Calorie

The amount of energy it takes to raise 1 gram of water (which is about 1 cubic centimeter) 1 degrees Celsius. Another unit of energy measurement is the Joule. One calorie is equivalent to 4.1868 Joules.

Capacitively Coupled Discharge Plasma

Plasma created by applying an oscillating, radio-frequency potential between 2 electrodes. Energy is coupled into the plasma by collisions between the electrons and the oscillating plasma sheaths. If the oscillation frequency is reduced, the discharge converts to a glow discharge.

Carbon dioxide (CO₂ )

A biproduct of combustion or respiration.  While CO₂ levels in the Earth’s atmosphere have fluctuated over many eons, recent investigations have revealed a steady and rapid rate of increase in atmospheric CO₂ during the past 100 years as a result of mankind’s industrialization.

Carrington longitude

A system of fixed solar longitudes rotating at a uniform synodic period of 27.2753 days (a sidereal period of 25.38 days). Carrington selected the meridian that passed through the ascending node of the Sun's equator at 1200 UTC on 1 January 1854 as the original prime meridian. The daily Carrington longitude of the central point of the apparent solar disk is listed (with other solar coordinates) in the Astronomical Almanac published annually by the U.S. Naval Observatory. Compare Bartels' rotation number.

CCD

Charge Coupled Device. A very sensitive electronic method of detecting light.

CCE-FU

The Consultative Committee for the EURATOM Specific Research and Training Programme in the Field of Nuclear Energy (Fusion). This has three sub-committees: FTC - Fusion Technology Committee; FPC - Fusion Physics Committee, and FIC - Fusion Industry Committee.

CDX-U

Current Drive Experiment - Upgrade. A small spherical TOKAMAK at Princeton. CDX-U Home Page

Centimeter burst

A solar radio burst in the centimeter wavelength range (1 to 10 cm or 0.01 to 0.1 m), or 30 000 to 3000 MHz in the frequency range.

  Central Electron Density

The electron density is the number of electrons per unit volume. This changes as one moves from the tokamak axis to the edge. The central electron density (the density in the center of the plasma) is important since it is in the center where most fusion reactions take place.

CEUSC

Culham EURATOM Steering Committee.

CFP

Community Fusion Programme. See EFDA.

Chain reaction

Rapidly accelerating multiple fission reactions resulting from the production of several neutrons by each fission reaction. An uncontrolled chain reaction within a sufficiently large amount of fission fuel (critical mass) can lead to an explosive energy release. 

Charge exchange recombination spectroscopy

Neutral atoms in the plasma (from, for example, a neutral beam) donate electrons to fully ionised impurity ions, producing hydrogen-like ions. As the electrons decay from excited states they emit photons from which the impurity temperature, rotation and density can be measured using conventional spectroscopy.

Charge Screening

see Debye Shielding.

Child-Langmuir Law

Description of electron current flow in a vacuum tube when plasma conditions exist that result in the electron current scaling with the cathode-anode potential to the 3/2 power

Chromosphere

(1)  a reddish layer in the Sun's atmosphere, the transition between the photosphere and the corona.

(2)  The layer of the solar atmosphere above the photosphere and beneath the transition region and the corona. The chromosphere is the source of the strongest lines in the solar spectrum, including the Balmer alpha line of hydrogen and the H and K lines of calcium, and is the source of the red (chromium) color often seen around the rim of the Moon at total solar eclipses.

Classical Confinement

Plasma confinement in which particle and energy transport occur via classical diffusion; best possible case for magnetically confined plasmas. See entry for classical diffusion below.

Classical Diffusion

In plasma physics, diffusion due solely to scattering of charged particles by Coulomb collisions stemming from electric fields of the particles. In classical transport (i.e. diffusion) the characteristic step size is one gyroradius (Larmor orbit) and the characteristic time is one collision time.

CLEO

Closed Line Experiment: A Culham facility to investigate stellarator and TOKAMAK plasmas at large aspect ratio (about 7). Now dismantled.

CoA

Contract of Association between UKAEA and EURATOM.

Coil

Single or multiple-turn winding of a conductor such as copper wire. When the conducting windings carry current, a magnetic field is produced which can be used for shaping or stabilizing a plasma, or for inducing plasma current.

Cold plasma

The plasma whose temperature is low enough to ignore the plasma resistivity. The kinetic effects of cold plasmas are negligible.

Cold Plasma Model

Model of a plasma in which the temperature is neglected. Many interesting plasma effects exist even in this approximation.

