Term	Definition
304 L	An alloy of austenitic stainless steel, with low carbon content in order to resist intergranular stress corrosion cracking.
316 N	An alloy of austenitic stainless steel.
316 SS	A generic abbreviation for 316 stainless steel, an alloy of austenitic stainless steel.
a	minor plasma radius
abnormal event	An unintended change of operating state of plant or component not causing unscheduled down-time.
abnormal operating conditions	Conditions imposed on the plant or a component deviating from normal operation.
abnormal operation	Operating procedures or operating state of a plant or a component deviating from normal operation. Deviations include operating procedures for start-up, current drive, and shut-down, which deviate from the operating procedures of normal operation, or operation at fusion power deviating from its nominal value.
AC	alternating current
activation	The process of inducing radioactivity by bombardment with neutrons or with other types of radiation.
additional heating	The application of beams of neutral particles and/or microwave radiation to the plasma from external apparatus so as to bridge the gap in heating between resistive heating due to plasma toroidal current (which gets weaker with increased temperature) and alpha-particle heating due to the slowing down of the helium reaction product in the plasma (which gets stronger with higher temperature).
AISI	American Iron and Steel Institute
Alfvén wave	A type of oscillation of plasma particles, consisting of transverse waves propagating along the magnetic field lines in a plasma. Also called magnetohydrodynamic wave.
alpha particle	a-particle, the nucleus of the helium-4 atom
alpha-power	a-power, the component of fusion reactor power which comes from the alpha particle reaction product.
anomalous transport	Energy and particle transport which is higher than predicted by neoclassical predictions.
ANSYS code	A commercial software package that provides a finite element analysis for performing, evaluating and presenting analysis operations in a wide variety of fields.
armour lifetime	The time an armour material (i.e a material facing the plasma) component can fulfil its functional purpose without exceeding its performance limitations. Lifetime can for instance be reduced by excessive erosion without redeposition in a suitable structural form.
armour-heat sink joint lifetime	The time the joint between the armour material (facing the plasma) and the underlying substrate (the “heat sink”) can be reliably predicted to fulfil its functional purpose. Such joints are typically affected by the number of operational cycles (leading to fatigue of the joint).
aspect ratio	The ratio between the major and (minimum) minor radii of the plasma torus
atomic mass	The atomic mass number, A, is the total of the number of protons and the number of neutrons in the atomic nucleus concerned. The atomic mass unit is defined as 1/12 the mass of a 12C atom, i.e 1.660438 x 10-27kg, or 931.478 MeV in equivalent energy units.
austenitic stainless steel	One of the three main phases of stainless steel – the others being martensitic and ferritic. Austenitic steels have face-centred cubic crystals i.e atoms at the corners and centre of each face of the cubic crystal, whereas the other phases are body-centred cubic (i.e atoms at the corners and at the cube centre). The phase depends on the composition – austenitic steels hava a high nickel and/or manganese content – whereas martensitic steels have low, and ferritic steels high chromium and/or molybdenum content.
auxiliary heating	Systems to heat the plasma and to drive current in it.
auxiliary heating power	auxiliary heating = additional heating. The power usually refers to that absorbed by the plasma, and the electrical power connected to the auxiliary heating system will usually be considerably higher, to account for reflection, absorbtion in transmission, and internal inefficiencies of generation.
availability	The percentage of time that a system or plant is available to execute/or executing its operating cycle.

Term	Definition
B	magnetic field
BA	Broader Approach
baffle elements	Limit the conductance between the divertor and the main chamber.
ballooning instabilities	Magnetohydrodynamic instabilities caused by the pressure gradient of the plasma.
barycenter	The centre of mass.
base programmes	The fusion programmes of the BA participants, in particular the physics programmes.
baseline	The set of documents making up, or appropriate to, the current design.
beam emission spectroscopy	BES, plasma diagnostics using light emission from a low energy neutral beam .
beamlines	The flight tube connecting the neutral beam box to the plasma torus. Sometimes, colloquially, the complete beam system including the beam box.
beta limit	Maximum plasma pressure that can be contained stably by the magnetic pressure generated by the confinement field.
Bohm diffusion	A rapid loss of plasma across magnetic field lines caused by microinstabilities. Theory formulated by the physicist David Bohm.
bolometer	An instrument that measures the energy of electromagnetic radiation in certain wavelength regions by utilizing the change in resistance of a thin conductor caused by the heating effect of the radiation. Also known as thermal detector.
bootstrap current	Plasma current induced by trapped particles in a tokamak /stellarator.
boronization	A technique for wall conditioning using boron to reduce the effective impurity in a tokamak .
breakeven	The point at which the fusion power produced by a reacting plasma equals the input power needed to sustain the plasma’s high temperature. To produce net power in a fusion reactor, according to the “Lawson criterion,” in a plasma of about 70 million degrees centigrade with a density of 1014 particles per cubic centimeter, the plasma must be contained for at least one second.
bremsstrahlung	Radiation emitted as a result of deflection (e.g. through near collisions) of rapidly moving charged particles.
bumper limiters	In tokamaks, any structures inside the vacuum vessel which protrude into the plasma space to intercept plasma which would otherwise strike other plasma facing components.
burn	The period of roughly constant and maximum fusion power during the plasma pulse.
burning plasma	A plasma significantly heated by alpha particles from internal fusion reactions. Significantly this usually means the alpha particle power is as large as the externally applied power.
business confidential information	Information containing know-how, trade secrets, or technical, commercial or financial information, which: 1) has been held in confidence by its owner; 2) is not generally known or available from other sources; 3) has not been made available by its owner to other parties without an obligation concerning its confidentiality, and 4) is not available to the receiving party without obligations concerning its confidentiality.

