The Fusion Glossary is a collection of terms used in fusion energy science, engineering, and industry — the working vocabulary of the field, compiled for communicators, partners, and readers of the 📝Commonwealth Fusion Systems library.

Glossary

A

📝AC loss — the heat a superconductor dissipates when its current or field changes, through hysteresis, coupling, and eddy-current mechanisms — the reason a zero-resistance conductor is not a zero-loss one.
📝Activation analysis — inferring neutron fluence from the radioactivity induced in deliberately exposed samples, the absolute yardstick against which neutron counters are calibrated.
📝Alcator C-Mod — the compact, high-field tokamak operated at MIT from 1993 to 2016, whose record-setting plasma pressure validated the high-field approach to fusion.
📝Alfvén eigenmode — a weakly damped plasma oscillation living in a gap in the shear Alfvén continuum, driven unstable by energetic particles such as fusion-born alphas.
📝Alpha heating — the self-heating of a fusion plasma by its own fusion-born alpha particles depositing their energy before escaping.
📝Alpha particle — the helium-4 nucleus produced by a fusion reaction, carrying 3.5 MeV in the D-T case.
📝Aneutronic fusion — fusion using fuels such as proton-boron that release little or no neutron radiation, at the cost of far higher required temperatures.
📝Anomalous transport — heat and particle loss from a plasma that exceeds collisional predictions, driven by plasma turbulence.
📝ARC — Commonwealth Fusion Systems' first commercial-scale fusion power plant, designed to deliver about 400 megawatts of clean power.
📝ARPA-E fusion programs — the U.S. Department of Energy's advanced-research programs that fund early-stage, high-risk fusion innovation.
📝ASDEX Upgrade — the tungsten-walled divertor tokamak at IPP Garching, in operation since 1991, on which the near-scrape-off-layer heat-flux scaling was measured.
📝Aspect ratio — the ratio of a torus's major radius to its minor radius, a basic descriptor of tokamak geometry.

B

📝Baffle — the structure closing a divertor's mouth against the main chamber, trapping neutrals so they can be radiated and pumped.
📝Ballooning mode — a pressure-driven plasma instability that develops on the outboard, weak-field side of a tokamak.
📝Banana orbit — the banana-shaped trajectory traced by particles trapped in a tokamak's non-uniform magnetic field.
📝Beryllium — a light metal used in fusion designs as a neutron multiplier and plasma-facing material.
📝Blanket — the structure surrounding a fusion plasma that absorbs neutrons, converts their energy to heat, and can breed fuel.
📝Bolometer — a diagnostic that measures the total electromagnetic power radiated by a plasma.
📝Bootstrap current — plasma current generated spontaneously by the plasma's own pressure gradients, reducing the need for external current drive.
📝Brayton cycle — the gas power-conversion cycle that drives every gas turbine, and the route by which a fusion plant's heat would become electricity.
📝Breeding blanket — a blanket containing lithium, designed to produce tritium when struck by fusion neutrons.
📝Bremsstrahlung — X-ray radiation emitted when electrons decelerate in the electric fields of ions, an unavoidable energy loss channel in hot plasmas.
📝Burning plasma — a plasma heated predominantly by its own fusion reactions rather than by external power.

C

📝Capacity factor — the ratio of the electricity a plant actually produces over a period to the maximum it could have produced running flat out, a key economic metric for fusion plants.
📝Central solenoid — the magnet stack in the center of a tokamak whose changing field drives the plasma current by induction.
📝Coated conductor — the layered architecture of second-generation HTS wire: a micrometres-thin REBCO film grown on a buffered metal ribbon and capped with silver and copper.
📝Commercial fusion — the emerging industry of privately financed companies working to deliver fusion electricity to the grid.
📝Confinement scaling laws — empirical formulas, such as the ITER H98 scaling, that predict how confinement time varies with machine size, field, and power.
📝Coulomb barrier — the electrostatic repulsion between positively charged nuclei that fusion conditions must overcome.
📝Critical current — the maximum current a superconductor can carry before losing superconductivity.
📝Critical magnetic field — the field strength above which a superconductor stops superconducting.
📝Critical temperature — the temperature below which a material becomes superconducting.
📝Cryogenics — the engineering of very low temperatures, used to cool superconducting fusion magnets.
📝Cryostat — the vacuum-insulated vessel that keeps a machine's superconducting magnets at cryogenic temperature.
📝Current drive — any method of sustaining plasma current without relying on the central solenoid's induction.

