Active gas handling system (at JET). Plant to remove and recover waste gases, including tritium. See cryopump.

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.

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.

See Active Gas Handling System.

See iron core.

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.

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.

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.

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.

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.

Analytic: algebraic solution of basic equations.
Computational: numerical solution of basic equations, using a computer. See Fokker-Planck Code, Grad-Shafranov equation, Monte Carlo, Neural network.

Measured heat loss is anomalously large compared to basic collisional theory of heat transport in toroidal plasmas ("neoclassical" theory). Particularly true for electrons.

Process by which the pumping power of a cryopump is increased so that it will pump faster and help withdraw helium ash from the vessel. A layer of argon frost is built up and a monolayer of helium attaches to each available argon atom.

A comprehensive tokamak fusion power plant study undertaken by a collaboration of US fusion laboratories in the early 1990s. Four designs were studied: ARIES-I, a device based on modest extrapolations from the present tokamak physics database; ARIES-II and ARIES-IV, two second stability devices which differed in their fusion power core composition, and ARIES-III, which, unlike the others, utilised the deuterium-helium-3 fusion reaction instead of the deuterium-tritium reaction.

ASDEX (Axisymmetric Divertor Experiment), a tokamak at the Max Planck Institut fur Plasmaphysik, Garching, Germany, was designed to study the effect of a double null divertor. The first H-modes were observed on ASDEX. The latest upgrade of this machine is called ASDEX-U, or ASDEX-Upgrade. It is intermediate in size between Compass-D and JET, and has the same magnetic configuration as these devices and as that planned for ITER. ASDEX-U Home Page

See helium ash.

Ratio of the major radius to the minor radius of the toroidal plasma; on JET and COMPASS, the aspect ratio is approximately 3 (as presently planned for ITER), on START it can be as low as 1.2 and MAST down to 1.3. See plasma geometry.

Unwanted regeneration caused by excessive gas load on a cryopump or by excess pressure in the parts of the machine held in vacuum.

Same as additional heating.

A local instability which can develop in the tokamak when the plasma pressure exceeds a critical value; it therefore constrains the maximum beta that can be achieved. It is analogous to the unstable bulge which develops on an over-inflated pneumatic inner tube. See resistive ballooning mode.

See trapping.

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.

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.

See poloidal beta.

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. See breeder.

Bohm transport is the anomalous diffusion associated with long wavelength plasma fluctuations and has the consequence that confinement times increase linearly with magnetic field. In gyro-Bohm transport, fluctuations have a shorter scale length, comparable with the ion gyro-radius, and consequently the confinement time increases quadratically with magnetic field. Gyro-Bohm transport is therefore more optimistic than Bohm for large scale devices like ITER.

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).

Application of a compound containing boron to the inner surface of a vacuum vessel to help reduce impurity radiation. See Glow Discharge Cleaning.

See energy break-even.

A term sometimes used to indicate that component of a fusion power plant used to "breed", or produce via nuclear reactions, tritium from the energetic neutrons released, for use as fuel in the power plant. The most commonly used reaction is:-

There is no relationship to the breeding process in fast reactors (fission power plants).

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.

An ignited plasma is said to be "burning".

Charge coupled device. A very sensitive electronic method of detecting light.

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.

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

Culham EURATOM Steering Committee.

Community Fusion Programme. See EFDA.

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

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

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.

Contract of Association between a European fusion laboratory and EURATOM, for example UKAEA and EURATOM.

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.

Compact Assembly: the Culham conventional aspect ratio tokamak facility. It has a similar magnetic geometry to JET and therefore plays a crucial role in scaling experimental results through JET to ITER.
(-C: with circular vacuum vessel) - operated from 1989-91
(-D: with D-shaped vacuum vessel) - operated from 1991-1999
Compass-D Home Page

Time taken for energy or particles to leave the plasma.

See transport scaling.

Engineering processes carried at low temperatures (usually below 120K). Cryogenics in fusion is used for cooling the superconducting magnets (if present) and for cryopumping.

A device (containing a coolant such as LHe or LN2) for trapping condensable gases from neutral beams or the plasma. When a layer of frost has built up the pump is regenerated. It is a form of vacuum pump. See argon frosting, pumping speed, cryosorption.

The process by which a cryopump removes gas in a vacuum.

A cryopump with an activated carbon surface capable of achieving faster pumping speed than a bare surface cryopump, to help in the pumping of lighter components such as hydrogen and helium.

See Close Support Unit.

