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Glossary

A brief explanation of the various terms used in the provided info can be found at the following www sites:

- http://www.fusion.org.uk/info/glossary/glossmain.htm

- http://plasmadictionary.llnl.gov/

- http://fusedweb.pppl.gov/Glossary/glossary.html

Here is a list of the most frequently adopted terms used in thermonuclear fusion research, focusing on terminology used in magnetic confinement fusion.

Check the glossary sites provided below for more complete info.

---A---

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.

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.

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 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 velocity: 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 waves: 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.

Alpha particle: 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.

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.

Auxiliary heating: See additional heating.

---B---

Banana orbits: See trapping.

Beta: Ratio of plasma pressure to magnetic field pressure. One of the figures of merit for magnetic confinement: the magnitude of the magnetic field pressure is determined by the expenditure on the field coils, etc., that generate it; since fusion reactivity increases with plasma pressure, a high value of beta is an indicator of good performance. The highest value of beta achieved in a large tokamak is about 13%, though much higher values are theoretically possible at low aspect ratio and have been achieved on START.

Beta limit: Maximum beta attainable, usually due to a deterioration in the confinement. The Troyon beta limit, which states that beta (in percent) cannot exceed g.I / a.B is often quoted. Here, g is the so-called Troyon coefficient, and has an value of around 3.5 for conventional tokamaks. (I is the plasma current in MegaAmps, a is the minor radius in metres, and B is the toroidal field in Tesla.) The normalised beta is given by beta.a.B / I, and (when quoted in percent) cannot exceed g.

Blanket: In a fusion power plant using deuterium-tritium fuel, the system surrounding the plasma vessel used to slow down the neutrons produced, so that the heat released can be used for electricity generation. In many designs, the blanket is also used to synthesise tritium (from the neutrons and a lithium compound) to use as fuel.

Bootstrap current: Theory predicted in 1970 that a toroidal electric current will flow in a tokamak which is fuelled by energy and particle sources that replace diffusive losses. This diffusion driven bootstrap current, which is proportional to beta and flows even in the absence of an applied voltage, could be used to provide the confining magnetic field: hence the concept of a bootstrap tokamak, which has no toroidal voltage. A bootstrap current consistent with theory was observed many years later on JET and TFTR; it now plays a role in design of experiments and power plants (especially advanced tokamaks).

Boronisation: Application of a compound containing boron to the inner surface of a vacuum vessel to help reduce impurity radiation.

Break-even: See energy break-even.

Bremsstrahlung: Radiation produced by the acceleration of charged particles. In a tokamak, by far the largest source of bremsstrahlung is the continuous deflection of the electron trajectories by the electrostatic fields of the ions.

Burn: An ignited plasma is said to be "burning".

---C---

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.

Close Support Unit: The Culham Close Support Unit (CSU) assists the EFDA Associate Leader for JET, and comprises professional staff seconded to Culham by the EFDA Associates. It has various roles concerned with the experimental programme, links with the Operator, and the coordination of enhancements to the JET Facilities, as well as providing an administrative role.

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.

Confinement time: Time taken for energy or particles to leave the plasma.

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.

Cyclotron frequency: 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.

---D---

DEMO: A demonstration fusion power plant which will resemble a commercial fusion power station as closely as possible. This stage in the progress from present-day machines to commercial fusion power is expected to be constructed after a Next Step device.

Density: In plasma physics "density" nearly always refers to the number density - the number of particles per unit volume (as opposed to the mass density - mass per unit volume).

Density limit: Plasma disruptions occur within tokamaks if the electron density is too high.

Deuterium: A stable isotope of hydrogen, whose nucleus contains one proton and one neutron. Deuterium plasmas are used routinely in present-day experiments.

DG-XII: The Research Directorate-General of the European Commission. Its mission is described here. The multi-annual Framework Programme provides the structure for the implementation of the Directorate-General's policy, by helping to organise and financially support cooperation between universities, research centres and industries.

Diagnostic: Apparatus used for measuring one or more plasma quantities (temperature, density, current, etc.). See CCD, charge exchange recombination spectroscopy, electron cyclotron emission, FIR, interferometry, Langmuir probe, laser ablation, magnetic diagnostics, neutral particle analyser, polarimetry, reflectometry, spectroscopy, survey spectrometer, Thomson scattering, soft X-rays.