D

📝D-D reaction — fusion of two deuterium nuclei, which needs higher temperatures than D-T and yields less energy per reaction.
📝D-T reaction — the fusion of deuterium and tritium into helium-4 and a neutron, releasing 17.6 MeV — the most attainable fusion reaction and the fuel of first-generation plants.
📝Debye length — the distance over which a plasma screens out electric fields, a fundamental plasma length scale.
📝DEMO — the class of demonstration power plants planned as the step between ITER and commercial fusion in public roadmaps.
📝deuterium — hydrogen's stable heavy isotope, extracted from seawater and one half of the D-T fuel pair.
📝DIII-D — the U.S. national fusion facility tokamak in San Diego, a workhorse of tokamak physics research.
📝Direct drive — inertial confinement fusion in which lasers strike the fuel capsule directly rather than heating a surrounding cavity.
📝Displacements per atom (dpa) — the standard measure of neutron damage, counting how many times each atom in a material is knocked from its lattice site.
📝Disruption — a sudden loss of plasma stability that terminates the discharge and dumps its energy into the machine.
📝Distributed fiber-optic sensing — measurement made continuously along a whole optical fiber rather than at discrete points, so a growing hotspot anywhere in a winding can be located.
📝Divertor — the region at the plasma edge that exhausts heat and particles, the highest heat-flux component in a tokamak.
📝Divertor detachment — an operating regime in which radiation and neutral gas cool the exhaust plasma before it strikes the divertor surface.
📝DOE Milestone-Based Fusion Development Program — the U.S. Department of Energy's public-private program that pays fusion companies for hitting negotiated technical milestones.

E

📝EAST — China's Experimental Advanced Superconducting Tokamak, notable for record-length plasma pulses.
📝Eddy current — a loop of current induced inside a conducting body by a changing magnetic field, flowing so as to oppose the change that created it.
📝Edge localized mode (ELM) — a repetitive eruption at the edge of a high-confinement plasma that expels bursts of energy toward the wall.
📝Electron cyclotron resonance heating (ECRH) — plasma heating by microwaves tuned to the electrons' gyration frequency.
📝Electron temperature — the temperature of a plasma's electron population, often quoted in keV.
📝Elongation — the ratio of a plasma cross-section's height to its width, which buys plasma current and confinement at the cost of vertical instability.
📝Energy confinement time (τ_E) — the characteristic time a plasma holds its heat before losing it, a core term of the Lawson criterion.
📝Engineering breakeven — the point at which a fusion plant produces more electricity than the entire facility consumes.
📝Error field — a small departure from perfect axisymmetry in a tokamak's field, left by imperfect coil winding and alignment, that can open a magnetic island and lock it.

F

📝Fall Line Fusion Power Station — the announced site of the first ARC plant in Chesterfield County, Virginia.
📝Fiber Bragg grating — a periodic index modulation written into an optical fiber core that reflects one wavelength and shifts it with strain and temperature, giving current-free local quench sensing.
📝Field-reversed configuration (FRC) — a compact plasma confinement concept whose fields close on themselves without a central hole.
📝Filament (blob) — the field-aligned coherent structures that carry most cross-field particle transport across the scrape-off layer.
📝First wall — the innermost material surface facing the plasma, which bears its radiation and particle flux.
📝Flattop — the steady phase of a plasma pulse after ramp-up, when current and field are held constant.
📝FLiBe — a molten fluoride salt of lithium and beryllium used in blanket designs as coolant and tritium breeder.
📝Flux expansion — the geometric widening of magnetic flux surfaces toward a divertor target, which spreads a fixed exhaust power over a larger wetted area.
📝Fuel recycling — the recovery and reuse of unburned fuel exhausted from the plasma each pulse.
📝fusion — the nuclear reaction merging light nuclei into heavier ones, releasing the energy that powers stars.
📝Fusion cross-section — the effective size of a nucleus as a fusion target, expressing how likely a collision is to fuse.
📝Fusion Industry Association (FIA) — the trade association of private fusion companies and their supply chain.
📝Fusion pilot plant (FPP) — a first-of-a-kind plant intended to demonstrate net electricity from fusion at modest scale.
📝Fusion power density — fusion power produced per unit plasma volume, which rises steeply with magnetic field strength.

