Absolute Plasma Instabilities
A class of plasma instabilities
with amplitudes growing with time at a fixed point in the plasma medium.
For comparison see convective instabilities.
Absorption
Light
can be absorbed by photoionization
of atoms (or photo-dissociation of molecules). In standard treatments, it
is assumed that the cross section σIon for this process is
independent of the intensity of the radiation. However, for intensities
below a certain threshold it is obvious that it will be reduced due to the
disturbing influence of plasma field fluctuations. One can adopt the
efficiency factor β(Ew)= Ew2/(Ew2
+ΔEp2), where Ew is the electric
field strength of the radiation (as given by the intensity I= Ew2/8π)
and ΔEp2 the effective plasma fluctuation field
(which depends on the charge density and particle velocity in relationship
to the wave frequency).
This
leads to the circumstance that the Optical Depth with regard to photoionization
is not proportional to the distance any more but has to be determined
through the integral τ(s)= σIon(0) .0∫sds'
N(s').β(Ew(s')) . Because generally the
intensity I (i.e. Ew2) is subject to the Exponential Absorption Law: I(s)= I0.e-τ(s).
The optical depth is determined by an integral equation which can be
solved numerically (see http://www.plasmafacts.de/nlabsorb.htm
).
Absorption line
In
spectroscopy, a characteristic wavelength of emitted radiation that is
partially absorbed by the medium between the source and the observer.
Absorption of Plasma Wave Energy
The loss of plasma wave energy to
the plasma particle medium. For instance, an electromagnetic wave
propagating through a plasma medium will increase the motion of electrons
due to electromagnetic forces. As the electrons make collisions with other
particles, net energy will be absorbed from the wave.
Acceleration
The
speeding-up of motion (or in general, any change of velocity, in magnitude
and/or direction). Fast electrons in the aurora, charged particles in the
radiation belt, cosmic rays etc., all require an acceleration process to
provide their high energy.
Accelerator
A
device which uses electric or magnetic fields to increase the speed (and
thus the energy) of charged particles. In magnetic fusion neutral beam
heating is used on present TOKAMAKs to heat the plasma to fusion relevant
temperatures. These accelerators boost large quantities (tens of amps in
current) of positively charged deuterium ions to 80-120 keV (thousand
electron Volts) to be injected into the plasma. Future accelerators for
magnetic confinement are expected to boost particle energies to -500 keV.
Studying the makeup of the nucleus, such as in high energy physics
research, requires accelerators capable of boosting a particle’s energy
into the million electron volt (MeV) range. Present-day accelerators can
produce particles with energies well above one hundred billion electron
volts allowing them to probe the constituents of nuclei.
Active region
A
localized, transient volume of the solar atmosphere in which plages,
sunspots, facula, flares, etc., may be observed. Active regions are the
result of enhanced magnetic fields; they are at least bipolar and may be
complex if the region contains two or more bipolar groups.
Additional heating
Heating
additional to Ohmic heating. Used to heat TOKAMAKs to temperatures at
which Ohmic heating is small. Usually uses neutral beams or
radio-frequency waves. Also called Auxiliary Heating. See electron
cyclotron resonant heating, ion cyclotron resonant heating, lower hybrid
heating.
Adiabatic Compression
Compression (of a gas, plasma,
etc.) not accompanied by gain or loss of heat from outside the system. For
a plasma in a magnetic field, a compression slow enough that the magnetic
moment (and other adiabatic invariants - see entry) of the plasma
particles may be taken as constant.
Adiabatic invariant
(1)
An invariant of a motion is a quantity which does not change as
time advances. For instance, the energy of a system is often an invariant
(for a swinging pendulum, or a planet and the Sun), and knowing that it
stays constant is a great help in calculating the motion. Adiabatic
invariants are quantities associated with approximately periodic motions
which almost do not change, and are similarly useful. They are important
in calculating the way ions and electrons move in a magnetic field.
(2)
Characteristic parameters which do not change as a physical system
slowly evolves; the most commonly used one in plasma physics is the
magnetic moment of a charged particle spiraling around a magnetic field
line.
Adiabatic Plasma
Part of plasma confinement system
which involves particles where the orbit radius and orbit period are small
compared to the characteristic scales of length and time. In such plasma
confinement the individual particles closely follow the magnetic field
lines by tightly cirling them. The motion of these particles can be
described by drift formalism and gyration centers. On average, such plasma
can be readily described by a well defined theory - magnetohydrodynamics -
MHD.