Term	Definition
CAD	computer aided design
CATIA	Computer-graphics Aided Three-dimensional Interactive Application, CATIA is a three-dimensional CAD modeling software with all the appropriate interactive features to aid the designer in rapid production of self-consistent engineering models and drawings.
CB	cold box
CD	current drive
central electron density	The plasma density on the toroidal axis in a tokamak.
central solenoid	The tokamak component that produces and sustains the plasma current which heats and shapes the plasma.
CERN	Centre Européen de Recherche Nucléaire (European Organization for Nuclear Research)
CfC, CFC	Carbon fibre composite, a material made from carbon in which carbon fibres are placed, sometimes with preferred orientations, to give added strength.
charge-exchange	A process in which there is a transfer of charge between two bodies during a collision between them (e.g. the collisional transfer of an electron from a neutral atom to a singly charged positive ion, the latter becoming neutral and the former charged).
CHF	critical heat flux
collective instabilities	Usually refers to Toroidal Alfven Eigenmode (TAE) instabilities and fishbone instabilities. TAEs can eject the alpha particles before they have slowed down and given their energy to the plasma, damaging the surrounding walls. Fishbone instabilities are caused by interaction between a major MHD instability and trapped particles, causing these trapped particles to be lost.
commissioning	The process during which plant or reactor components and systems, after construction, are made operational and verified to be in accordance with design assumptions and to have met the performance criteria.
common-cause failure	The failure of a number of devices or components to perform their functions as a result of a single specific event or cause.
compliant material	Any material which has a flexible or flowing characteristic which enables it to maintain contact (and conductivity) between two surfaces which form a gap.
confinement	Restriction of a hot plasma to a given volume as long as possible by magnets and pinch effects.
confinement enhancement factor	The factor by which the plasma energy confinement time, calculated from a particular scaling law, would need to be enhanced in order to achieve a particular effect.
confinement time	The amount of time the plasma is contained by magnetic fields before its energy leaks away.
controlled ignition	A sustained equilibrium burn of a plasma with no auxiliary heating, other than for optimizing the plasma behavior. A state where energy release by thermonuclear reactions is sufficient to balance energy losses and is free of spontaneous excursions of thermonuclear power.
controlled thermonuclear fusion	The process in which very light nuclei, heated to a high temperature in a confined region, undergo fusion reactions under controlled conditions, with the associated release of energy which may be harnessed for useful purposes.
cooldown	Usually refers to the cooling period necessary to remove heat from a large superconducting magnet system to lower the temperature to the operating point.
“Cordey” pass	The point in operating density and temperature which minimizes the auxiliary power needed to heat a plasma to controlled ignition.
core confinement	Energy confinement related particularly to the plasma core (i.e the region within the separatrix).
correction coils	Coils whose purpose is to compensate small errors in the confining magnetic field arising from fabricating misalignments.
CP	contact person
critical heat flux	(CHF), the heat flux at which there is a departure from nuclear boiling and steam bubbles coalesce to form an insulating layer which leads to very rapid heating and large loss of heat transfer.
cross section	Measure the probability of interaction between an atomic nucleau, atom, or molecule and an incident particle, have the dimensions of area, and the appropriate unit for expressing them is the square meter.
cryogenic	Term applied to very low-temperature substances and apparatus.
cryogenic technologies	Technologies used to liquify helium and nitrogen (e.g. gas expansion) providing a low temperature coolant for magnets, vacuum pumping panels, etc., at around 4K and 80K respectively.
cryoplant	The plant used to liquify helium and nitrogen to cool magnets, vacuum pumping panels, etc.
cryopump	A vacuum pump system using panels cooled by liquid helium.
cryostat	A vacuum vessel capable of being evacuated at room temperature built around a superconducting tokamak, which provides thermal insulation to maintain the magnets at low temperature.
CS	See “central solenoid ”
CTB	coil terminal box
CTE	coefficient of thermal expansion, the fractional increase in length of a body per degree temperature rise
current drive	A means for producing the toroidal plasma current.
current drive	By pointing neutral particle beams roughly tangentially to the plasma major circumference, or by oscillating the microwave heating system antennae in a particular phase relationship with each other, toroidal plasma current can be driven non-inductively. These schemes can also enhance the ability of the plasma to drive its own toroidal current (the “bootstrap” effect). All these schemes weaken the need for inductively driven plasma current, and stretch the burn time, possibly to steady state.
CVD-W	Chemical Vapour Deposition of tungsten onto a surface.
CW	cold worked (i.e. material property); continuous wave (i.e. radiofrequency wave)
CX	charge exchange, the method by which energetic particles in the scrape-off layer around the plasma core transfer energy to neutral particles in front of the divertor plate, thereby protecting the plates and spreading the heat loads.

Term	Definition
D-alpha light	(D-alpha), radiation as decaying from the n=3 –> n=2 transition in deuterium atoms (near 6500 Å).
D-shaped plasma	A toroidal plasma whose cross-section is in the form of a “D”.
DC	direct current
DD reactions	Reactions between nuclei of deuterium, to release energy by nuclear fusion, and in which helium nuclei and neutrons, or tritium and protons, are produced with roughly equal probability.
decay heat	The heat produced from the radioactive decay of fission or neutron activation products.
decay time	The time taken for the activity of a material to decay to the point at which the material can again be treated as if it is not activated.
decommissioning	The process by which the facility is permanently taken out of operation at the end of the plant lifecycle with adequate regard for the health and safety of workers and the public and protection of the environment.
decontamination	The removal or reduction of radioactive contamination by a physical and / or chemical process.
DEMO	Demonstration fusion reactor. The next experimental device to follow ITER, and predecessor of a prototypical commercial-sized fusion reactor. DEMO would generate electricity at the level of a few hundred MW and utilise all technologies necessary for a commercial device. It does not have to produce electricity economically, but it does have to demonstrate convincingly that with some scaleup it could be economical. Reliable operation, after a shakedown period, is essential.
design value	A numerical value for which the plant or a component is designed. Incorporates all margins (e.g. design margins, safety factors, tolerances) which might be required to cope with uncertainties.
designated institution	The organisation selected by a voluntary contributor to design and procure equipment from a manufacturer and pay for it from their budget.
deuterium	An isotope of hydrogen with one neutron and one proton in the nucleus.
deuterium-tritium plasma	(D-T plasma), a plasma consisting of a mixture of deuterium and tritium.
deviation	A departure from normal operating states. A departure from the design during manufacturing.
diagnostics	Equipment for determining (diagnosing) the properties and behaviour of a plasma during an experiment.
discharge (or release)	The release of radioactive or toxic substances (effluents) to the environment.
dispersion strengthened	(DS), a material which has enhanced strength by means of small granules of another material uniformly dispersed within it.
disruption	A gross instability which gives rise to an abrupt temperature drop and the termination of the plasma. See “plasma disruption”.
divertor	The system for power and particle exhaust in a tokamak in which the plasma at the edge is “diverted” to a region remote from the main plasma by suitably modifying the magnetic field at the plasma edge.
divertor channel	The region of the divertor into which field lines in the plasma scrape-off layer are conducted.
divertor configuration	The arrangement and material composition of of plasma facing components and pumping slots on the divertor cassette.
divertor physics	Plasma physics associated with toroidally confined plasmas with a null point in the confinement region designed to remove impurities from the interior of the plasma column.
double-null	A tokamak plasma with two null points . See also “divertor physics”.
double-null equilibrium	A tokamak plasma configuration in which the field lines in the scrape-off layer intersect the plasma chamber walls or divertor at the top and bottom of the chamber. The poloidal field therefore has a field null at the top and bottom of the plasma core, and both nulls are within the plasma chamber. A single null configuration has only one null within the plasma chamber.
down-time	The time for which the device cannot be operated, for various reasons.
driven current	Current produced in a plasma to extend or maintain the pulse length of the inductive material.
DT reactions	Reactions between the nuclei of two isotopes of hydrogen, deuterium and tritium, to release energy by nuclear fusion, and in which helium nuclei and neutrons are produced.
dump resistors	Large current-carrying resistors which dissipate large amounts of electrical energy over a short period of time by ohmic heating.
duty factor	The ratio of working time to total time for an intermittently operating device.
dwell time	The time in each operational pulse which is not burn time.