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

Current per unit cross-sectional area.

The variation of plasma current density within the plasma, usually expressed as a function of the distance from the magnetic axis (see profile).

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

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

Core X-Ray Spectroscopy.

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.

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

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.

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).

Plasma disruptions occur within tokamaks if the electron density is too high. See Hugill diagram, Greenwald density.

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

An alternative fusion reaction to deuterium-tritium:-

Since the fuel is not radioactive and few neutrons are produced (from subsequent reactions between the daughter products), this reaction has safety and environmental advantages over D-T. However, it is harder to achieve, with higher temperatures (three times), densities and confinement times required for power plant operation.

The most likely fusion reaction to be used in future fusion power plants:-

Deuterium-tritium plasmas have already been used to produce significant amounts of fusion energy in both the JET and TFTR devices.

The Directorate-General for Research 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. The current Sixth Framework Programme covers the period 2002 - 2006.

See deuterium-helium-3 reaction.

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.

A flux loop surrounding the poloidal cross-section of the plasma, for measuring the toroidal flux.

The random flow of heat (or particles) down a thermal (or density) gradient.

See Doublet-III.

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. See Hugill diagram.

Graphical or algebraic expression of the number of particles within each subdivision of the energy range, as a function of the average energy of each subdivision.

Divertor and Injection Tokamak Experiment at Culham. Now dismantled, but experiments that were performed on DITE to investigate disruption control are receiving renewed interest, particularly for Next Step devices.

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. See H-mode, pumped divertor, radiative divertor, single/double null.

See single/double null.

A tokamak run by the General Atomic Company, San Diego, in which the plasma geometry can be varied over a very wide range. See reverse shear. DIII-D Home Page

Kinetic theory which accurately describes plasma processes which have spatial scales much larger than the particle Larmor radii. See gyrokinetic theory.

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.

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

Experimental campaigns on JET involving the use of deuterium and tritium fuel. DTE-1 started in April 1997. The preliminary tritium experiments were in November 1991.

See Deuterium-Tritium reaction.

See Electron Cyclotron Emission.

See Electron Cyclotron Resonant Heating.

See Electron Cyclotron Current Drive.

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.

See European Fusion Development Agreement.

Non-inductive current drive using ECRH.

Radiation emitted by electrons as a result of their cyclotron motion around magnetic field lines. Used to measure electron temperature.

Additional heating method using RF waves to accelerate electrons orbiting in the magnetic field. See high field ECRH launch.

See Edge Localised Mode.

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

Extra-Mural Research. EMR contracts are placed with universities for students to work at Culham.

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.

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.

See Confinement Time.

The magnetic coils of a tokamak are designed to give the desired magnetic field configuration. Inevitably however the finite number of coils and imperfections in their construction lead to unwanted deviations from this configuration known as error fields. These can lead to disruptions and are of particular concern for larger tokamaks. See ripple.

The European Atomic Energy Community. See DG-Research.

The agreement that coordinates the work within the European Union and Switzerland on controlled thermonuclear fusion. EFDA has been running since 1 January 1999 and has been extended to the present time, beyond its original expiry date of 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.

The rotation of the plane of polarisation of light passing through a magnetised plasma. See polarimetry.

The fast Alfven wave exists over a broad frequency spectrum, from the ion cyclotron range of frequencies (ICRF) where its character is electromagnetic, down to magnetohydrodynamic frequencies. Its velocity is comparable to the Alfven velocity. The fast Alfven wave is used routinely for high-power (about 20 MW) ICRF heating on JET, as it is efficiently absorbed in the plasma by the mechanism of ion cyclotron resonance. Although usually stable in tokamaks, the wave can be excited by energetic ion populations. See magneto-acoustic cyclotron instability.

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.

Use of measurements of plasma parameters to change the control or heating of the plasma to maintain the desired conditions.

See CCE-FU.

The coils of a magnetic confinement device that produce the magnetic field required to stabilise and shape the plasma. They may be resistive or superconducting. See TF and PF coils, error fields, cryopump.

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

Far Infra-Red (e.g. wavelength about 0.2 - 1 mm) part of the electromagnetic spectrum. FIR lasers are used to measure the magnetic field and plasma density.

Rapid bursts of MHD activity sometimes observed when neutral beam heating is used in tokamaks (fishbone refers to the shape of the bursts in magnetic field when plotted as a function of time).

Constant current (refers to shape of the graph as a function of time).

A large, single-turn coil of wire, used to measure the magnetic flux enclosed by it.