Advanced TOKAMAKs
TOKAMAKs
are naturally pulsed devices because the plasma current is driven by
inductive means (by a transformer). However it is possible that so-called
"Advanced TOKAMAKs" are feasible: these would operate
continuously with the current driven by a combination of non-inductive
external drive and the natural pressure-driven currents that occur in
plasmas. They would require careful optimisation of pressure and
confinement. They are being studied both theoretically and experimentally
(at Culham, JET and elsewhere) as continuous operation is highly desirable
for fusion power production and their relatively small size results in a
more economical power plant than an ITER-like design. See reverse shear.
AE
index
A
geomagnetic index describing the auroral electrojet.
Afterglow
See Plasma Afterglow
Airglow
A
faint luminescence of the night sky originating in photochemical reactions
in the upper atmosphere. Also referred to as geocoronal emission.
Alfven gap modes
The
toroidal nature of TOKAMAK plasmas produces gaps in the otherwise
continuous spectrum of Alfven waves, which are populated by discrete,
undamped Alfven gap modes. These modes could be easily destabilised by
resonant energy transfer from energetic particles (e.g. alpha particles
from fusion reactions.
Alfven Ion Cyclotron instability
(AIC)
An electromagnetic
microinstability near the ion cyclotron frequency; driven by the ion loss
cone in a mirror device.
Alfven resonance layer
The
Alfven resonance layer can be located at any desired position between the
plasma center and the plasma edge as shown in fig. below. It depends on
the appropriate choice of frequency and mode numbers.
Alfven speed/velocity
(1)
The velocity of
propagation of Alfven waves in the direction of the magnetic field; it is
proportional to the magnetic field strength, and inversely proportional to
the square root of the ion density.
Alfven
speed,
where,
r
= ni mi = ion mass density
B
= steady magnetic field (e.g. for hydrogen B
= 1 Tesla)
mi = mass of ion (e.g. for hydrogen, mi
= 1.67 x 10-27 kg)
ni
= ion number density (e.g. for hydrogen, ni
= 1019/m3)
(2)
Phase velocity of the
Alfven wave; equal to the speed of light divided by the square root of 1
plus the ratio of the plasma frequency to the cyclotron frequency. Also
see Alfven waves.
Alfven time
The
time taken for an Alfven wave to travel one radian in the toroidal
direction. This is a measure of the time-scale on which Alfvenic MHD
effects can occur.
Alfven waves
(1)
A fundamental
plasma phenomenon, which is primarily magnetohydrodynamic in character:
oscillation of the magnetic field and, in some cases, plasma pressure. In
TOKAMAKs, these waves are typically strongly damped (i.e. they would
spontaneously decay if externally excited). See also fast Alfven wave.
(2)
Electromagnetic waves that are propagated along lines of magnetic
force in a plasma. Alfen waves, named after plasma physicist and Nobel
Prize winner Hannes Alfen, have frequencies significantly less than the
ion cyclotron frequency, and are characterized by the fact that the
magnetic field lines oscillate with the plasma.
Alfvén, Hannes Olof Gösta
A pioneering Swedish plasma
physicist(born: May 30, 1908; died: April 2, 1995), Hannes Alfvén won the
1970 Nobel Prize in Physics for contributions and fundamental discoveries
in magnetohydrodynamics.
For further biographical
information,
please visit: http://www.alfvenlab.kth.se/hannes.html
Alpha particle
(1)
A type of fast ion emitted by many heavy radioactive nuclei, such
as uranium. Actually, the nucleus (atom stripped of all electrons) of the
gas helium.
(2)
He 2+, a positively charged particle consisting of two protons and
two neutrons; denoted by the Greek letter, alpha (a); a helium-4 nucleus.
An alpha particle is a typical product of fusion reactions.
(3)
The nucleus of a helium atom, consisting of two protons and two
neutrons bound together. In a fusion power plant, energetic
alpha-particles (as well as neutrons) will be created by the fusing of
deuterium and tritium nuclei. The heating which is provided by these
alpha-particles as they slow down due to collisions will be essential in
achieving ignition.
Alternating current (AC)
Electrical
current in which the direction of charge flow reverses
periodically. Electric
energy in the U.S. operates at a periodic frequency of 60 Hertz (cycles
per second).
Ambipolar plasma diffusion
Diffusion process in which build up
of spatial electrical charge creates electric fields (see ambipolar
potential), which cause electrons and ions to leave the plasma at the same
rate.
Ambipolar plasma potential
Electric fields that are
self-generated by the plasma and act to preserve charge neutrality through
ambipolar diffusion.