Term	Definition
earthing	See “grounding”.
EB	electron beam, a means of welding or cutting materials.
ECCD	Electron cyclotron current drive, see ECRH. Momentum can also be transferred to the plasma and if directed properly it can contribute to driving plasma current.
ECH	electron cyclotron heating, see “ECRH”
ECRH	Electron cyclotron resonance heating, the application of external power to the plasma at one of the frequencies at which waves propagate in the plasma. The EC resonance is the frequency with which electrons orbit the field lines, so enhancing that oscillation adds energy to the plasma ions by subsequent collisions.
edge fluctuations	Fluctuations in either density or electric field occurring at the boundary of a plasma.
edge localized mode	(ELM), the rapid transition between H-mode and L-mode; fluctuation at the plasma boundary associated with magnetohydrodynamic instabilities.
edge physics	Plasma physics devoted to studying the boundary of a particular plasma.
EF	equilibrium field
electro-mechanical load	Totality of all electromagnetic and mechanical loads.
electromagnetic loads	Movement of the current-carrying plasma induces currents flowing in continuous conducting paths in the surrounding structure. These induced currents create mechanical loads on the structures themselves and their joints. The loads can be especially large during rapid plasma current change, such as startup and ramp-down of plasma current, or in the various cases of disruption or vertical displacements of the plasma.
electron beam	A stream of electrons moving with the same velocity and direction in neighbouring paths and usually emitted from a single source such as a cathode.
electron cyclotron current drive	(ECCD), a technique to drive current noninductively by using radio-frequency waves at the electron cyclotron frequency.
electron cyclotron frequency	The frequency (usually microwave) of radio waves which have the same frequency as electrons orbiting magnetic field lines, such as those in a tokamak.
electron cyclotron resonance heating	(ECRH), a mode of heating a plasma by resonant absorption of energy based on the waves induced in the plasma at the cyclotron frequency of electrons.
ELM	edge localized mode, low amplitude oscillation and energy loss at the plasma edge caused by MHD instability. Various types can occur, some causing more energy loss than others.
ELMy H-mode	A period of improved energy confinement in the plasma accompanied by relatively low level instabilities localised at the plasma edge which help to maintain thermal equilibrium in the plasma. See also H-mode.
empirical scaling formulas	Formulas relating key parameters related to change in plant characteristics which are derived primarily from observation of the plant rather than from a deep physical understanding or theoretical model of the underlying driving processes.
energy confinement time	The ratio of instantaneous plasma energy content to the net power flow into the plasma required to maintain that energy content.
energy confinement time scaling law	A usually empirical scaling formula, based on major machine parameters , which predicts the energy confinement time for, other experiments and conditions.
energy transport	The thermal conductivity and convection rates in a plasma.
enhanced particle modes	EPMs, mhd activity in which certain particles species are accelerated.
EOB	end of burn
equatorial plane	The horizontal level determined by one half of the height of the toroidal field coils.
equatorial port extension	Equatorial ports of the main vacuum vessel each consist of a hole in the vessel itself, a “port stub” which is welded to the rim of the hole and a straight section, the “port extension”, welded to the port stub.
equilibration times	The time it takes for electrons and ions to come into thermal equilibrium.
event sequences	Accident states defined by a postulated initiating event, its further pathways of propagation, and the corresponding consequences.
external heating sources	Devices which provide auxillary heat to the plasma to raise it to burn temperature. See “auxiliary heating”.

Term	Definition
fail-safe	The behaviour of a component or system, following a failure (either internal or external). If a given failure leads directly to a safe condition, the component or system is termed fail-safe with respect to that failure.
failure modes and effects analysis	A qualitative systems analysis technique in which the different failures of components are identified and the consequences of those failures on the system or plant as a whole are investigated.
FEM	finite element model
FEM stress contours	Finite Element Model, stress contours are a graphical way to represent the value of the stress resulting from the analysis of a component by the Finite Element Model. The stress contours are lines that cover the surface (or a section) of the model, with each line representing a “constant stress” line.
ferromagnetic inserts	Plates of ferromagnetic steel included within the vacuum vessel in the shadow of the TF coils to counteract TF ripple.
FFMEA	functional failure modes and effects analysis
first mirrors	Components (usually of plasma diagnostics or heating systems) which reflect visible light and which are the first material surface seen by plasma radiation.
first wall	FW, the interior surface of a tokamak closest to the plasma.
flat-top pulse length	The duration a plasma is in quasi-steady state.
FMEA	failure modes and effects analysis
fuel pellets	Small slugs of frozen fuel in the 3-6 mm diameter range fired frequently (between 50 and 7 Hz respectively) into the plasma to maintain sufficient fuel density in the plasma core.
Functional Analysis	A method for identifying functional requirements and analyzing performance requirements and dividing them into discrete tasks or activities. It involves the identification and decomposition of the primary system functions into sub-functions at ever increasing levels of detail.
functional FMEA	FFMEA, functional failure modes and effects analysis, an extension of the FMEA method applied to functions rather than systems. It starts from a list of plant functions, identifies the ways these functions could fail and then investigates the consequences of such failures on other functions and eventually on plant safety or reliability.
functional isolation	Prevention of influences from the mode of operation or failure of one circuit or system on another.
fusion	The merging of two light atomic nuclei into a heavier nucleus, with a resultant loss in the combined mass. Fusion is accompanied in general by the release of energy. See also: “controlled thermonuclear fusion”.
fusion performance	Usually means the level of power amplification, Q, or the energy confinement time.
fusion power	The power output from fusion reactions, i.e. in neutrons and alpha-particles.
fusion reactor	Usually, a device harnessing fusion power to create electrical power.
FW	first wall