See field lines

A computer code to describe the velocity distribution of plasma particles allowing for collisional relaxation and plasma heating. Calculates distribution functions.

See poloidal beta.

See CCE-FU.

See DG-Research.

See CCE-FU.

Supplying a fusion plasma with the necessary reactants. See pellet injection.

Wave theory which includes complete accounting of wave energy (transmitted, reflected and absorbed, including energy transferred to other waves) for studying RF heating.

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.

The main components of a fusion power device, which for a tokamak include the limiter or first wall, divertor, blanket, shield, PF coils, TF coils, current drive system, cryostats and structural components. Items such as buildings, turbines, heat exchangers etc., which are not specifically fusion-related are not included.

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

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

Mathematical product of density, temperature and energy confinement time. A measure of approach to energy break-even and ignition. See also Lawson Criterion.

See Glow Discharge Cleaning.

A means of sealing the inner surface of the vacuum vessel in order to stop the out-gassing of impurities from the wall; a current is passed through a gas (usually one of hydrogen, methane, helium, trimethyl boron or diborane) so that the gas molecules form a covering layer (the actual layering process involved and its effectiveness depends on the gas being used).

A second-order, non-linear elliptic partial differential equation that describes the relationship between the plasma current distribution and pressure in terms of magnetic flux. This equation is readily solved numerically by computer, and the calculated plasma geometry and profiles that satisfy it comprise a plasma equilibrium.

The Greenwald normalised density is given by n₂₀.pi.a²/I_p, where n₂₀ is the electron density expressed in units of 10²⁰m^-3, a is the plasma minor radius in metres, and I_p 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.

See Bohm transport.

Version of kinetic theory in which the radius of Larmor gyration is not assumed to be small. An essential theory for investigating fine-scale instabilities which might be responsible for driving turbulence, which may in turn be responsible for anomalous transport.

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

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.

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

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.

Current which flows outside the confined plasma region, in the scrape-off layer. During a VDE, the plasma makes contact with a limiter and starts to be scraped off, causing a fraction of the plasma current to flow along the field lines. The current path can intercept a material surface, where the current will take the path of least resistance, causing intense heating and deformation of the material. The effects of halo currents may not be axisymmetric, and can cause great damage. They are a potential concern for Next Step devices.

A stellarator configuration in which the coils resemble distorted, non-planar TF coils - no continuous helical coils or tokamak-like PF coils are present. The Helias (HELIcal Advanced Stellarator) has been proposed to be the most promising stellarator concept for a power plant, with a modular engineering design and optimised plasma, MHD and magnetic field properties. The Wendelstein VII-X device is based on a five field-period Helias configuration.

A means of imparting a helical magnetic wave into a plasma to produce a driven current and is potentially valuable for sustaining toroidal and spherical devices. The helicity of a toroidal plasma is related to a linkage of toroidal and poloidal magnetic fields, and is approximately conserved throughout a discharge. If additional helicity can be injected, the plasma current can be sustained or even increased.

A stellarator configuration in which a helical coil is used to confine the plasma, together with a pair of PF coils to provide a vertical field. TF coils are sometimes also used to control the magnetic surface characteristics.

A large stellarator of heliotron configuration, built in Kyoto, Japan.

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. See argon frosting.

Condition in which the plasma energy is a significant fraction of the energy in the magnetic field. An alternative measure is when the plasma energy is comparable to the energy in the poloidal magnetic field (i.e. poloidal beta is approximately unity). See beta.

Electron cyclotron resonant microwaves launched from the inside of the plasma torus. This allows higher density plasma to be heated.

A plot of 1/q against nR/B (where q is the safety factor at the edge of the plasma, n is the average electron density, R is the plasma major radius and B is the toroidal field at the plasma centre), showing the boundary of tokamak operation as limited by disruptions. The density and q limits are clearly seen. (In the following Hugill diagram n is in units of 10¹⁹ electrons per cubic metre.)

International Atomic Energy Agency.

See Ion Cyclotron Current Drive.

Ion Cyclotron Emission. Analogous to ECE. Observed in JET and TFTR as a suprathermal signal, apparently driven by collective instability of energetic ion populations such as fusion products and injected beam ions.

See Inertial Confinement Fusion

See Ion Cyclotron Resonant Heating.

See Ion Cyclotron Resonant Heating.

In the context of magnetohydrodynamics, "ideal" implies that the magnetic field and the plasma always move together. For this to occur, the electrical resistivity of the plasma must be negligible.