Diffusion, thermal (or particle): The random flow of heat (or particles) down a thermal (or density) gradient.

Disruption, disruptive instability: A complex phenomenon involving MHD instability which results in rapid heat loss and termination of a discharge. Plasma control may be lost, triggering a VDE in which the apparatus may be damaged, particularly in large machines. This phenomenon places a limit on the maximum density, pressure and current in a tokamak.

Divertor: A magnetic field configuration affecting the edge of the confinement region, designed to divert impurities/ helium ash to a target chamber. Alternative to using a limiter to define the plasma edge.

Drift orbits: Particle motion is in the direction of the magnetic field, but, particularly for fast particles, electric fields and gradients of the magnetic field give rise to an additional drift perpendicular to the magnetic field.

Driven current: Plasma current produced by a means external to the plasma (e.g. a transformer, neutral beams, RF waves).

---E---

ECR(H): See Electron Cyclotron Resonant Heating.

EFDA: See European Fusion Development Agreement.

Electron cyclotron resonant heating: Additional heating method using RF waves to accelerate electrons orbiting in the magnetic field.

ELM: Edge Localised Mode. An instability which occurs in short periodic bursts during the H-mode in divertor tokamaks. It causes transient heat and particle loss into the divertor which can be damaging.

Elongation: Ratio of the plasma cross-sectional height to its cross-sectional width. See vertical displacement event, plasma geometry.

Energetic particle: In terms of energy, the particles in a plasma can be divided into two classes. The more numerous class (thermal particles) consists of particles whose distribution in energy can be characterised by a temperature typically in the range 1-30 keV for modern tokamaks. The less numerous class (energetic particles) contains particles of significantly higher energy (up to several MeV). Energetic particles can be created by fusion reactions, neutral beam injection, or RF heating.

Energy break-even: When heating power = total fusion power produced (i.e. alpha power plus neutron power for a D-T plasma). The fusion performance of a power plant is denoted by Q, which is the ratio of the energy of the fusion products to that used to heat the plasma. Break-even corresponds to Q=1, ignition corresponds to Q=infinity. A burning plasma has Q>1.

European Fusion Development Agreement: The agreement that coordinates the work within the European Union and Switzerland on controlled thermonuclear fusion. EFDA runs from 1 January 1999 to 31 December 2002. One of the activities covered by the EFDA is the collective use of the JET facilities by the European Fusion Associations; the JET Implementing Agreement defines the terms under which that can proceed. Further information.

---F---

Fast wave current drive: Current drive produced by a fast wave, as opposed to a lower hybrid wave. The wave can penetrate the plasma more easily than a lower hybrid wave.

Field coils: The coils of a magnetic confinement device that produce the magnetic field required to stabilise and shape the plasma.

Field lines, flux surfaces: Imaginary lines marking the direction of a force field. These define surfaces, to which the particles are approximately constrained, known as flux surfaces.

Flat-top current: Constant current (refers to shape of the graph as a function of time).

Fuelling: Supplying a fusion plasma with the necessary reactants.

Fusion: A fusion reaction occurs when two light nuclei (ions) approach each other so closely that their Coulomb (charge) repulsion is overcome, allowing the nuclei to fuse. The total mass of the fusion products is lower than that of the two original nuclei; the difference is converted to kinetic energy which is distributed between the products. Methods being investigated in an attempt to harness this potentially huge source of energy include magnetic confinement fusion and inertial confinement fusion. See Lawson Criterion, deuterium-tritium reaction, deuterium-helium-3 reaction.

Fusion product: The product of a fusion reaction, for example an alpha-particle or neutron in a deuterium-tritium plasma.

Fusion reactivity: Fusion reaction rate per ion. For present typical tokamak conditions, it increases with the density and temperature of the plasma.

Fusion triple product: Mathematical product of density, temperature and energy confinement time. A measure of approach to energy break-even and ignition.

---G---

Greenwald density: The Greenwald normalised density is given by n20.pi.a2/Ip, where n20 is the electron density expressed in units of 1020m-3, a is the plasma minor radius in metres, and Ip is the plasma current in Mega Amperes. In many tokamaks this value does not exceed 1, so the Greenwald density is a measure of the density limit for a tokamak.