Term	Definition
gas puffing	By releasing puffs of fuel or impurity gas from valves into the plasma chamber it is possible to fuel the outer regions of the plasma. Fuel pellets are used to fuel deeper into the plasma.
glass fibre	Glass can be made in the form of fibres whic can be used to strengthen materials, make strong materials, or provide insulating properties. The material thus constructed is known as fibre-glass. Glass fibre can also be used to communicate using light due to the internal reflection of a beam of light passing lengthways along the fibre. Such fibres can be many kilometers in length.
glass-kapton	An especially strong combination of glass fibres and kapton, a polyimide electrically insulating high mechanical strength film.
graphical user interface	Computer interface utilising icons representing files or folders (directories) of files, drop-down or pull-down menues of commands, and requiring the user to navigate through these primarily using a pointer controlled by a mouse.
grounding	A large conducting body, such as the earth, used as a return for electric currents and as an arbitrary zero of potential.
gyro-Bohm like	A thermal diffusion coefficient which is smaller than Bohm diffusion.
Gyrotron	A type of radiofrequency power transmission tube (valve) used to produce electromagnetic waves in the GHz range for plasma heating in the range of the plasma electron cyclotron resonance (i.e the frequency at which electrons orbit the magnetic field lines)

Term	Definition
H&CD	heating and current drive
H-alpha studies	A diagnostic technique to study the plasma boundary by observing the alpha line (6563 A wavelength) of the hydrogen emission spectrum.
H-mode	High mode, a regime of operation attained during auxiliary heating of divertor tokamak plasmas when the injected power is sufficiently high. A sudden improvement in particle confinement time leads to increased density and temperature, distinguishing this mode from the normal L-mode, “low mode” .
halo currents	When the plasma column is displaced vertically a current is induced flowing from the plasma through the vacuum vessel (shortest way) and thence back into the plasma. It thus contributes to delaying the plasma vertical movement (Faraday’s law).
heat flux	The total flow of heat in heat exchange
heat sink	An element of a composite material or structure which acts as the main attractor of heat in the system. Thus in a cooled body the coolant is the ultimate heat sink, though often a solid material included for its good thermal conduction to the ultimate heat sink is often known as the heat sink and this is the role it plays among the solid elements.
heat sink lifetime	The time the underlying substrate (the “heat sink”) of a plasma-facing component can be reliably predicted to fulfil its functional purpose (and therefore not need replacement).
heavy-ion beam probe	A plasma diagnostic which probes the plasma with a beam of heavy ions.
helical twist	A spiral twist of constant form and usually of constant radius. Thus the magnetic field lines in the tokamak have helical twists around the plasma, forming magnetic surfaces on which the twist pitch is constant.
helicity safety factor	Also known as the plasma safety factor, this is aproximately the ratio rBt/RBp (where r, R are minor , major radii of the torus, and Bp, Bt are poloidal and toroidal field strengths respectively). The safety factor is 2 x the number of times a field line circles the major axis of the plasma torus before going once round the minor axis of the plasma torus
HFS	high field side
high-heat flux	Power conducted or radiated from the plasma with loads exceeding 0.5 MW per square meter.
HT	Home Team
HTS	high temperature superconductor
HVAC	heating, ventilation and air conditioning

Term	Definition
IGBT	insulated gate bipolar transistor
ignition	See “controlled ignition”.
IM	initial magnetisation
implementing agency	The organisations charged by Euratom and Japan to implement the Broader Approach Agreement. These are F4E (European Joint Undertaking for ITER and the Development of Fusion Energy, also known as Fusion for Energy) and QST (National Institutes for Quantum and Radiological Science and Technology).
impurities	Atoms of unwanted elements in the plasma usually originating from the surrounding walls.
in-vessel components	The in-vessel components comprise the blanket, the divertor, the fuelling and internal pumping systems, the port plugs, and diagnostic sensors mounted directly on the vessel.
inboard	A region located further toward the tokamak vertical centreline than the object referred to.
inboard plasma shape	The shape of the plasma inboard (i.e towards the machine centre) of the major circumference. This is strongly affected by the nearest poloidal field coils, usually the central solenoid.
inductively driven plasmas	The basic means of driving toroidal current in the tokamak plasma uses the fact that most field lines created by the central solenoid pass down its bore and do not return on themselves until they pass outboard of (i.e radially beyond) the plasma. This “inductive linkage” between the solenoid and plasma allows a change in current in the solenoid to drive current in the plasma (Maxwell’s Laws).
inherent safety characteristic	Achieving safety by eliminating a specified hazard by means of the choice of material or design concept.
inspection	Examination, observation or measurement to determine the conformance of materials, parts, components, systems, structures, as well as processes and procedures, with defined requirements.
instability	A plasma state in which any small perturbation is amplified to a considerable change in the equilibrium of the system, leading to disruptions.
internal mode	Magnetohydrodynamic instabilities which occur at the center of a tokamak plasma which cannot be controlled or suppressed by external means.
internal transport barrier	ITB, a magnetic field configuration which slows down energy conduction transverse to it. In the establishment of the H-mode, an ITB is established just inside the separatrix, helping to hold heat inside the plasma. Other ITBs have been found also inside the plasma core, although the conditions for their creation and their usefullness is not yet fully understood.
ion	An atom which has become charged as a result of gaining or losing one or more orbiting electrons. A completely ionized atom is one stripped of all its electrons.
ion-Berstein wave	A plasma wave named after the discoverer, I.B. Bernstein.
ionization	The removing or adding of an electron to a neutral atom, thereby creating an ion.
IR	infra-red
ISO	International Organization for Standardization
isotope	One of several versions of the same element, possessing different numbers of neutrons but the same number of protons in their nuclei.
isotopic content	The mix of given isotopes in a mixture of isotopes, for instance the amount of tritium and other hydrogenn isotopes in a waste gas stream.
isotopic mass	The mass of each atom of a material in atomic mass units (see above). Thus the plasma isotopic mass for fusion is 2.5, since the fuel is basically a 50/50 mix of deuterium (2 amu) and tritium (3 amu).
IT	information technology

Term	Definition
JA	Japan
JAEA	Japan Atomic Energy Agency
JT 60U	Japan Tokamak – 60 Upgrade, more commonly referred to as JT60 Upgrade
JT-60SA	Japan Tokamak 60 Super Advanced

Term	Definition
kapton	A polyimide electrically insulating high mechanical strength film.
kink mode	A magnetohydrodynamic instability causing a plasma column to deform into a “kink”.

