the FAQ.
==================================================================
BBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBB
# B: variable used for Magnetic Field
# B: chemical symbol for the element boron; see entry
# Be: chemical symbol for the element beryllium; see entry
@ BCSS: Blanket Comparison and Selection Study (no entry)
@ BHP: Biological Hazard Potential; see entry
@ BPX: Burning Plasma eXperiment; see entry
@ BTU: British Thermal Unit; see entry
@ BWR: Boiling Water Reactor (fission); see entry
* Background Radiation: Level of environmental radation due to
"background" sources. Background sources can be natural, such
as cosmic rays and natural radioactive elements (principally
radon, but including other elements such as isotopes of potassium
(which people get substantial amounts of in foods like bananas)).
They can also be man-made, such as from fossil-fuel combustion,
everyday leakage from nuclear activities, and leftover from
atmospheric nuclear weapons tests. Background radiation is
usually distinguished from acute radiation, such as from medical
x-rays, nuclear accidents, radioisotope therapy, or other short-term
doses. The man-made contribution to background radiation is
quite small compared to the natural contribution; medical uses
dominate human exposure to acute radiation.
& Backscattering: Deflection of incident particle / radiation
through an angle greater than 90 degrees relative to the original
direction of motion/propagation.
* Ballooning Instability: See Ballooning Mode
* Ballooning Mode: A mode which is localized in regions of
unfavorable magnetic field curvature ("bad curvature") and
which becomes unstable when the force due to pressure
gradients (grad p) is greater than the mean magnetic
pressure force (grad B^2)/(8*pi).
* Banana Orbit: The fast spiraling of an charged particle around a
magnetic field line is accompanied by a slow movement ("drift") of
the center of the sprial. Projected onto a poloidal plane, the drift
orbit has the shape of a banana. These orbits are responsible for
neo-classical diffusion (see entry).
$ Barn: Unit of area equal to 1x10^-24 square centimeters
(or 1x10^-28 square meters). Commonly used in describing
cross sections of atomic, nuclear, and particle interactions.
(see cross section).
* Baseball Coils: Used in magnetic-mirror geometries to
produce a minimum-B configuration; so-called because of their
resemblance to the characteristic shape of lacing on a baseball.
* Beam: stream of particles or electromagnetic radiation
travelling in a single direction.
* Beam-Beam Reaction: Fusion reaction which occurs from the
collision of two fast ions originating in injected neutral beams.
* Beam Dump: A mass of shielding material which absorbs
the energy of a beam of particles or electromagnetic radiation.
* Beam-Plasma Reaction: Fusion reaction which occurs from the
collision of a fast beam ion with a thermal plasma ion.
* Beam Splitter: Optical device for dividing a beam of
electromagnetic radiation into two or more separate beams.
* Beam-Wall Reaction: Fusion reaction which occurs from the
collision of a fast beam ion with an ion embedded in or adsorbed
onto the reactor wall.
* Bean-Shaped Plasma: A toroidal plasma indented on the inboard
side (that is, on the side with the "donut hole"); results in
additional stability to ballooning modes (see entry). Moderate
indentation (does, can, may?) provide access to the
second-stability region (high beta). (see relevant entries)
! Becquerel, Antoine-Henri: French scientist and discoverer of
radioactivity; co-winner of Nobel Prize. (See Curie)
$ Becquerel: Unit of radioactivity equal to 1 disintegration per
second. (see Curie)
* Bellows: Flexible mechanical structure with walls like those
of an accordion.
* Bernstein Mode: Type of mode which propagates perpendicular
to the equilibrium magnetic field in a hot plasma. The waves
have their electric field nearly parallel to the wave propagation
vector (nearly longitudinal). The modes propagate in
frequency ranges lying between integer harmonics of the
electron cyclotron frequency. Named after Ira Bernstein.
& Beryllium: (Be) Element with atomic number 4 (four protons).
May be useful in multiplying fusion neutrons to enhance tritium
production in a lithium blanket; rather hazardous to handle.
(See relevant terms mentioned.)
* Beta, or beta-value: Ratio between plasma kinetic pressure and
magnetic-field pressure; proportional to the ratio between plasma
kinetic energy density and magnetic field energy density. Beta
is usually measured relative to the total, local field
(loosely called beta toroidal), but sometimes the plasma pressure
relative to only the poloidal component of the field (beta poloidal)
or relative to some external field (like the maximum field at the
magnetic coils) is more useful. There is also a normalized beta
(beta_N) of interest when discussing the beta limit (see entry).
(lots of help from Art Carlson with the above.)
"Because the cost of a reactor is strongly influenced by the
strength of the magnetic field that must be provided, beta values
are directly related to the economics of fusion power production.