Gyrotron: Device used for generating high power microwaves in the electron cyclotron range of frequencies (about 50 - 200 GHz).

---H---

H factor: Ratio of the energy confinement time for a given set of plasma conditions to the theoretical value calculated from an L-mode scaling law. For a plasma in the H-mode the H-factor is typically around 2, that is, the confinement time is enhanced by a factor of two compared to the normal L-mode.

H-L transition: Change from H-mode to L-mode (usually quite sudden).

H-regime (mode): A High confinement regime that has been observed in tokamak plasmas. It develops when a tokamak plasma is heated above a characteristic power threshold, which increases with density, magnetic field and machine size. It is characterised by a sharp temperature gradient near the edge (resulting in an edge "temperature pedestal"), ELMs, and about a 100% increase in energy confinement time compared to the normal L-regime.

Helium ash: Fusion reactions in a deuterium-tritium plasma produce energetic alpha-particles (helium nuclei), which heat the plasma as they slow down. Once this has happened, the alpha-particles have no further use: they constitute helium ash, whose removal and replacement by deuterium-tritium fuel is required to prevent dilution of the plasma.

---I---

IAEA: International Atomic Energy Agency.

ICCD: See Ion Cyclotron Current Drive.

ICF: See Inertial Confinement Fusion.

ICRF: See Ion Cyclotron Resonant Heating.

ICRH: See Ion Cyclotron Resonant Heating.

Ignition condition: Condition for self-sustaining fusion reactions: For a deuterium-tritium plasma, ignition occurs when the heating of the plasma by the alpha-particles is equal to the heat loss. See energy break-even.

Impurities: Ions, other than the basic plasma ion species, which are unwanted as they lose energy by radiation and dilute the plasma.

Impurity radiation: Radiation of energy by impurities, which is lost by the plasma. This is undesirable in a power plant because more fusion energy would have to be produced to offset this loss. The radiation rate increases with the mass of the impurity ions, which is why the plasma facing surface is often coated with carbon, boron or other low Z material.

Inertial confinement fusion: The use of high-powered lasers or other beam devices to implode a pellet of material to such high densities that fusion occurs.

Internal transport barrier: A transport barrier within the plasma, as opposed to one near the plasma edge.

Ion cyclotron current drive: Non-inductive current drive using ICRH.

Ion cyclotron resonant heating: Additional heating method using RF waves at frequencies (about 20-50 MHz) matching the frequency at which ions gyrate around the magnetic field lines.

ITB: See Internal Transport Barrier.

ITER: International Tokamak Experimental Reactor. The proposed Next Step tokamak to be built in Cadarache, France.

---J---

JET: Joint European Torus, the largest tokamak in the world, is sited at Culham. See advanced tokamak, bootstrap current, preliminary tritium experiment, DTE-1, DTE-2, fast Alfven wave, iron core, reverse shear, EFDA, JET Operation Contract.

JT-60U: JT-60U is the flagship tokamak of the Japanese magnetic confinement programme, similar in size to JET. It is sited at Naka and run by JAERI. A much larger tokamak is planned, JT-60SU.

---K---

Kinetic theory: A detailed mathematical model of a plasma in which trajectories of constituent electrons and ions are described. More complex than fluid and two-fluid theories, it is necessary in the study of RF heating and some instabilities, particularly when energetic particles are involved.

---L---

L-H transition: Change from L-mode to H-mode (usually quite sudden).

L-regime (mode): As opposed to the H-regime. The "normal" Low confinement regime of additionally heated tokamak operation.

Larmor radius: Radius of the gyratory motion of particles around magnetic field lines.

Lawson criterion: A plasma that will generate more energy through fusion reactions than is required to create and sustain it, must satisfy the Lawson criterion. This states that the mathematical product of ion density and energy confinement time must be above a certain value, dependent on the reaction in question. For deuterium-tritium fusion this value is about 2_1020 m-3s.

Low aspect ratio: Low ratio of major to minor radius (say about 1.3 as in MAST as compared to a JET value of about 3). Same as tight aspect ratio and small aspect ratio.