Term	Definition
L-H threshold power	During plasma heating, in order to overcome conduction losses, a certain amount of external heating must be applied to establish the H-mode internal transport barrier which places the plasma core in a mode of improved energy confinement. This heating power is referrred to as the L-H threshold power.
L-mode	Low mode, 1) The normal behavior of a plasma undergoing ohmic heating; that is, as the plasma’s temperature climbs higher, the confinement of the plasma deteriorates. 2) A confinement regime in a tokamak in which the confinement time is low, named after discovery of the H-mode (high mode) of operation.
Landau Damping	Damping of a plasma oscillation wave which occurs in situations where the particles of the plasma are able to increase their average velocity at the expense of the wave, and thus to damp it out, even in cases where the dissipative effects of collisions are unimportant.
Langmuir probe	A probe used for measuring the plasma temperature (named after I. Langmuir who first employed it and developed the theory for interpretation of the measurements).
laser	An acronym for light amplification by stimulated emission of radiation. An optical device that amplifies and concentrates light waves, emitting them in a narrow, intense beam.
laser scattering device	Formally known as a “Thomson scattering device”; a diagnostic device used to measure electron temperature in a plasma by directing laser light into the plasma. The laser’s photons scatter off the electrons, spreading in a manner proportional to the electron temperature.
Lawson criterion	The minimum physical conditions of plasma temperature (T), plasma density (n), and confinement time (t) needed for the production of net power in fusion. It is expressed as nt>f(T), where f(T) has a minimum value of around 2 X 1020 sec/m3 for the deuterium-tritium reaction.
LHe	liquid helium
limiter	(Nuc. Eng.). An aperture which defines the boundary of a plasma and protects the vacuum vessel from damage by contact with the hot plasma.
limiter system	Mechanical structures used to define the plasma boundary.
lip seal	A metallic flange or “lip” extending around the perimeter of a component which must be hermetically sealed to another component. Sealing of the “lips” is accomplished by welding, and unsealing by grinding or cutting the lip seal weld.
LN	liquid nitrogen (cryogenic), low nitrogen (material)
long-lead time	A date prior to the estimated physical construction start to ensure availability at the time needed so as to not delay the construction performance.
lost alpha detectors	Detectors used for measuring unconfined alpha particles.
low field regions	Regions of lower (magnetic) field suitable for siting conductor joints, for instance, as the forces experienced during the plasma cycle will be minimised there. These are usually regions furthest away from the bore of the coil.
low-Z material	In tokamaks, any material with a low atomic number, such as beryllium, carbon, boron, etc.
LTM	long term maintenance

Term	Definition
magnetic bottle	1) The containment of a plasma during thermonuclear experiments by applying a specific pattern of magnetic fields. 2) The magnetic field used to confine a plasma in controlled fusion experiments.
magnetic fusion	The use of magnetic fields to confine a plasma that is undergoing fusion.
magnetic quench detection	A detection system to protect a superconducting magnet from going to its “normal state”.
magnetic shear	The change in direction of the magnetic field from one nested magnetic surface to another.
magnetohydrodynamic	MHD, the study of the motions of an electrically conducting fluid in the presence of a magnetic field. The motion of the fluid gives rise to induced electric currents which interact with the magnetic field which in turn modifies the motion. The phenomenon has applications to magnetic fields in space and and to the possibility of generating electricity.
magnetohydrodynamic instabilities	See “kink mode”, “pinch effect”, and “sawteeth”.
maintainability	1) A system effectiveness concept, measures the ease and rapidity that a system or equipment can be restored to operational status after failing. 2) The probability that an item of hardware or software will be restored to a specific condition within a given period of time.
major plasma radius	The centre of the last closed flux surface at the height of the maximum width of the plasma. The major radius of the torus.
MARFE	multifaceted asymmetric radiation from the edge of tokamak plasmas
MDAS	mass data acquisition system
MDSS	mass data storage system
mechanical structure	The structure providing external support for the tokamak magnets against electromagnetic and mechanical loads.
metadata	Data defining where a data object resides and defining other attributes about the data object (e.g., date of creation or design status).
MHD	magnetohydrodynamic
mhd instability	Instabilities driven by the energy of the magnetic field and by the momentum of charged particles in the plasma. Small displacements of plasma density from the equilibrium position can reinforce that displacement until a further compensating force counteracts that reinforcement or the plasma equilibrium configuration is disrupted. Various modes of such displacement in the toroidal and poloidal planes can be stabilised so that the bulk plasma remains in position.
microwave scattering and reflectometer	A diagnostic system used in tokamaks for fluctuation measurements.
MIG	metal in gap
minor plasma radius	The small radius of the plasma torus.
Mirnov loops	Conducting wire loops used in tokamaks to measure magnetohydrodynamic instabilities.
mockup	A piece of hardware reproducing some or all of the features of an actual component to be used in ITER.
model coil	Coils used for testing manufacturing processes, using a design exhibiting key features of the ITER magnet coils.
MG	motor-generator
monoblock	A type of construction of plasma-facing components in which blocks of plasma-facing material are threaded onto coolant pipes through a central hole. A variant is the saddle block, in which the hole intersects the block edge on the side away from the plasma.
monster sawteeth	Very large internal magnetohydrodynamic oscillations with sawteeth-like amplitude variations occurring at the centre of a tokamak plasma.
motional Stark effect	MSE, a diagnostic system used for measuring the safety factor on axis in a tokamak.
MOU	matching optics unit
MSE	motional stark effect
MT	Magnet Technology Conference
MTBF	mean time between failures
MTL	main transmission line
MTTR	mean time to repair

Term	Definition
n	plasma density
NBI	neutral beam injection/injector
NbTi	Flexible superconductor made of niobium titanium compound suitable for use up to 10T with helium coolant at 4.5 K.
ND(T)	non-destructive (testing)
negative ion sources	Sources of negative deuterium ions used in Neutral beam Injection. Hence N-NBI, P-NBI.
neoclassical tearing modes	NTMs, an MHD stability caused by the formation of magnetic islands (loops of magnetic flux) near integral or rational q (safety factor) surfaces.
neoclassical transport	(Nuc. Eng.) The departure from classical transport in a tokamak due to toroidal effects as predicted by theory.
neutral beam current drive	NBCD, noninductive current drive using high energy neutral beam injection.
Neutral Beam Injection	Neutral beam injectors on ITER use negative deuterium ions from an arc discharge between tungsten cathodes and the source body anode. These are accelerated into a gas neutraliser. Neutral particles continue on into the plasma while charged ions are deflected, collected and recycled.
neutral beams	A beam of high-energy atoms used to heat a plasma. As the atoms are neutral, the beam is not affected by magnetic fields.
neutral particle beam	NPB, a concentrated, nearly unidirectional flow of neutral particles.
non-circular cross section	A plasma configuration which has a higher vertical height than horizontal width.
non-inductive current drive	Methods of driving a current in a tokamak other than with a transformer.
normalised beta	The ratio of plasma kinetic pressure to magnetic pressure, multiplied by the product of minor plasma radius (m) and toroidal magnetic field (T) at the plasma centre, and divided by the plasma current (MA).
null point	zero magnetic field
numerical codes	Computer codes ( to distinguish them from codes of practice, such as structural design codes).