Beta is usually expressed as a percentage, with 5% generally
believed to be the minimum value required for an economical
fusion reactor." - from the PPPL WWW page on PBX-M.
See also: pressure, kinetic pressure, magnetic pressure,
second stability.
* Beta-Normal: Beta-N, the normalized beta, is beta relative to
the beta limit (see below).
* Beta-Poloidal: Beta-P is the same as the ordinary beta, except
only the poloidal field is used in calculating the magnetic field
pressure. Beta-P is > 1 in many modern tokamaks.
* Beta Emission: Form of nuclear decay where a neutron splits
into a proton plus electron plus neutrino set. The proton
stays in the nucleus but the electron ("beta ray") is ejected.
* Beta Limit, also called Troyon Limit: If the plasma pressure in
a tokamak becomes too high, the so-called ballooning modes become
unstable and lead to a loss of confinement (sometimes catastophic,
sometimes not). The exact value at which this occurs depends
strongly on the magnetic field B, the plasma minor radius a, and
the toroidal plasma current I, such that maximum value of the
normalized beta, beta_N=beta*B*a/I, is around 4% (with B in Teslas,
a in meters, and I in Mega-amperes). The exact value depends on
details of the plasma shape, the plasma profiles, and the safety
factor. (Beta entries provided by Art Carlson.)
* Beta Particle / Beta Ray: Original term used for electrons
(and positrons) ejected from decaying nuclei via beta emission.
(Label derives from the old days when we had various kinds of
radiation emission, and they were labeled alpha, beta, and
gamma (the first letters of the Greek Alphabet) because no one
really knew what any of them were.)
* Beta value: See "beta" just above.
* Biasing: [from Art Carlson] The vacuum vessel of a tokamak
(or other device) has a variety of structures--limiters, divertor
plates, the wall itself. These are usually mechanically and
electrically connected, but it is possible to bias (charge) them to
different voltages relative to each other. This allows some control
over the electric fields and currents around the plasma, which can
influence, for example, the thickness of the scrape-off-layer, the
transition between L- and H-mode, and the equilibrium configuration.
Biasing experiments are being done on DIII-D, TEXTOR, and TdeV.
* Binary Collisions: Collisions involving only two particles;
multiparticle collisions (eg, three-body collisions) are usually
neglected/approximated...
* Binding Energy: Energy required to separate two objects;
conversely, energy released when two objects are allow to bind
together. Equivalent to the mass defect (see entry) via E=mc^2.
* Biological Hazard Potential (BHP): Measure of the hazard posed by
a given quantity of radioactive material in which the variation in
biological effects of the various elements are accounted for.
(See also integrated biological hazard potential, IBHP)
& Biot-Savart Law: General formula for determining the magnetic
field due to a steady line (not space) current. Related to Ampere's
Law.
* Blanket: a region surrounding a fusion reactor core within
which the fusion neutrons (if any) are slowed down, heat
is transferred to a primary coolant, and tritium is bred
from lithium (if tritium is used as fuel). In hybrid
applications, fertile materials (U-238 or Th-232) are located
in the blanket for conversion into fissile fuels.
* Bohm diffusion: A rapid loss of plasma across magnetic field
lines caused by microinstabilities. Theory formulated by the
physicist David Bohm. From Chen's book
(see bibliography): "Semiempirical formula for the diffusion
coefficient given by Bohm in 1946 (noted by Bohm, Burhop, and
Massey, who were developing a magnetic arc for use in uranium
isotope separation)." Bohm diffusion was proposed (not derived
from first principles) to scale as 1/B rather than the 1/B^2
scaling predicted by classical diffusion. A 1/B scaling results
from assuming that particles diffuse across field lines at an
optimum rate (effective collision frequency=cyclotron frequency).
The 1/B scaling is observed (approximately) in most reactors.
(Acknowledgements to Philip Snyder) See also: diffusion,
microinstabilities, field lines...
* Boiling Water Reactor (BWR): Class of fission reactor where
water is used as a coolant and allowed to boil into steam.
(I don't remember much more about it - any help out there?)
& Boltzmann constant: k = 1.38 x 10^-16 erg/degree. This
is the ratio of the universal gas constant to Avogadro's number.
It is also used to relate temperatures (Kelvin) to energies (ergs
or Joules) via E = (constant of order unity) * kT.
& Boltzmann Distribution: See Maxwell-Boltzmann distribution;
distribution function.
* Boltzmann Equation: Fundamental equation in kinetic theory
which describes the evolution of the distribution function.
(See also Vlasov equation.)