G

📝Gas puffing — fueling or edge-cooling a plasma by injecting neutral gas at the boundary.
📝Gigawatt thermal vs gigawatt electric — the distinction between a plant's raw heat output (GWth) and the electricity it delivers after conversion losses (GWe).
📝Greenwald limit — the empirical density ceiling above which tokamak plasmas tend to disrupt.
📝Gyrokinetics — the reduced kinetic theory used to simulate plasma turbulence by averaging over particles' fast gyration.
📝Gyroradius (Larmor radius) — the radius of a charged particle's spiral around a magnetic field line.
📝Gyrotron — the high-power microwave source used for electron cyclotron heating.

H

📝H-mode — the high-confinement regime, discovered in 1982, in which a transport barrier forms at the plasma edge and roughly doubles confinement.
📝Halo current — current that leaves a disrupting plasma through its cool outer edge and returns through the vessel structure, driving large mechanical forces.
📝Helium ash — the thermalised helium left after fusion alphas deposit their energy, inert cargo that dilutes the fuel unless it is pumped out.
📝High-Temperature Superconducting (HTS) Magnets — fusion magnets wound from high-temperature superconductor, enabling roughly 20-tesla fields in compact machines.
📝Hohlraum — the small metal cavity that converts laser light to X-rays in indirect-drive inertial fusion.
📝HTS tape — the thin, flexible steel-backed ribbon coated with superconductor from which high-field fusion magnets are wound.

I

📝Ignition — the condition in which fusion self-heating sustains the reaction with no external heating, achieved in a laboratory for the first time at NIF in 2022.
📝Impurity pumping — the removal of helium ash and seeded impurities from the divertor, governed by enrichment and by the neutral pressure the divertor sustains.
📝Impurity radiation — energy radiated by non-fuel ions in the plasma, harmful in the core but deliberately useful in the exhaust.
📝Impurity seeding — the controlled injection of impurity gases to radiate away exhaust power before it reaches surfaces.
📝Inductive current drive — driving plasma current by transformer action from the central solenoid, inherently pulsed.
📝Inertial confinement fusion (ICF) — fusion achieved by compressing a fuel capsule so quickly that its own inertia confines it, as at NIF.
📝INFUSE — the U.S. Department of Energy program that pairs private fusion companies with national-laboratory expertise.
📝Integrated modeling — coupling transport, equilibrium, heating, pedestal, and boundary codes into one workflow iterated to self-consistency; how a machine that does not yet exist acquires a number.
📝Interferometry — a diagnostic that measures plasma density from the phase shift of a beam passing through it.
📝Ion cyclotron resonance heating (ICRH) — plasma heating by radio waves tuned to the ions' gyration frequency.
📝Ion temperature — the temperature of a plasma's ion population, the one that directly sets the fusion rate.
📝Isotope separation — the industrial extraction of one isotope from another, as in producing deuterium from seawater.
📝ITER — the 35-nation experimental tokamak under construction in France, designed to produce 500 megawatts of fusion power at Q of 10.

J

📝JET — the Joint European Torus in the UK, which set the record for total fusion energy from D-T fuel before retiring in 2023.
📝JT-60SA — the fully superconducting Japan–Euratom tokamak at Naka, built as ITER's satellite and first operated in 2023.

K

📝keV — kiloelectronvolt, the plasma physicist's temperature unit, where 1 keV corresponds to about 11.6 million degrees Celsius.
📝Kink instability — a current-driven instability that bends the whole plasma column like a twisted rope.
📝KSTAR — South Korea's superconducting tokamak, a leader in long-duration high-temperature plasma operation.