Lower hybrid current drive: Non-inductive current drive using lower hybrid heating.

Lower hybrid heating: Heating produced by a lower hybrid wave.

Lower hybrid (LH) wave: A plasma wave of frequency between the ion and electron cyclotron frequencies. It has a component of electric field parallel to the magnetic field, so it can accelerate electrons moving along the field lines.

---M---

Magnetic axis: The magnetic surfaces of a tokamak form a series of nested tori, of decreasing minor radius. The central "torus" defines the magnetic axis.

Magnetic confinement fusion: The use of strong magnetic fields to confine plasma so as to allow fusion reactions to occur within it.

Magnetic islands: Islands in the magnetic field structure caused either by externally applied fields or internally by unstable current or pressure gradients.

Magnets: See field coils.

Major radius: Distance from the centre of the torus (to the centre of the plasma cross-section).

MARFE: Multi-faceted Asymmetric Radiation From the Edge. A thermal instability sometimes observed near the edge of tokamak plasmas.

MCF: See Magnetic Confinement Fusion.

MHD (magnetohydrodynamics): A mathematical description of the plasma and magnetic field, which treats the plasma as an electrically conducting fluid. Often used to describe the bulk, relatively large-scale, properties of a plasma.

MHD and other plasma instabilities: Unstable distortions of the shape of the plasma/magnetic field system.

Microinstabilities: Instabilities with characteristic lengths similar to the particle Larmor radii, rather than to the tokamak dimensions. These are thought to be responsible for the fine-scale turbulence in tokamaks, and hence anomalous transport.

Minor radius: Half the horizontal extent of the plasma cross-section. In the context of plasma profiles, the radial distance from the centre of the plasma.

Mode number: Characterises the wavelength of an instability.

---N---

NBI: Neutral Beam Injection. See Neutral Beam.

Neo-classical: Classical collisional plasma transport theory, corrected for toroidal effects. The neoclassical theory predicts the existence of the bootstrap current.

Neutral beam: A beam of high velocity neutral atoms injected into the plasma to impart momentum to the plasma ions. Neutral beam injection is a method of providing additional heating and current drive.

Neutrons: Neutral elementary particles. Products of deuterium-tritium and other fusion reactions.

Next step: Next generation of experimental facility, or facilities, after present large tokamaks.

---O---

Ohmic heating (OH): Inductive heating by using a transformer to drive a current in the plasma. This is necessarily pulsed.

Operating limits: See tokamak operating boundaries.

Optimised shear: Adjusting the current profile to optimise tokamak performance.

---P---

Passing particles: See trapping.

Pellet injection: Fuelling a plasma by firing into it a stream of small pellets of frozen material (e.g. deuterium) at high velocity.

PF coils: The coils that produce the external poloidal field in a tokamak (in addition to that produced by the plasma current), used to control the plasma geometry.

PINI: Positive Ion Neutral Injector - the main component of a neutral beam injection system.

Plasma: If we increase the temperature of a gas beyond a certain limit, it does not remain a gas: it enters a regime where the thermal energy of its constituent particles is so great that the electrostatic forces which ordinarily bind electrons to atomic nuclei are overcome. Instead of a hot gas composed of electrically neutral atoms, we have two co-mingled populations composed of oppositely charged particles - electrons and ionised nuclei. This is a plasma, and it is neither solid, liquid, nor gas.

Plasma confinement: Retention of plasma within a region, including the physics of heat and particle losses from the plasma.

Plasma current: The current flowing toroidally through the plasma in a tokamak.

Plasma equilibrium: A plasma with given profiles and geometry in force balance with the magnetic field.

Plasma pressure: Proportional to the product of plasma density and temperature. In magnetic confinement devices, this outward pressure is counterbalanced by magnetic forces.

Plasma radiation: Radiation (i.e. energy loss via electromagnetic waves) from a plasma is due to a number of processes.