Term	Definition
ohmic heating	OH, heating resulting from the resistance a medium offers to the flow of electric current. In a plasma subjected to ohmic heating, ions are heated almost entirely by the transfer of energy from the hotter electrons.
ohmic heating system	OH system, resistive heating in a tokamak by use of an induction coil.
operating margins	Each parameter describing an operating system has a nominal value and a limiting one at which operation would cease or otherwise be deficient. The differences between the nominal parameter values prevailing and the limiting ones are referred to as “operating margins”
operating parameter	An algebraic magnitude characterizing the plant or a component operating state,e.g., temperature (t), fusion power (P), electrical current (I) and magnetic field (B).
operating point	A point in n-dimensional space, where each dimension measures the value of a key operating parameter, identified with a particular type of operation.
operating space	The set of valid operating points in n-dimensional operating parameter space.
operating state	When the plant or component is in operation. The plant is in operating state during the entire pulse length.
operating time	The sum of pulse lengths achieved within a certain calendar time at normal and abnormal operation.
operation availability	See “availability”
operational domain	see “operating space”
operational limits and conditions	A set of rules establishing parameter limits, the functional capability and the performance level of its equipment and personnel for safe operation.
operational reliability	The probability that a system will perform as specified when used in the manner and for the purpose intended.
outboard	A region located further away from the tokamak vertical centreline than the object referred to.
overturning moment	Toroidal field coil moment (in a tokamak) about the device radius from out of plane forces arising from vertical fields and from the plasma field.

Term	Definition
pancake winding	A method of winding a conductor for a magnet (c.f. layer winding). A conductor is wound in a circumferential direction rather than along the axis of a magnet, and forms a unit module like a pancake. ‘Pancakes’ are wound separately, stacked together, and then jointed electrically (pancake-to-pancake joint) in a series.
particle exhaust	Various methods of exhausting particles from a plasma, such as with a divertor or pumped limiter.
passive component	A component which does not need any external input to operate.
PBS	plant breakdown structure
pedestal physics	The transport barrier that occurs just inside the magnetic separatrix in H-mode provides a thin layer where the pressure increases sharply. At the top of this profile “pedestal” density and temperature values serve as boundary conditions for the core profiles. This may be very important if the core temperature gradient turns out to be constrained. The energy content of the pedestal is generally about one third of the total core energy content, and scales differently from the core global scaling. Pedestal physics is a study of this region.
pellet injector	A device that shoots small frozen quantities of hydrogen isotopes at high speed into the inner regions of a hot plasma. This method has some penetration advantages over conventional gas injection.
PF	poloidal field
PFC	plasma-facing component
physical separation	Separation by geometry (distance, orientation, etc.), by appropriate barriers, or a combination thereof.
PID	plant intergration document
PL	Project Leader
plant subsytem	Each component forms part of some functional subsystem of the “plant”. There are about 20 major subsystems, covering every part of the tokamak, the buildings, and all peripheral plant out to the point of interaction with the environment around the plant.
plasma	The fourth state of matter. A collection of electrically charged electrons and ions that is approximately “charge neutral” macroscopically.
plasma confinement phenomena	Phenomena which affect plasma core energy confinement. The energy confinement time in plasma has been characterised empirically by various machine parameters (e.g. size and current). Explaining why confinement scales in this way requires an understanding of how the different physical processes affecting confinement might interact. This involves a study and modelling of the possible phenomena, and then comparison with experimental results.
plasma current	The electrical current going the long way around the torus.
plasma disruption	A rapid deposition of plasma energy resulting from the loss of plasma confinement to part of the plasma-facing structure as a result of instabilities.
plasma elongation	The ratio of the height of the plasma cross section to its width.
plasma facing components	Tokamak components which directly interact with the plasma, and are subject to high heat fluxes. Typical plasma facing components include the first wall, limiter and divertor.
plasma processes	Plasma confinement, heating, and energy and particle loss, in the central plasma core, across transport barriers, and in the plasma edge region.
plasma profiles	The variation in amplitude of plasma parameters (most often density and temperature of the electrons or ions) across the minor radius of the plasma.
plasma purity	The degree to which the plasma is contaminated by items other than the fuel species (deuterium and tritium). The plasma may be contaminated by impurities coming fromthe wall, deliberately or accidentally fed into the bulk plasma, or as a result of the fusion reactions themselves (e.g.helium). Measured by the parameter Zeff.
plasma shutdown	The orderly process of extinguishing the plasma at the end of a plasma burn pulse, involving reduction of the plasma thermal energy, and reduction of plasma current to zero.
plasma temperature	Temperature expressed in degrees K (thermodynamic temperature) or electron volts (kinetic temperature). A measure of the random kinetic energy (energy of motion) of the ions or electrons present.
plasma volume	The volume encompassed by a given plasma in a tokamak or stellarator.
PM	Project Manager
PMI	plasma-material interaction
PMY	professional-man-year, see “PPY”
P-NBI	positive ion neutral beam injector
poloidal direction	Movement in the vertical plane intersecting the plasma torus along projections in that plane of any of the tokamak’s nested toroidal flux surfaces.
poloidal field	PF, the magnetic field generated by an electric current flowing in a ring.
poloidal field coils	Components of a tokamak that assist in stabilizing the plasma.
port axis direction	The radial symmetry axis of the port extension.
port plugs	The vacuum vessel contains a large number of ports. During operation these are filled with specialised equipment for plasma diagnostics, heating, vacuum pumping, limiters, etc.. Each item of this equipment is housed in a specialised port plug which provides a consistent interface with the adjacent vessel and blanket, adequate shielding, and links for port plug services to external supplies.
POS	plasma operation state
position/reaction force detection sensors	A system of mechanical or piezoelectric driven strain gauges.
postulated initiating events	Identified events that lead to anticipated operational occurrences or accident conditions and their consequential effects.
power tubes	Radio-frequency heating systems produce electromagnetic waves of the required power and frequency using high power valves (“tubes”). e.g., the gyrotron, used for ECRH. This consists of an electron gun, an acceleration chamber, a resonance chamber immersed in a strong magnetic field produced by superconducting magnets, and finally a collector to accommodate any energy not transmitted. An electron beam is accelerated into the strong magnetic field. The electrons gyrate at high speed round the field lines and emit an electromagnetic wave with the gyro-frequency. The wave is strongly amplified in the resonance chamber and then transmittted out of the cavity to the target.
PPM	professional-person-month
PPY	professional-person-year
prescribed limits	Limits established or accepted by the Regulatory Body.
primary coolant	The fluid circulated through the tokamak to remove heat.
PS	power supply
pulse length	The interval of time when fusion reactions are taking place in the plasma.