A magnetohydrodynamic instability of the central region of a tokamak. This, or its close relative the resistive internal kink mode, may be involved in the sawtooth disruptions which occur in most tokamaks.

International Energy Agency

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.

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

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. See Glow Discharge Cleaning.

The prevention of impurities from entering the plasma. See Divertor, Boronisation.

The use of high-powered lasers or other beam devices to implode a pellet of material to such high densities that fusion occurs. Further ICF information, hosted by Los Alamos

See MHD (and other) instabilities.

Studies of the interference patterns created by laser beams passing through the plasma can provide information about certain parameters such as density.

Denoted by the symbol l_i. A measure of how peaked the plasma current profile is. A plasma with a high internal inductance has most of its current near the centre of its cross-section.

A type of MHD instability that can occur within the central region of the plasma (where q<1) reducing the peak temperature and density.

An instability which breaks magnetic field lines and reconnects them with a different topology to reduce the system to a lower energy state. These events are frequently observed in MAST and other spherical tokamaks, where they appear to replace the potentially more serious disruptive instability.

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

A wave which only exists in a hot plasma and is supported by the ions. It propagates at right angles to the magnetic field, when it is undamped, at harmonics of the ion cyclotron frequency. The is also an electron Bernstein wave which propagates at harmonics of the electron cyclotron frequency. See magneto-acoustic cyclotron instability.

Non-inductive current drive using ICRH.

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

Infra red part of the electromagnetic spectrum.

See Internal Reconnection Event.

An iron core may be used to improve the flux linkage between the primary coils and the plasma in a tokamak driven by transformer action. However, toroidal asymmetries are introduced, and loss of equilibrium may occur when the iron saturates. These disadvantages do not occur in an air-cored tokamak, although such machines suffer from poor flux linkage and require careful design of the primary winding distribution to avoid stray fields in the vicinity of the plasma. JET is an example of an iron-cored tokamak, and COMPASS is a typical air-cored tokamak.

See magnetic islands.

See Internal Transport Barrier.

International Tokamak Experimental Reactor. The proposed Next Step tokamak. ITER Home Page

Technical Advisory Committee for ITER.

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, AGHS, EFDA, JET Operation Contract. EFDA-JET Home Page

See European Fusion Development Agreement.

The contract between EURATOM and UKAEA for the operation of the JET facilities within the framework of the EURATOM-UKAEA Contract of Association. The JOC provides for the UKAEA to be responsible for the operation, maintenance and safety of the JET facilities on behalf of the other parties to the JIA.

JET Implementing Agreement; see EFDA.

See Jet Operation Contract.

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. See reverse shear. JT60-U Home Page

Oscillation which is unstable as a result of the energy distribution of ions or electrons.

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. See drift and gyro kinetic theories.

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

As opposed to the H-regime. The "normal" Low confinement regime of additionally heated tokamak operation.

Electrical probe inserted into the edge of a plasma for measurements of density, temperature and electric potential.

The world's largest stellarator, situated in Nagoya, Japan, at the National Institute for Fusion Studies. LHD started operating in March 1998. LHD Home Page.

An ideal mode is an MHD instability in which the magnetic field lines move with the plasma (implying zero electrical resistivity). A large scale mode has a wavelength which is a significant fraction of the plasma dimensions.

Radius of the gyratory motion of particles around magnetic field lines.

Use of lasers for disintegration and ionisation of objects injected into the plasma.

See Thomson scattering diagnostic.

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. See also Fusion triple product.

See Lower Hybrid Current Drive.

See Large Helical Device.

Liquid helium or supercritical helium used in fusion to cool superconducting magnets and cryopumps.

See Lower Hybrid Heating.

See internal inductance.

A material surface within the tokamak vessel which defines the edge of the plasma and thus avoids contact between the plasma and the vessel. Alternative to using a divertor to define the edge.

Radiation due to the decay from excited states by electrons. Excitation of line radiation arises from electron impact and from recombination into excited states.

Liquid nitrogen used to protect LHe circuits from heat ingress and in cryopumps to trap water vapour or carbon dioxide that may leak into the vacuum vessel.

MHD modes that cease rotating (though they can still grow).

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.

Non-inductive current drive using lower hybrid heating.

Heating produced by a lower hybrid 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.

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

The use of strong magnetic fields to confine plasma so as to allow fusion reactions to occur within it. See tokamak, stellarator, reverse field pinch, Z-pinch, theta-pinch.