Poloidal beta: The poloidal component of the plasma beta - a measure of plasma confinement by currents flowing poloidally in the plasma. In a tokamak, the poloidal beta is equal to 1 if no poloidal currents are flowing. In this case, the plasma is confined entirely by the toroidal plasma current crossed with the poloidal magnetic field it induces. If the poloidal beta is zero, the plasma current lies entirely parallel to the magnetic field, and there is no magnetic confinement - a region of plasma where this applies is known as a force-free region. If the poloidal beta is greater than one the plasma is confined mostly by the poloidal current crossed with the toroidal field, and the toroidal current merely serves the purpose of centering the plasma. This latter configuration is sometimes referred to as a high beta device.

Poloidal field: Component of the magnetic field parallel to the minor circumference. The poloidal field is essential for confinement and, in a tokamak, is generated by the plasma current (c.f. stellarator); this is in contrast to the larger toroidal field, which is generated externally.

Power plant: A fusion power station, producing economically-feasible amounts of electricity from fusion reactions (see blanket). The term "reactor" is sometimes used for such a device, or for any fusion device in which the plasma is ignited.

Power threshold: The L-H transition (and also improved performance regimes related to reverse shear) occurs when the power exceeds a certain threshold value - the power threshold.

Profile control: Control of instabilities by controlling the radial variation of pressure, density or current.

Pumped divertor: Divertor field lines directed into a pumped chamber surrounding the target plate.

---Q---

Q: See energy break-even.

q: See safety factor.

q limit: The safety factor at the edge of the tokamak plasma must be greater than 2 in order to avoid disruptions. (This limit becomes more restrictive at tight aspect ratio.)

---R---

Radiative divertor: A divertor configuration in which most of the power is radiated before it reaches the target plates, thereby avoiding intolerable heat loads on these plates.

Reactor: See power plant.

Relaxation: The evolution of a turbulent plasma to a lower energy state.

Resistive instability: Instability due to diffusion and rearrangement of magnetic field lines. When the plasma resistivity is small, these instabilities have a slow growth rate.

Resonant ions/electrons: Resonance occurs when one of the characteristic frequencies of particle motion in the plasma (for example, the cyclotron frequency) matches the frequency of some applied perturbation (for example, an RF wave).

RF (waves): Radio-Frequency (electromagnetic waves): important frequencies generally between 20 MHz and 200 GHz. RF waves are often used to provide additional heating and current drive.

---S---

Safety factor, q: Number of turns the helical magnetic field lines in a tokamak make round the major circumference for each turn round the minor circumference. Denoted by the symbol q. There is no connection with the ordinary sense of the word "safety".

Sawtooth: A cyclically recurring instability which affects the central region of tokamak discharges. The temperature periodically falls abruptly, then slowly recovers. The jagged trace produced by plotting temperature against time gives the instability its name.

Sawtooth crash: The rapid collapse of the central temperature in a tokamak which is sawtoothing.

Scaling laws: Empirical or theoretical expressions for how various plasma phenomena (e.g. confinement, power threshold, etc.) vary with the tokamak conditions using a range of free parameters to be fixed by "best fits" of the scaling law to tokamak data. They are particularly useful for predicting the performance of future tokamaks.

SEAFP: The European Safety and Environmental Assessment of Fusion Power, which reported in December 1994. A second study, SEAFP-2, updated the previous results and reported in 1999; see the list of downloadable papers.

Single/double null: Points of zero poloidal magnetic field where the separatrix crosses itself. Usually put above and/or below the plasma (but sometimes put in the plane of the torus). Most divertor configurations have either one or two nulls, known as X-points.

Small aspect ratio: Same as low aspect ratio.

Stability theory: The theory of how small perturbations to a system evolve in time. Spontaneous growth is instability. Instabilities can saturate at some small amplitude, in which case they degrade confinement, or grow uncontrollably, in which case confinement is lost (e.g. a disruption occurs).

Steady-state power plant: A continuously (as opposed to cyclically) operated power plant.

Stellarator: A toroidal magnetic confinement device whose poloidal field is generated by external helical coils (unlike the tokamak where it is generated by an internal current induced by transformer action). The absence of a plasma current gives stellarators significant potential advantages over tokamaks as fusion power plants (no disruptions, no current drive, and no stability control system). There are a number of different stellarator configurations; see torsatron, heliotron, helias. In general, stellarators have not been as successful as tokamaks though a considerable level of research continues, notably in Germany, Spain, USA, Russia and Japan.