Term	Definition
q-value	A magnetohydrodynamic safety factor describing the number of times field lines need to pass round the major circumference of the plasma before they close around the minor circumference; q=(R/a)(BT/BP)
QA	quality assurance
QC	quality control
QM	quality management
QST	National Institutes for Quantum and Radiological Science and Technology
quality assurance	QA, all those planned and systematic actions necessary to provide assurance when a facility, structure, system or component will perform satisfactorily and safely in service. The prevention of defects.
quality control	QC, all actions necessary to verify conformance to requirements, and to control nonconformities or deficiencies. Quality Control is an inspection function, the detection and control of deficiencies.
quench	See quench time. Also, the procedure by which a superconducting magnet starts behaving as a normal conductor with resistive heating, usually due to a source of local heating or poor coolant flow. The magnet quench is handled by a discharge system to remove the magnetic energy as fast as is prudent.
quench time	Usually, the time needed to extinguish the plasma current.

Term	Definition
R	major plasma radius
R&D	research and development
radiated power	Power radiated from the plasma by various mechanisms – line radiation (due to emission of quanta as energetic electrons decay to lower states in partly ionised atoms near the plasma edge), bremmstrahlung (“braking radiation” due to acceleration/decelleration of ionised particles as they come near to collision) and synchrotron radiation (due to particles orbiting the magnetic field lines).
ramp-down time	The time necessary to terminate the plasma and quench the burn, involving a phase of decrease (ramp-down) of plasma temperature, followed by a decrease (ramp-down) of plasma current.
ramp-up rates	Rate of change of plasma current or temperature during their respective ramp-up times.
ramp-up time	The time necessary to initiate the plasma and heat it up to burn temperature, involving a phase of increase (ramp-up) of plasma current, followed by an increase (ramp-up) of plasma temperature.
rampdown rates	Rate of change of plasma current or temperature during their respective ramp-down times.
realtime observations	Continuous display of the value of a plasma parameter at the current moment. In contrast to observations of plasma behaviour and parameters which are determined some time after their actual occurrrence, e.g by post-processing of diagnostic data gathered during a plasma pulse.
relative resistance ratio	(Elec.) RRR, the ratio of the resistance of a piece of a material to the resistance of a piece of specified material, such as annealed copper, having the same dimensions and temperature.
remote handling	Handling of tools or components by machines with the controls at a remote location.
remote maintenance	Maintenance and modification of the radioactive elements and components of the tokamak using machines and tools controlled remotely to avoid human exposure to radioactivity.
residual heat	Heat which is produced after the reactions have been terminated. See also “decay heat”.
resistive wall mode	The type of discharges proposed for steady state operation are not stable without a perfectly conducting wall at the plasme edge. In ITER and JT-60SA, with the resistive wall some distance from the edge, the result is that RWMs, a type of MHD instability, occur and grow on the timescale of the magnetic field penetration through the wall. The external control coils are therefore used to control these modes.
retrofit	To furnish or provide with new equipment or parts unavailable at the time of original manufacture or construction.
reversed shear	A variation in magnetic shear which changes sign as one moves across the plasma.
RF	radio frequency
RH	remote handling
RO	(Technical) Responsible Officer
“rolled up”	(Idiom.) The act of summing detailed data into more general summary data.
RRR	relative resistance ratio
runaway electrons	Those electrons in a plasma that gain energy from an applied electrical field at a faster rate than they lose it through collision with other particles. These electrons tend to “run away” in energy from the remainder of the plasma.

Term	Definition
saddleblock	see “monoblock”
safety actuation system	The collection of equipment required to accomplish the necessary safety actions when initiated by the protection system.
safety factor	1) (Elec.) The amount of load, above the normal operating rating, that a device can handle without failure. 2) An overload factor in design to ensure safe operation.
safety systems	Systems important to safety, provided to assure the safe shutdown of the plant or to limit the consequences of anticipated operational occurrences or accident conditions.
sawtooth, sawteeth	Magnetohydrodynamic instabilities that create oscillations in a plasma, which result in plasma disruptions.
sawtooth stabilization	Various methods of stabilizing sawteeth oscillations (which induce disruptions in the plasma) in tokamaks.
scaling laws	Laws stating that if two quantities are proportional and are known to be valid at certain orders of magnitude, then they can be used to calculate the value of one of the quantities at another order of magnitude.
scheduled down time	The down time of the plant according to plan.
scheduled non-operating state	When the plant or component is down according to plan for the length of the scheduled down time.
scrape-off layer	SOL, the region of plasma between the last closed flux surface and the surrounding walls.
second stability regime	A theoretically predicted regime of a plasma which can sustain very high plasma pressures.
second stable core regime	The second stability regime in the central region of a plasma.
separatrix	The last closed flux surface.
shear	See magnetic shear. Also mechanical shear, due for example to twisting of a body, a stress in the plane parallel to the twisting force.
shut-down time	The time span between the end of burn and the end of the plasma state; part of the operating time.
SI	(Système International), International Standards
single failure	A random failure which results in the loss of capability of a component to perform its intended safety functions. Consequential failures resulting from a single random occurrence are considered to be part of the single failure.
single-null equilibrium	See double-null equilibrium
sintered	Agglomeration in metal (or ceramic particles) by heating and compression.
SN	single null (divertor)
SOB	start of burn
SOF	start-of-current flattop
SOFE	IEEE Symposium on Fusion Engineering
SOFT	Symposium on Fusion Technology
SOL	scrape off layer
spectroscopy system	A plasma diagnostic system based on observing the light emission from the plasma.
SS	stainless steel
stagnation point	(Fl. Mech.) A point in a field of flow about a body where the fluid particles have zero velocity with respect to the body.
steady state operation	The operation of the plasma in a way in which termination of the pulse is not determined by plasma behaviour, but is rather a choice of the operator. Operation which in principle can continue indefinitely.
steady-state	(Phys.) The condition of a body or system in which the conditions at each point do not change with time, that is, after initial transients or fluctuations have disappeared. In dynamic equilibrium, with entropy at its maximum.
superconducting coil	Magnetic coils which use superconductors that have zero resistivity when cooled below the critical temperature.
superconducting magnets	Superconducting magnet coils which circumscribe the torus to confine the plasma within and away from its inner surface.
superconductivity	The flow of electric current without resistance in certain metals and alloys at temperatures near absolute zero.
superconductor	A type of electrical conductor that permits a current to flow with zero resistance.
supercritical	Helium will remain liquid in a bath at 1 atmosphere pressure provided the temperature does not rise above 4.2K. If the JT-60SA coils are placed in such a coolant bath and a high pulse of heat ensues in their operation, most of the helium must be vented to avoid large overpressures. To avoid this, the coils operate with pumped supercritical helium, just above the critical temperature, which retains a large measure of the heat transfer properties of liquid helium, without the risk of overpressure.
superinsulation	(Chem. Eng.) A multilayer insulation for cryogenic systems, composed of many floating radiation shields in an evacuated double-wall annulus, closely spaced but thermally separated by a poor-conducting fibre.
support systems	Systems which provide services to a main plant system or systems. The term is relative. Thus tokamak support systems include heating and cooling, whereas support systems for the heating system include their electrical and cooling systems.
surface contamination	Contamination that is the result of the deposition and attachment of foreign materials to a surface.
synchrotron radiation	Radiation emitted by very fast, charged particles in a magnetic field as a result of their natural gyration in that field. Particle gyrations are at the cyclotron frequency.
system availability	A figure-of-merit useful in quantifying system effectiveness, normally facility up-time divided by planned up-time.