Collision Frequency

The concept of a collision frequency is probably the most important one in plasma physics (and the physics of gases in general) when it comes to assessing the significance of the individual physical processes. It is defined as

                                                                              ν_c= N^.σ_c(v)^.v ,

where N is the volume density of the background medium and σ_c(v) the cross section of the particle with velocity v for the type of collision being considered (e.g. Coulomb collisions, radiative recombination, collisional excitation).

Despite the random nature of collisions, ν_c can be considered as an exact quantity because the large number of particles usually assures that the average is very sharply defined within relatively short time scales and small volumes. This allows therefore an exact assessment of the importance of the individual collisional processes and also a comparison with the physical time scales like the Atomic Decay Probability or Plasma Frequency. (see also Mean Free Path, Level Population, Plasma).

Collisionality

A measure of how frequently collisions occur in a TOKAMAK plasma. A collisionality of unity corresponds to a trapped particle performing a single banana orbit before being scattered.

Collisionless Plasma Model

Model of plasma where the density is low enough, and/or the temperature is high enough, that collisions can be neglected because the plasma time scales of interest are shorter than the particle collision times.

Combustion

The ‘burning’ or oxidation of fuel using oxygen and heat.

Comet

A small body of ice and dust which orbits the Sun.

COMPASS(-C)(-D)

Compact Assembly: the Culham conventional aspect ratio TOKAMAK facility. It has a similar magnetic geometry to JET and therefore plays a crucial role in scaling experimental results through JET to ITER.

Confinement

The containment of plasma particles and energy within a container for some extended period of time. A fusion reactor must confine the fuel plasma for a long enough time at high enough density and temperature in order to be economically feasible. See Lawson Criterion.

Confinement time

(1)  Time taken for energy or particles to leave the plasma.

(2)  The characteristic time that plasma can be contained within a laboratory experimental device using a magnetic field, a particle’s own inertia, or by other methods (e.g. electric field). The electron and ion particle confinement time is often distinguished from the energy confinement time of the plasma.

Confinement time scaling laws

See transport scaling.

Continuous Medium

The usual description of the Scattering of Radiation is based on the assumption of scattering by individual particles. However, this concept breaks down if the medium becomes 'continuous', that is if the distance between the scattering particles becomes less than the wavelength of the radiation (analogous to the specular reflection from a surface). The usual effects of scattering (i.e. spatial redistribution of radiation) disappear in this case as the scattering phase function becomes sharply peaked into the forward direction. Density gradients of the medium will then result in a quasi- refraction effect (the refraction of light in the earth's atmosphere is likely to be of this type).
There is also evidence that this aspect is of relevance for collisional excitation of atomic states by electrons or ions as the relevant cross section is apparently enhanced if the medium becomes continuous with regard to the wavelength of the equivalent radiative transition (see http://www.plasmafacts.de/striapot.htm).

Continuum

Optical radiation arising from broadband emission from the photosphere.

Continuum Radiation

Various processes are listed in the textbook literature that are capable of producing a radiation continuum. Of these, the free-free processes like Bremsstrahlung or Synchrotron Radiation, although apparently supported by observational evidence, can be discounted as fictitious: the emission of radiation can not be explained in a logically consistent manner by the acceleration of charged particles, as it would make the emission dependent on the state of motion of the observer. The dynamic changes associated with the emission would therefore become a subjective quantity, which is logically not acceptable in the same sense as the mutual force between two objects can (by definition) not depend on the state of motion of the observer (principle of relativity). It can furthermore be ruled out that the physical objects which cause the acceleration provide a preferred reference frame, because any force is either a function of the coordinates alone (coulomb force, gravitational force) or a function of the coordinates and the velocity (Lorentz force). The overall acceleration would therefore still be ambiguous depending on the state of motion of each of the interacting particles due to the presence of third bodies.

The only true continuum is produced by the recombination of electrons with ions, which results in a continuum according to the energy characteristics of the free electron spectrum and the recombination cross section.
However, the discrete atomic spectrum may form a quasi- continuum if the lines are sufficiently broadened. This happens in particular for high plasma densities and/or highly excited atomic states . There is theoretical and observational evidence that under these conditions the 'continuum' of blended lines is many orders of magnitude more intense than the actual recombination continuum (see for instance http://www.plasmafacts.de/index.html#A5). (for the latter aspect see also Bohr-Einstein Radiation Formula).

Controlled Thermonuclear Fusion

Laboratory experimental plasmas in which light nuclei are heated to high temperatures (millions of degrees) in a confined region that results in significant enough fusion reactions under controlled conditions to be able to produce energy.