Ampere (amp, A)
A
unit of measure of electrical current named after Andre M. Ampere. In the
meter-kilogram-second (mks) system, a one amp current (I) is produced by
the flow of one coulomb of charge per second. A current of one Ampere is
produced in a wire of one Ohm resistance (W) with a potential difference
(V) of one Volt. (see Ohm, resistance, Volt)
Ampere’s law
A
mathematical function relating the magnetic field to the electrical
current in a wire. The law
states that the line integral of the magnetic inductance (B) around a
closed loop is equal to the product of the permeability of free space (mo
= 4p 10-7 Henries/meter) and the electrical current (I) passing through
the area bounded by the path.
Amplitude
Maximum
value of the displacement in a periodic motion or oscillation.
Analytic/computational modelling
Analytic:
algebraic solution of basic equations. Computational: numerical solution
of basic equations, using a computer. See Fokker-Planck Code, Grad-Shafranov
equation, Monte Carlo, Neural network.
Angstrom (Å)
Units
of dimension used to describe length scales typical of atoms. 1 Å = 0.1
billionth of a meter (10-10m). The radius of the hydrogen atom is 0.529 Å.
Anomalous plasma diffusion
Particle or heat diffusion in a
plasma that is larger than what would predicted from theoretical
predictions of ‘classical’ plasma phenomenon. Classical diffusion and
neo-classical diffusion are the two well-understood diffusion theories,
although neither is adequate to fully explain the experimentally observe
magnitude of ‘anomalous’ diffusion.
Anomalous transport
Measured
heat loss is anomalously large compared to basic collisional theory of
heat transport in toroidal plasmas ("neoclassical" theory).
Particularly true for electrons.
Ap index
A
daily index determined from eight Ap index values.
A
mean, 3-hourly "equivalent amplitude" of magnetic activity based
on K index data from a planetary of 11 Northern and 2 Southern Hemisphere
magnetic observatories between the geomagnetic latitudes of 46 and 63
degrees. ap values are given in units of 2 nT.
Aphelion
The
point on the path of a Sun-orbiting body most distant from the center of
the Sun. Compare perihelion.
Apogee
The
point on the path of an Earth-orbiting satellite most distant from the
center of the Earth. Compare perigee.
Arc
See plasma arc
Arc Plasma
(1)
A type of electrical discharge between two electrodes;
characterized by high-current density within the plasma between the
electrodes.
(2)
A form of
electric discharge, can generate heat with a high energy density of
several ten thousand degrees in a short period of time. In order to
develop effective application of arc plasma, we are investigating the
possibility of using arc plasma for volume reduction of low activity
wastes from nuclear power stations.
Arcade
A
series of magnetic loops, overlying a solar inversion line.
Arch filament system (AFS)
A
system of small arched linear-absorption features connecting bright,
compact plages of opposite polarity. An AFS is a sign of emerging bipolar
magnetic flux and possibly rapid or continued growth in an active region.
ARIES
A
comprehensive TOKAMAK fusion power plant study undertaken by a
collaboration of US fusion laboratories in the early 1990s. Four designs
were studied: ARIES-I, a device based on modest extrapolations from the
present TOKAMAK physics database; ARIES-II and ARIES-IV, two second
stability devices which differed in their fusion power core composition,
and ARIES-III, which, unlike the others, utilised the deuterium-helium-3
fusion reaction instead of the deuterium-tritium reaction.
ASDEX
ASDEX
(Axisymmetric Divertor Experiment), a TOKAMAK at the Max Planck Institut
fur Plasmaphysik, Garching, Germany, was designed to study the effect of a
double null divertor. The first H-modes were observed on ASDEX. The latest
upgrade of this machine is called ASDEX-U, or ASDEX-Upgrade. It is
intermediate in size between Compass-D and JET, and has the same magnetic
configuration as these devices and as that planned for ITER. ASDEX-U Home
Page
Aspect ratio
Ratio
of the major radius to the minor radius of the toroidal plasma; on JET and
COMPASS, the aspect ratio is approximately 3 (as presently planned for
ITER), on START it can be as low as 1.2 and MAST down to 1.3. See plasma
geometry.
Aspect ratio
In magnetic confinement fusion
plasmas with toroidal geometry: ratio of the major diameter of the plasma
from side-to-side across the entire plasma ring (including the open center
of ‘dough nut’ geometry) divided by the minor diameter width of a
slice taken through solid width of one side of the ‘dough nut’. In
inertial confinement fusion plasmas: ratio of the diameter of the
‘pea-sized’ fusion-fuel capsule’s divided by capsule’s outer shell
thickness.