L

📝L-H transition — the abrupt switch from low- to high-confinement mode as heating power crosses a threshold.
📝L-mode — the baseline low-confinement regime of tokamak operation.
📝Langmuir probe — an electrode diagnostic that measures density and temperature at the plasma edge.
📝Lawson criterion — the threshold condition on density, temperature, and confinement time that a plasma must exceed for net fusion energy.
📝Levelized cost of energy (LCOE) — the lifetime cost of a plant divided by its lifetime electricity output, the standard yardstick for comparing power sources.
📝Limiter — a material structure that defines the plasma edge in machines without a divertor.
📝Lithium — the light metal from which fusion neutrons breed tritium in a blanket.
📝Lorentz force — the force a magnetic field exerts on moving charge and, summed over a winding, the structural load that governs high-field magnet design.
📝Low-activation materials — structural materials chosen so neutron bombardment leaves only short-lived radioactivity.
📝Low-temperature superconductor — the earlier generation of superconductors, such as niobium-tin and niobium-titanium, that require cooling near absolute zero and cap fields well below HTS levels.
📝Lower hybrid current drive — driving plasma current with radio waves at the lower hybrid frequency.

M

📝Magnetic confinement fusion (MCF) — the approach that holds a fusion plasma with magnetic fields, as in tokamaks and stellarators.
📝Magnetic flux surface — one of the nested toroidal surfaces traced by field lines, on which plasma pressure equalizes.
📝Magnetic island — a break in nested flux surfaces where field lines reconnect, degrading confinement.
📝Magnetic mirror — a linear confinement concept that reflects particles between regions of strong field.
📝Magnetic proton recoil — neutron spectrometry that converts neutrons to recoil protons in a foil and momentum-analyses them magnetically, giving an absolutely calibrated spectrum.
📝Magnetic shear — the change in field-line pitch between flux surfaces, which stabilizes many instabilities.
📝Magnetics diagnostics — the coil-based measurements of plasma current, position, and shape used in real-time control.
📝Magnetized target fusion — a hybrid approach that compresses a pre-magnetized plasma to fusion conditions.
📝Magnetohydrodynamics (MHD) — the theory treating plasma as a conducting fluid, the framework for large-scale stability.
📝Massive gas injection — disruption mitigation by firing a large quantity of noble gas into the plasma so its stored energy radiates over the whole wall instead of landing on a small area.
📝MAST Upgrade — UKAEA's spherical tokamak at Culham, the first machine anywhere to operate a Super-X divertor.
📝MIT Plasma Science and Fusion Center — the MIT research center from which Commonwealth Fusion Systems spun out in 2018.
📝Molten salt — the class of ionic melts used as high-temperature nuclear coolant and, in fusion, as tritium breeder.
📝Monte Carlo neutronics — stochastic particle-transport simulation that tracks sampled neutron histories to predict dose, nuclear heating, activation, and breeding.

N

📝National Ignition Facility (NIF) — the U.S. laser facility that achieved the first laboratory fusion ignition in December 2022.
📝Neoclassical tearing mode — a tearing instability seeded and sustained by the loss of bootstrap current inside a magnetic island.
📝Neoclassical transport — the irreducible collisional transport level set by toroidal geometry.
📝Neutral beam injection (NBI) — heating a plasma by firing beams of fast neutral atoms into it.
📝Neutron, 14 MeV — the fast neutron carrying most of the D-T reaction's energy, which escapes the magnetic field and deposits its energy in the blanket.
📝Neutron activation — the process by which neutron capture makes reactor materials radioactive.
📝Neutron damage — the structural degradation of materials under sustained neutron bombardment, a central materials-qualification challenge.
📝Neutron diagnostics — instruments that measure fusion power directly by counting the neutrons produced.
📝Neutron flux — the intensity of neutron flow through a surface, which sets shielding and damage rates.
📝Neutron multiplier — a material such as beryllium that releases extra neutrons when struck, boosting tritium breeding.
📝Neutron spectrometer — an instrument measuring the energy distribution of fusion neutrons rather than only their rate, revealing ion temperature and fuel ion ratio.
📝No-insulation coil — an HTS magnet winding technique that omits turn-to-turn insulation so current can bypass local defects, improving quench tolerance.
📝Non-inductive current drive — sustaining plasma current by waves, beams, or bootstrap current instead of induction, the route to steady-state tokamaks.
📝NRC byproduct-materials framework — the U.S. Nuclear Regulatory Commission's 2023 decision to regulate fusion under the materials-licensing regime used for accelerators, not the reactor regime used for fission.
📝NSTX-U — Princeton's spherical torus upgrade, which operated for ten weeks in 2016 before a coil failure began a long rebuild.