Coils of wire in various configurations, placed around the outside of a plasma to measure the local magnetic field or related quantities. See flux loop, Mirnov coil, Rogowski coil.

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

See Shear.

This instability results from an exchange of energy between the fast Alfven wave (or magneto-acoustic wave) and an ion Bernstein wave which has a source of free energy through the presence of a population of energetic (non-thermal) ions, e.g. fusion products. The instability occurs for propagation perpendicular to the equilibrium magnetic field. See super Alfvenic velocity.

See field coils.

Distance from the centre of the torus (to the centre of the plasma cross-section). See plasma geometry.

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

Close to the transition from stability to instability.

Mega Amp Spherical Tokamak, currently operating at Culham. MAST follows on from the pioneering START device. MAST Home Page

See Magnetic Confinement Fusion.

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.

Unstable distortions of the shape of the plasma/magnetic field system. See Alfven gap mode, ballooning instability, disruption, edge localised mode, ideal internal kink modes, internal kink, internal reconnection event, kinetic instability, large scale ideal modes, magneto-acoustic cyclotron instability, marfe, microinstabilities, mode number, neo-classical tearing modes, peeling mode, profile control, resistive ballooning modes, resistive instability, resonant magnetic perturbation, sawtooth, TAE modes, tearing modes, vertical displacement event.

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.

Half the horizontal extent of the plasma cross-section. In the context of plasma profiles, the radial distance from the centre of the plasma. See plasma geometry.

A small cross-section, multiple-turn coil of wire, used to measure the component of the local magnetic field perpendicular to the plane of the coil. These coils are generally referred to as "pickup" coils when the output voltage is integrated - this is proportional to the magnetic field. Fluctuations in the field are best measured by using the unintegrated signal - the coils are then referred to as "Mirnov" coils.

Another word for wave or oscillation in a plasma. Also used, independently, in place of the word regime.

Characterises the wavelength of an instability.

A statistical technique used in numerical calculations where one of a number of events may occur many times, each with a certain probability.

Particles moving transversely to a magnetic field experience an electric field. This gives rise to Stark splitting of spectral lines which can be interpreted to reveal the local magnetic field inside the tokamak. This is a major diagnostic on some tokamaks to deduce the current profile.

See Motional Stark Effect.

See Neutral Beam.

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

The magnetic island produced by a tearing mode perturbs the bootstrap current which further amplifies the island and degrades confinement or leads to a disruption. This is the neo-classical tearing mode.

A computer code or electronic circuit that uses incoming data to give plasma parameters, etc., having previously been "trained" on a series of examples by analogy with the learning processes involved in the human brain.

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. See charge exchange recombination spectroscopy, energetic particle, fishbones, plasma rotation.

An instrument comprising an array of microchannel plate detectors that measure the energies of neutral particles leaving the plasma. The thermal ion temperature and the fast ion spectrum can be determined.

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

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

See additional heating and current drive (non-inductive).

See beta limit.

See Neutral Particle Analyser.

A spherical tokamak currently operating at Princeton, USA. NSTX is a similar size to MAST, but of different design - the two devices will run complementary experimental programmes. NSTX Home Page

See Neo-classical Tearing Mode.

Inductive heating by using a transformer to drive a current in the plasma. This is necessarily pulsed.

See tokamak operating boundaries.

Adjusting the current profile to optimise tokamak performance (e.g. see reverse shear).

See Confinement Time.

See trapping.

An edge MHD instability which exists when the current density at the plasma edge is non-zero. It may be associated with ELMs.

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

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.

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

See Mirnov coil.

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.

Retention of plasma within a region, including the physics of heat and particle losses from the plasma. See transport.

The current flowing toroidally through the plasma in a tokamak. See poloidal field.

A plasma with given profiles and geometry in force balance with the magnetic field. See Grad-Shafranov equation.

The position and shape of the plasma cross-section. See major radius, minor radius, aspect ratio, elongation, triangularity.

Physical quantities which characterise the plasma and can be deduced experimentally: e.g., density, temperature, current, confinement time, beta, geometry.

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

Radiation (i.e. energy loss via electromagnetic waves) from a plasma is due to a number of processes. See Bremsstrahlung, Line radiation, Recombination radiation, Synchrotron radiation, Impurity radiation.

Bulk rotation of the plasma in the toroidal or poloidal direction. Neutral beam injection can cause plasma rotation in the toroidal direction at velocities of typically 100 km/s.

Measurement of the rotation of the plane of polarisation of light; used to measure the local magnetic field and thus the safety factor. See Faraday rotation.