Synchrotron radiation: Radiation due to electrons spiralling around magnetic field lines. Relativistic effects are important and increase the total emitted power substantially.

---T---

TAE modes: Toroidal Alfven Eigenmodes. Similar to Alfven gap modes.

Temperature: A measure of thermal energy in units of degrees or electron volts (1 eV is about 10000 degrees Celsius).

TF coils: The coils that produce the toroidal field in a tokamak.

TFTR: The Tokamak Fusion Test Reactor at Princeton was the largest US device, and performed a major campaign using deuterium and tritium fuel between 1993 and 1997. TFTR had a relatively high magnetic field of 5 T and a circular cross-section.

Thermal particles: As a result of collisional energy exchange, the energy of most plasma particles falls within a distribution which can be described by a single temperature (typically 1 to 30 keV for tokamaks). These are the thermal particles, as distinct from energetic particles which lie outside the thermal equilibrium.

Tokamak: The most successful device yet found for magnetic confinement of plasma. Its magnetic field is made up from helical lines of force on toroidal surfaces, and is generated both by external field coils and by the current in the plasma. The word tokamak comes from a Russian acronym for toroidal magnetic chamber.

Tokamak operating boundaries: The set of plasma parameters (e.g. density, current, pressure), beyond which it is impossible to operate a tokamak. Careful use of plasma cross-sectional shapes and current profiles can increase the operating regime.

Toroidal field: The largest component of the magnetic field of a tokamak, produced by coils external to the plasma, which is in the toroidal direction.

Transport: The processes by which particles and energy in the centre of the plasma are lost to the edge of the plasma.

Transport barrier: In certain operational scenarios (e.g. the H-mode) a region of low transport exists giving rise to a steep pressure gradient. Such a region is referred to as a transport barrier.

Transport scaling: The magnitude of heat transport may be expressed, empirically or theoretically, in terms of a simple functional dependence on a few plasma parameters. This enables one to deduce how the heat transport varies (scales) in response to changes in the value of these parameters.

Trapped particles: The outside (larger major radius portion) of a tokamak plasma has a lower magnetic field than the inside. Particles with low velocity parallel to the magnetic field may not have sufficient energy to enter the higher field (inside) region and become trapped on the outside. They are not free to circulate toroidally but instead bounce back and forth, performing so-called banana orbits.

Trapping: Particle trapping occurs over the central region of most tokamaks, where the collisional mean free path is longer than the distance a particle must travel along a field line to get from top to bottom of the torus (the connection length). It results from the conservation of total particle energy and magnetic moment. If a particle's motion is nearly perpendicular to the magnetic field, it can become trapped in an orbit that does not complete a revolution in either the toroidal or poloidal direction. Since this orbit has a finite width due to the particle's drift across the magnetic field it is known as a banana orbit. Particles not trapped in this way are called passing particles.

Tritium: An isotope of hydrogen, whose nucleus consists of one proton and two neutrons. Tritium does not occur naturally, because it is unstable to radioactive decay. For this reason, special tritium handling technology is required whenever the use of deuterium-tritium plasmas is contemplated, as in JET and future fusion power plants.

Turbulence: Randomly fluctuating, as opposed to coherent, wave action. For example, the turbulent surface of water beneath a waterfall can only be described in terms of its averaged properties, such as the scale and duration of fluctuations; whereas a more systematic description can be given to waves on the surface of a still pond.

---UVW---

UKAEA Culham Division: The division of UKAEA responsible for fusion research. Culham Division operates the Compass-D, START and MAST tokamaks, and holds the JET Operation Contract.

VDE, Vertical displacement event: A plasma instability during which the whole plasma moves up (or down) away from its equilibrium position. High elongation plasmas are more prone to this motion due to the more strongly shaped magnetic field required to produce them. Unless it is controlled by a feedback system the plasma will be rapidly lost as it collides with the vessel, causing its current to flow through the wall and other components.

Wendelstein 7-X (W7-X): A large advanced stellarator under construction in Germany to follow its present W7-AS device, based on the Helias configuration.

---XYZ---

X-point: See single/double null.

Acknowledgement:

This glossary is a subset of the UKAEA glossary compiled by Peter Knight