Term	Definition
TBC	to be confirmed
TBD	to be determined
teaching-playback procedure	A procedure usually used with remote handling systems whereby operations are carried out using automatic equipment under manual control, and the actions are recorded to be carried out in future under automatic control after initial setup. Similar to the creation of a macro on a computer.
technical specifications and drawings	Drawings and specifications consistent with the procurement plan and providing the necessary level of detail to support the procurement process.
TF	toroidal field
TF ripple	Because a discrete number of toroidal field coils are used, the toroidal field experienced towards the outboard side of the plasma fluctuates in value at a given radius from the vertical machine centreline, being higher at a given radius in the shadow of each coil than between the coils. This “ripple” in the field traps particles and provides a mechanism for power loss. Thus maintaining a low ripple is a priority, and ferromagnetic material inserts are placed in the shadow of the TF coils, as these weaken the ripple.
thermal energy	Energy due to the relatively high temperature of a medium relative to its surroundings.
thermal loads	Heat loads experienced by a body.
thermal plasma	A plasma in which the vast majority of collisions occur due to random rather than directed impact (e.g. due to a beam of incoming particles). In this case the plasma particles adopt a Maxwellian distribution of velocities and the energy content of the plasma is thus determined by the plasma temperature.
thermo-hydraulic load	The combined load from the pressure of the coolant and the heat source.
thermonuclear fusion	Fusion at high temperature with a significant release of energy.
thixotropy	The propery of certain gels of becoming fluid when agitated and of reverting back to a gel when left to stand.
threshold power	If the power input to a fusion fuel plasma exceeds a certain level then a high enough temperature is reached that heating from fusion reactions plus the external power exceeds the power lost by radiation and conduction, and the plasma temperature continues to rise. If this threshold is not reached then the temperature does not rise despite continued external heating.
TIG welding	Tungsten inert gap welding.
tokamak	(toroidalnaya kamera + magnitnaya katushka), a device for containing a plasma inside a torus chamber by using a combination of two magnetic fields–one created by electric coils around the torus, the other created by intense electric current in the plasma itself.
tokamak building	The main building housing the tokamak and its main peripheral equipment.
toroidal Alfvén eigenmode	TAE, a branch of magnetohydrodynamic instabilities associated with Alfvén waves propagating in the toroidal direction.
toroidal direction	In a ring doughnut, the direction parallel to the large circumference.
toroidal field	TF, a magnetic field generated by a currrent flowing in a solenoid around a torus.
toroidal field coils	TF coils, these coils confine a plasma by creating a magnetic bottle for confinement.
toroidal magnetic field on axis	The magnetic field at the centre of the small cross section of the plasma.
toroidal system	Name given to the general class of “doughnut-shaped” magnetic devices in which magnetic lines of force closing in on themselves. Stellarators and tokamaks are examples.
toroidal vessel	A torus-shaped chamber used to confine plasma.
torus	A magnetic component (a coil or a transformer) made in the shape of an anchor ring. With this construction, most of the magnetic field is contained within the core and leakage is minimal. Thus, there is little or no interaction with adjacent components and circuits. Also toroid.
Townsend avalanche	(Phys.) A cumulative ionization process in which the ions and electrons of one generation undergo collisions that produce a greater number of ions and electrons in succeeding generations.
transport	The rate at which desired material is carried through any section of a processing plant, e.g. isotopes in isotope separation.
triangularity	The amount of inward asymmetry in a noncircular-shaped plasma.
triple product	The confinement time, density, and temperature product used to characterize the quality of confinement.
tritium	The third isotope of hydrogen, containing one proton and two neutrons in the nucleus.

Term	Definition
ultimate heat sink	The atmosphere, a body of water or ground water (or a combination of these), to which generated heat is transferred.
ultra-high-quality vacuum	See vacuum pumps. Ultra high quality vacuum has a gas pressure lower than 1 µbar .
unscheduled down time	The down time of the plant not according to plan.
unscheduled non-operating state	When the plant or component is down according to plan for the length of the unscheduled down time due to an off-normal event.

Term	Definition
vacuum pumps	Pumps which remove gas, usually air, from a chamber, leaving it under vacuum. A variety of types are used, depending on the quality and degree of the vacuum needed.
Vacuum Vessel	VV, establishes a confinement barrier in a tokamak and limits the heat flux to the toroidal field coils, and provides low-impurity volume for the reacting plasma.
vertical displacement event	VDE, a sudden shift in the vertical position of a tokamak plasma, frequently associated with a plasma disruption.
vertical position control	See vertical stability control
vertical stability control	A plasma whose vertical cross section is elongated vertically is unstable vertically if it is displaced from its nominal equilibrium position. A vertical field is applied to the plasma region to control this. If the plasma moves vertically it is necessary to tune this vertical field to force the plasma position back to the equilibrium. This is achieved using a feedback control system on the currents in the poloidal field coils, linked to plasma position sensors.
very high mode	VH-mode, a very high mode of energy confinement, first discovered in the DIII-D tokamak.
voluntary contributor	A European government which agreed to underwrite part of the additional costs of implementing the European half of the Broader Approach Agreement (France, Italy, Spain, Germany, Belgium).
VTL	vacuum transmission lines
VV	vacuum vessel

Term	Definition
work breakdown structure	WBS, a tree-like structure with unique identification numbers consisting of levels used to plan and organize the work of the project. At the top level the same numbers are used in part to identify component systems and structures within the plant, and together these form the project breakdown structure.
work flow management	In information technology, a work flow management application that assists system users in determining the task on which to work, accessing relevant information, controlling the necessary sequence of steps for completion of the task, passing data, decisions, and relevant information among those working on the same task, and provides task status information, resulting in improved process quality, productivity, and monitoring and reporting of critical factors.

Term	Definition
X-point	Point (or points) at which the core plasma, private flux region, and scrape-off layer plasma(s) meet on the separatrix.

Term	Definition
Z	atomic number
Zeff	The effective charge of a plasma.