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.

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.

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.

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.

Three plasma discharges on JET, in November 1991, into which, for the first time in a tokamak, a significant amount of tritium was injected. The power liberated from fusion reactions (about 2 MW for about 2 seconds) was in accordance with theoretical expectations.

See transformer drive.

Variation of plasma parameters with minor radius.

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

The year-by-year scheme of work undertaken by UKAEA Culham Division, as agreed with the UK Engineering and Physical Sciences Research Council.

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

The rate in litres/second at whch a cryopump or vacuum pump removes gas from a vacuum space.

See energy break-even.

See safety factor.

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.) See Hugill diagram.

Arises when there is a charge imbalance in the plasma.

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.

See power plant.

Radiation produced when an (impurity) ion recombines with an electron.

In most tokamaks the pulse length is much longer than the particle confinement time. Thus on average each plasma ion goes to the wall or limiter and returns to the plasma many times during the length of the discharge. This process is called recycling.

Use of reflected microwaves to measure density.

The process by which the cryopumped gases on a cryopump are released (by warming) and recovered in the AGHS. See auto-regeneration.

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

A class of ballooning mode which would be stable in the absence of resistivity, but can be unstable in its presence. Related to, but topologically different from, tearing modes.

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

The tendency to resist the flow of electric current, thereby dissipating energy. Plasmas are very good conductors of electric current, so that as a first approximation their resistivity can often be neglected. In this case, "ideal" magnetohydrodynamics may be applied.

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).

An externally applied magnetic perturbation matched to the spatial structure (and optionally the frequency and phase) of an instability.

A toroidal magnetic confinement device in which the poloidal and toroidal fields are of comparable magnitude. To maintain stability the toroidal field reverses close to the edge of the plasma when a critical plasma current is exceeded.

In a tokamak the current density is usually greatest at the magnetic axis, in which case the safety factor rises from the centre to the edge of the plasma. Using non-inductive current drive and/or the bootstrap current the current density can be made to be greatest away from the centre. In this "reverse shear" case, the safety factor has a minimum away from the plasma centre. Using reverse or low shear ("optimised shear") some tokamaks, notably DIII-D and TFTR in the US and more recently JT-60U in Japan and JET, have shown greatly improved plasma performance. Reverse shear is an attractive option for advanced tokamak scenarios.

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.

See Reverse Field Pinch.

A periodic oscillation in the toroidal field due to the finite number of TF coils in a tokamak.

See Resonant Magnetic Perturbation.

A magnetic diagnostic used to measure currents. A Rogowski is essentially a long, thin, many-turn solenoid, with the return wire passing back along the axis. The solenoid is itself formed into a closed loop by bringing the ends very close together. The voltage measured across the ends of the coil is proportional to the rate of change of current flowing through the closed loop. Rogowskis are used to measure the plasma current, the field coil currents and any halo currents present.

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".

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.

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

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.

The small amount of residual plasma between the "edge" of the plasma (defined by the limiter or the separatrix) and the tokamak vessel.

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.

A second region of stability against ballooning modes, which occurs at high plasma pressure gradient. It can be obtained either by adjusting the required pressure and q profiles in a tokamak of usual cross-section, or by shaping the plasma boundary to a "bean" shape. Second stable plasmas have a high poloidal beta, which leads to a high bootstrap current. This has potential cost savings for a fusion power plant as a less powerful current drive system would be required.

A theoretical approach where the behaviour of some of the key quantities is deduced from experiment, rather than a priori.

Boundary separating magnetic field lines that intersect the wall (open lines) and the closed magnetic field lines that never intersect the wall (closed lines).

Refers to either the variation of plasma flow (flow shear) or safety factor (magnetic shear) with minor radius. If the type of shear is not specified, it usually means magnetic shear.

Major component of a fusion power plant, which reduces the neutron flux reaching the TF coils and beyond. This minimises the radiological impact of the neutrons, and their heating of the coils which, if superconducting, need to remain at liquid helium temperatures.

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.

The set of equations which represent a plasma as a magnetised, electrically conducting fluid with the usual fluid properties of viscosity, thermal conductivity, etc. The possibility of distinct behaviour of electrons and ions (i.e. two "fluids") is not included.

Same as low aspect ratio.

Hot plasmas radiate X-rays, which can be imaged using special cameras to give information about the plasma geometry and density and temperature profiles.

The detection and analysis of the spectrum of radiation emitted by a plasma. This can yield information abo