Astronomical unit (AU)
(1)
The mean distance between the Earth and Sun, equal to 214.94 solar
radii or 1.496E+11m.
(2)
The mean Sun-Earth distance, a unit of distance widely used in
expressing distances in the solar system. 1 AU = km = miles.
Astrophysical plasmas
Astrophysical plasmas include the
sun and other stars, the solar wind and other stellar winds, large parts
of the interstellar medium and the intergalactic medium, nebulae, and
more. Planets, neutron stars, black holes and some neutral hydrogen clouds
are not in a plasma state. Approximately 99% of the observable universe
can be described as being in a plasma state.
Astrophysics
The
branch of astronomy and physics that deals with the physics of
astronomical objects.
Atmosphere
(1)
The gaseous envelope surrounding a planet or star.
(2)
The layer of gas surrounding the earth or other planets. The upper
atmosphere is the region of Earth's atmosphere above the troposphere
(which extends to about 20 km). Regions of the upper atmosphere are the
stratosphere, mesosphere and thermosphere.
Atom
Smallest
unit of an element; consists of a nucleus containing one or more protons
(and possibly neutrons), surrounded by an equal number of electrons.
Atomic
Decay Probability
Atoms
in excited states are not stable but decay to lower levels more or less
rapidly. For dipole allowed transitions the problem can be treated as a
radiatively damped oscillator with an amplitude given by the quantum
mechanical Overlap Integral
<r>i,k for the lower state i=(m,l') and the upper state k=(n,l)
involved. By equating the decay rate of the energy of a classical
oscillator of frequency νi,k
with the quantum mechanical decay rate Ai,k.h.νi,k
(h=Planck constant), one obtains for the atomic decay probability Ai,k=
16π.e4/(3c3h).νi,k3.<r>i,k2
. (one should note that this expression is smaller by a factor 1/4
compared to the usual value quoted in the literature which is however
derived inconsistently from statistical equilibrium considerations).
In
general, <r>i,k can only be evaluated numerically, but
for large values of the principal quantum numbers m and n it can be
approximated by a power law in terms of these parameters, with the result
Am,n= 1.3.109.m-1.8.(n-1)3.2
[sec-1] (m,n >>1). A numerical summation over all lower
levels m reveals furthermore that the Total
Average Atomic Decay Probability (i.e. the average lifetime) of level
n can be approximated by An= 1.1.109(n-1)-3.6
[sec-1] (n >>1). (neglection of the angular momentum
quantum number l in these
approximations may lead to an error up to a factor 2 in the absolute
values for Am,n and An, the relative values for
fixed l should be accurate to within a few percent however).
Atomic energy
Energy
derived from the mass-to-energy conversion that occurs as a result of
nuclear reactions.
Atomic
mass unit (AMU)
A
unit of mass equal to exactly 1/12 the mass of a carbon atom of mass
number 12. 1 amu = 931.48 MeV = 1.661 x 10-27 kg.
Atomic number (Z)
The
number of protons in the nucleus of an atom.
Attitude (of a satellite)
The
direction in which the satellite is oriented in space.
Aurora
(1)
A sporadic, faint visual phenomena associated with geomagnetic
activity that occurs mainly in the high-latitude night sky. Auroras occur
within a band of latitudes known as the auroral oval, the location of
which is dependent on geomagnetic activity. Auroras are a result of
collisions between atmospheric gases and precipitating charged particles
(mostly electrons) guided by the geomagnetic field from the magnetotail.
Each gas (oxygen and nitrogen molecules and atoms) gives out its own
particular color when bombarded, and atmospheric composition varies with
altitude. The auroral altitude range is 80 to 1000 km, but typical auroras
are 100 to 250 km above the ground; the color of the typical aurora is
yellow-green, from a specific transitions of atomic oxygen. Auroral light
from lower levels in the atmosphere is dominated by blue and red bands
from spectral line of atomic oxygen. The patterns and forms of the aurora
include quiescent arcs, rapidly moving rays and curtains, patches, and
veils.