Convection

The bulk transport of plasma (or gas) from one place to another, in response to mechanical forces (for example, viscous interaction with the solar wind) or electromagnetic forces.

Convective instabilities

Plasma wave’s amplitude increasing as the wave propagates through space without necessarily growing at a fixed point in space. For comparison see absolute instabilities.

Corona

(1)  The outermost layer of the solar atmosphere, characterized by low densities (<10E+9 per cubic cm or 10E+15 per cubic m and high temperatures (>10E+6 K). It is not visible from the Earth except during a total eclipse of the Sun or by use of a special telescope called a coronagraph.

(2)  The outermost layer of the Sun's atmosphere, visible to the eye during a total solar eclipse; it can also be observed through special filters and best of all, by X-ray cameras aboart satellites. The corona is very hot, up to 1-1.5 million degrees centigrade, and is the source of the solar wind.

(3)  The very hot outer layer of the Sun's atmosphere, composed of highly diffused, superheated, ionized gases, and extending into interplanetary space. The hot gasses in the solar corona form the solar wind. See also: The Dynamic Sun.

(4)  The outermost part of a star's atmosphere, the corona characterized by high temperatures and low densities relative to the stellar photosphere. The corona is home to many plasma phenomena. The origin of the high temperature in the corona remains an outstanding mystery in solar physics.

Coronagraph

An optical device that makes it possible to observe the corona at times other than during an eclipse. A simple lens focuses an image of the Sun onto an occulting disk that prevents the light from the solar disk from proceeding farther along the optical path, effectively providing an artificial eclipse.

Coronal hole

(1) An extended region of the corona, exceptionally low in density and associated with unipolar photospheric regions having "open" magnetic field topology. Coronal holes are largest and most stable at or near the solar poles, and are a source of high-speed solar wind. Coronal holes are visible in several wavelengths, including solar x-rays and the He 1083 nm emission line.

(2) An extended region in the corona, exceptionally low in density and associated with unipolar magnetic photospheric regions having "open" magnetic field topology. Coronal holes are largest and most stable at or near the solar poles, and a source of high-speed solar wind. Coronal holes are visible in several wavelengths but most notably in solar x-rays.

Coronal loops

A typical structure of enhanced corona observed in EUV lines and soft x-rays. They are sometimes related to H alpha loops. Coronal loops represent "closed" magnetic topology.

Coronal mass ejection (CME)

(1) A vast region of hot, dense, and high speed solar wind propagating away from the Sun.

(2) An observable change in coronal structure that (1) occurs on a time scale between a few minutes and several hours and (2) involves the appearance of a new, discrete, bright white-light feature in the coronograph field of view. They are associated with the large-scale, closed magnetic structures in the corona. At times of coronal mass ejections large quantities of material (10^15 - 10^16 g) are sporadically ejected from the Sun into the interplanetary space. The speed of the leading edge of the coronal mass ejection may vary from 50 km/s to 1200 km/s. Average speed is about 400 km/s. The average heliocentric width is about 45 degrees. The largest geomagnetic storms are caused by coronal mass ejections.

(3) A huge cloud of hot plasma, expelled sometimes from the Sun. It may accelerate ions and electrons, and may travel through interplanetary space as far as the Earth's orbit and beyond it, often preceded by a shock front. When the shock reaches Earth, a magnetic storm may result.

(4) A transient outflow of plasma from or through the solar corona. CMEs are often but not always associated with erupting prominences, disappearing solar filaments, and flares.

(5) Sporatic ejection of plasma and magnetic field from the Sun, usually observed by optical telescopes with the Sun blocked off. An event can eject tens of trillions of tons of predominantly hydrogen plasma at hundreds of killometers per second, sometimes reaching more than 1000 km/s. Also called solar mass ejection.

Coronal neutral line

The line in the corona that separates solar magnetic fields of opposite polarity. It is calculated from solar observations of the photospheric magnetic field. Extension of the neutral line radially outward by the solar wind defines the current sheet in the heliosphere.

Coronal rain

 Material condensing in the corona and appearing to rain down into the chromosphere as observed in H alpha at the solar limb above strong sunspots.

Coronal streamer

(1)  A feature of the white light corona that looks like a ray extending away from the sun out to about one solar radius, having an arch-like base.

(2)  A large-scale structure in the white-light corona often overlying a principal inversion line in the solar photospheric magnetic fields. (See helmet streamer.)

Corrected geomagnetic coordinates

A nonspherical coordinate system based on a magnetic dipole axis that is offset from the Earth's center by about 450 km toward a location in the Pacific Ocean (15.6 N 150.9 E). This "eccentric dipole" axis intersects the surface at 81N 85 W, and 75 S 120 E.