* Bootstrap Current: Currents driven in toroidal devices by
neo-classical diffusion (see entry). They may amount to a
substantial fraction of the net current in a tokamak reactor,
thus lengthening the pulse time or decreasing the power needed
for current drive.
& Boron: (B) Fifth element (Z=5) in the periodic table; has
5 protons; potential use as an aneutronic fuel. (See FAQ section
1, part on reactions.) Also useful as a neutron-absorber.
* Boronization: Energy confinement in a fusion plasma depends
strongly on the average atomic number (Z) of the elements in the
plasma. Boronization refers to a process whereby boron (atomic
number 5) is injected into a plasma and used to coat the walls
of the reactor; the effect is that impurities from the reactor
walls which enter the fusion plasma are primarily boron (which
has a fairly low Z) rather than the higher-atomic-number metals
typically used in reactor structures. Boronization has been
associated with improved fusion plasma performance. Boronization
is an example of Wall Conditioning. See also Boron, atomic number,
wall conditioning, impurities.
* Bounce Frequency: The average frequency of oscillation of
a particle trapped in a magnetic mirror as it bounces back
and forth between its "turning points" in regions of high
magnetic field. (See also trapped particle, turning points,
banana orbit).
* Boundary Layer: In fluid flow, a narrow region next to a
fixed boundary or surface where the fluid velocity rapidly
changes from zero to some finite value. The term has been
generalized to situations with similar mathematics.
* Branching Ratio: In a fusion reaction involving two nuclei,
there are typically a variety of possible sets of products which
can form. The branching ratio for a particular set of products
is the probability that that set of products will be produced.
* Breakeven: there are several types:
Commercial: When fusion power can be converted into enough
electric power to power the reactor and generate enough
electricity to cover the costs of the plant at economically
competitive rates. (?)
Engineering: When enough energy can be generated from the
fusion power output to supply power for the reactor and
generate a surplus; sort of commercial breakeven without
the economic considerations. (?)
Scientific: When fusion power = input power; Q=1.
(See also Lawson Criterion)
Extrapolated - projected for actual reactor fuel using
an alternative fuel.
Actual - determined using the actual fusion fuel to be
used in the reactor (typically DT).
* Breeder Material: In D-T fusion, refers to lithium or
lithium-containing substances which are placed in the
blanket to convert the fusion neutrons back into tritium,
using nuclear transmutation of lithium isotopes.
* Breeder Reactor: Class of nuclear reactor (could be fission
or fusion) which uses some of the nuclear byproducts (generally
neutrons) to transmute non-fuel materials to new materials which
can be used for fuel in other reactors, in such a way that
the reactor creates more fuel than it consumes (breeding).
Term usually refers to reactors which breed fission fuel.
Use of breeder reactors would greatly extend the fuel supply for
nuclear fission energy, but also creates additional opportunities
for diversion of fissile materials to weapons production and
could exacerbate proliferation of nuclear weapons.
& Bremsstrahlung: (German for "Braking Radiation") Electromagnetic
radiation from a charged particle as it slows down (decelerates),
or as it changes direction due to near collisions with other particles.
Similar to synchrotron radiation (see also). In a plasma
bremsstrahlung occurs when electrons (which are lighter and generally
move faster) collide with ions (which are heavier and generally move
slower); the acceleration/deceleration of the electrons causes them
to radiate bremsstrahlung.
& Brewster's Angle: The angle of incidence at which
electromagnetic waves reflected from a dielectric medium are
completely polarized perpendicular to the plane of incidence;
the component polarized parallel to the plane of incidence is
completely transmitted.
* British Thermal Unit: Unit of energy needed to raise a pound of
water by one degree fahrenheit; equal to 252 calories or 1055 Joules.
(See also calorie, joule). Not part of the metric system.
> Bumpy Torus: I believe this concept tries to combine mirror
concepts with toroidal ones. My understanding is that it is
essentially a series of mirrors stuck end to end and bent into
a ring. - Albert Chou (corrections / enhancements welcome!)
* Bunching: A technique for spatial compression of a pulse
in a beam of charged particles.
* Bundle Divertor: Divertor concept where a toroidal field
coil extracts a "bundle" of toroidal field lines (flux) and
forms a separatrix in the toroidal field. (Hard to do
and tends to mess up axisymmetry of the torus; not used much.)
> Burning Plasma eXperiment (BPX): Proposed U.S. successor to TFTR;
never funded. See also: CIT, TPX.
* Burnout: Rapid reduction in the neutral particle density
in a plasma discharge; occurs when the ionization rate (which
converts neutrals to ions and electrons) exceeds the rate
of recombination (which converts ions to neutrals) and the
rate of influx of neutral particles.
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