(2)
A sporadic,
faint visual phenomenon associated with geomagnetic activity that occurs
mainly in the high-latitude night sky. Auroras occur within a band of
latitudes known as the auroral oval, the location of which is dependent on
geomagnetic activity. Auroras are a result of collisions between
atmospheric gases and precipitating charged particles (mostly electrons)
guided by the geomagnetic field from the magnetotail. Each gas (oxygen and
nitrogen molecules and atoms) gives out its own particular color when
bombarded, and atmospheric composition varies with altitude. Since the
faster precipitating particles penetrate deeper, certain auroral colors
originate preferentially from certain heights in the sky. The auroral
altitude range is 80 to 1000 km, but typical auroras are 100 to 250 km
above the ground; the color of the typical aurora is yellow-green, from a
specific transition of atomic oxygen. Auroral light from lower levels in
the atmosphere is dominated by blue and red bands from molecular nitrogen
and molecular oxygen. Above 250 km, auroral light is characterized by a
red spectral line of atomic oxygen. To an observer on the ground, the
combined light of these three fluctuating, primary colors produces an
extraordinary visual display. Auroras in the Northern Hemisphere are
called the aurora borealis or "northern lights." Auroras in the
Southern Hemisphere are called aurora australis. The patterns and forms of
the aurora include quiescent arcs, rapidly moving rays and curtains,
patches, and veils.
(3)
Called aurora borealis in the northern hemisphere and aurora
australis in the southern hemisphere, aurorae are emissions by atmospheric
atoms and molecules after being excited by electrons precipitating from
the nightside magnetosphere. A visible manifestation of magnetospheric
substorms. Also see (e.g) Dendy, Richard, ed. _Plasma Physics: an
Introductory Course_, Cambridge University Press, 1993.
Aurora (short for polar aurora)
A
glow in the sky, seen often in a ring-shaped region around the magnetic
poles ("auroral zone") and occasionally further equatorward. The
name comes from an older one, "aurora borealis," Latin for
"northern dawn," given because an aurora near the northern
horizon (its usual location when seen in most of Europe) looks like the
glow of the sky preceding sunrise. Also known as "northern
lights," although it occurs both north and south of the equator.
Aurora
(Southern/Northern Lights)
The
bright emission of atoms and molecules in the polar upper atmosphere that
appears as permanent, ring shaped belts called the auroral oval around the
north and south geomagnetic poles. It is associated with a global
electrical discharge process caused by energetic particles impinging on
the upper atmosphere of Earth. See also: Auroras:
Billboards for Electric Space.
Auroral electrojet
A
current that flows in the ionosphere in the auroral zone.
Auroral
oval
(1)
An elliptical band around each geomagnetic pole ranging from about
75 degrees magnetic latitude at local noon to about 67 degrees magnetic
latitude at midnight under average conditions. It is the locus of those
locations of the maximum occurrence of auroras and widens to both higher
and lower latitudes during the expansion phase of a magnetic substorm.
(2)
The pattern of auroral light around the north and south magnetic
poles. The auroral oval expands and contracts over a period of hours and
days, depending on geomagnetic activity.
(3) The region in which aurora can be seen at any single time, as observed (for instance) by satellite cameras. It resembles a circle centered a few hundred kilometers nightward of the magnetic pole, and its size varies with magnetic activity. During large magnetic storms it expands greatly, making auroras visible at regions far from the pole, where they are rarely seen.
Auto-ionization
Contrary
to established opinion, atoms or molecules can ionize each other through
collisions even if their translational energy is smaller than the
ionization energy. This is because bound electrons can collide with each
other when two atoms come together and one of these may gain enough energy
in the process to become ionized, leaving the other with correspondingly
less energy in the atom (this is a purely classical process and does not
affect the quantum mechanical states the electrons occupy; one has to
remember that the quantum mechanical wave function for a given energy is
finite everywhere in space and allows the electron therefore to have any
classical energy). This process should be strongly temperature dependent,
having its highest efficiency if the corresponding velocity of the
approaching atoms is equal to the velocity of the bound electrons (i.e.
about 108 cm/sec). For smaller velocities the electron orbits
will have time to adjust themselves mutually to the field of the other
atom and ionizing collisions will become less likely.
The
proposed process could be the explanation for the relatively high plasma
density of the nighttime F- region of the earth's ionosphere. This would
lead to an effective cross section of 10-20 cm2 in
this case (which is characterized by atom velocities of 105
cm/sec). In general this mechanism should result in a significant degree
of ionization even in the absence of any UV- radiation sources, which
should be highly relevant for some astrophysical problems like star
formation (see http://www.plasmafacts.de/index.html#A8).
Auxiliary heating
Same
as additional heating.
Avogadro’s number (NA)
NA
= 6.023 x 1023. The number of molecules in one mole, where one mole
contains the molecular weight of a substance. As an example, 1 mole of
carbon with AMU 12 weighs 12 g and contains 6.023 x 1023 atoms of carbon.