Cosmic ray

(1)  Electromagnetic radiation of extremely high frequency and energy; cosmic rays usually interact with the atoms of the atmosphere before reaching the surface of the Earth; some cosmic rays come from outside the solar system while others are emitted from the Sun and pass through holes in the corona.

(2)  Nuclear and subatomic particles moving through space at speeds close to the speed of light. They are thought to come from stars in the Milky Way galaxy.

Coulomb Collision

Particle collisions where the Coulomb Force (electrical-force attraction or repulsion) is the governing force that results in deflections of the particles away from their initial paths.

Coulomb Explosion

When a sufficiently intense laser irradiates a group of atoms (gas cluster, object, target, etc.), the electric field of the laser beam can drive some or all of the electrons off the atoms. With the electrons gone, the resulting group of ions explodes due to the Coulomb repulsion of the positive charges.

Coulomb Scattering

Integration of the well known Rutherford formula over the scattering angle leads to the Total Cross Section for Coulomb Scattering

                                        σ_c(E)= 5/16^.Z^2.e⁴/ε² ,

with ε the energy of the scattered particle in the center of mass system, e its charge and Z the charge number of the target particle. This form is different from the usual result quoted in the literature which contains the additional Coulomb- Logarithm factor. The latter can however be shown to be due to an incorrect Energy Loss- weighting function in the integration of the Rutherford formula.

Coulomb(C)

Unit of electrical charge named after Charles A. de Coulomb. In the meter-kilogram-second (mks) system one Coulomb equals the quantity of electricity transferred by a current of one ampere in one second. Electrons and protons each have exactly the same, although opposite, charges equal to 1.602 x 10-19C.  See Ampere.

Culham Science Centre

The UK centre for fusion research, performed by UKAEA Culham Division. Culham Science Centre is owned and managed by UKAEA, and as well as the EURATOM/UKAEA fusion programme and JET, it has a number of high-tech companies as tenants, especially AEA Technology. Culham is situated on the A415, east of Abingdon, Oxfordshire, UK.

Current density

Current per unit cross-sectional area.

Current drive

Any of a number of means to maintain or increase electrical current in a plasma by using external devices such as neutral beam or radio frequency (RF) power generators.

Current drive (non-inductive)

A method of driving plasma current (in a TOKAMAK) that does not depend on transformer action (e.g. by using RF waves or neutral beams); necessary for a continuously operated power plant, since transformer action is cyclic. Also being applied to control instabilities and to optimise confinement. See electron cyclotron current drive, fast wave current drive, helicity injection, ion cyclotron current drive, lower hybrid current drive.

Current ramp-up (down)

The increase (decrease) of plasma current either at the start of operation (ramp-up) or during operation to modify the current profile for performance investigations.

Curvature drift

Single-particle drift motion arising from the centrifugal pseudo-force felt by the particle as it travels along a curved magnetic field line. See Drift Motion for more information.

Cusps

See polar cusps.

Cusps (of the magnetosphere)

Two regions of weak magnetic field, on the sunward boundary of the magnetosphere, one on each side of the equator. They separate magnetic field lines closing on the front from those swept into theearth's magnetotail.

Cutoff frequency

Frequency beyond which a plasma wave ceases to exist or changes its nature.

Cyclotron frequency

(1)  Charged particles in a magnetic field have a natural frequency of gyration in the plane perpendicular to the field - the cyclotron frequency. For electrons in a TOKAMAK, the cyclotron frequency is typically a few tens of GHz, and for ions, a few tens of MHz. Also see Larmor Frequency.

(2)  Number of times per second that a particle orbits a magnetic field line. The frequency is completely determined by the strength of the field and the particle's charge-to-mass ratio. (Often, and incorrectly, called the Larmor frequency. The cyclotron or gyrofrequency is twice the Larmor frequency of precession.)

Cyclotron Radius

Radius of orbit of charged particle about a magnetic field line. Also called gyroradius, Larmor radius.

Cyclotron Resonance

Charged particles in a magnetic field will resonate with an electric field (perpendicular to the magnetic field) that oscillates at the particle’s cyclotron frequency, or harmonics of the particle’s cyclotron frequency.

Cylindrical approximation

An approximation to the true TOKAMAK geometry in which the torus is cut and straightened, so that the toroidal direction becomes the cylinder axis. There are two directions of symmetry: along the axis (the "toroidal" direction) and about the axis (the "poloidal" direction).