O

📝Ohmic heating — the resistive heating a plasma receives from its own current, which weakens as the plasma gets hotter.
📝Operational space — the bounded region of parameters in which a device can actually run, with every physics and engineering limit satisfied at once.

P

📝Pancake winding — the flat-spiral coil geometry stacked to build a magnet, wound as single or double pancakes.
📝Pedestal — the steep pressure gradient at the edge of an H-mode plasma on which core performance stacks.
📝Pellet injection — fueling the plasma core by firing frozen fuel pellets into it at high speed.
📝plasma — the fourth state of matter, an ionized gas that conducts electricity and can be confined by magnetic fields.
📝Plasma beta — the ratio of plasma pressure to magnetic pressure, a measure of how efficiently a field is used.
📝Plasma breakdown — the moment at startup when neutral gas ionizes and becomes a conducting plasma.
📝Plasma current — the megaampere-scale current flowing in a tokamak plasma that generates the confining poloidal field.
📝Plasma rotation — the bulk spinning of a plasma, which suppresses turbulence and stabilizes some modes.
📝Plasma-facing components — the armored surfaces — first wall and divertor — that directly face the plasma.
📝Poloidal field — the magnetic field component running the short way around the torus, produced mainly by the plasma current.
📝Poloidal field coil — the external ring coils that shape and position the plasma.
📝POPCON — the plasma operating contour plot, a design chart contouring the auxiliary power a steady plasma would need across density and temperature, with the physics limits drawn across it.
📝Power purchase agreement (PPA) — a long-term contract to buy a plant's electricity, the instrument by which fusion power is first sold.
📝Pulse length — the duration of a single plasma discharge, with duty cycle expressing the fraction of time a pulsed plant is producing.

Q

📝Q (fusion energy gain factor) — the ratio of fusion power produced to heating power supplied, with Q greater than 1 meaning net energy from the plasma.
📝Q_plasma vs Q_engineering — the distinction between plasma-level gain and whole-plant gain after every subsystem's consumption is counted.
📝Quench — the sudden loss of superconductivity in a magnet, which converts stored magnetic energy to heat.
📝Quench protection — the detection and energy-extraction systems that keep a quench from damaging a magnet.

R

📝Radiation hardening — designing components to survive neutron and gamma exposure; also, in materials science, the irradiation-driven rise in yield strength that costs a metal its ductility.
📝RAFM steel — reduced-activation ferritic-martensitic steel, the leading structural material for fusion blankets.
📝Ramp-up — the phase of a pulse in which plasma current climbs to its flattop value.
📝REBCO — rare-earth barium copper oxide, the high-temperature superconductor at the heart of high-field fusion magnets.
📝Reflectometry — probing a plasma like radar, reflecting swept microwaves from a cutoff layer to yield density profiles and turbulence measurements through a single small port.
📝Remote handling — robotic maintenance of activated reactor components that humans cannot approach.
📝Resonant magnetic perturbation (RMP) — small applied 3-D fields used to suppress edge localized modes.
📝RF heating — the family of radio-frequency and microwave techniques for heating plasma and driving current.
📝Runaway electron mitigation coil — a passive in-vessel coil driven by a disruption's own collapsing current, which breaks up the flux surfaces that would otherwise confine a runaway beam.
📝Runaway electrons — electrons accelerated to relativistic energy during disruptions, capable of damaging the wall.

S

📝Safety factor (q) — the number of toroidal circuits a field line makes per poloidal circuit, the central parameter of tokamak stability.
📝Sawtooth oscillation — the periodic collapse and recovery of core temperature in a tokamak.
📝Scientific breakeven — Q of 1: fusion power out of the plasma equal to heating power in, first exceeded by NIF in 2022.
📝Scrape-off layer — the thin plasma layer outside the separatrix where field lines end on material surfaces.
📝Screening current — the persistent current circulating within a REBCO tape's width as the surrounding field changes, adding a field of its own that distorts a magnet's designed field.
📝Separatrix — the last closed magnetic surface, dividing confined plasma from the exhaust region.
📝Shielding — the material layers that protect magnets and workers from neutron and gamma radiation.
📝Soft X-ray imaging — a diagnostic that views the hot core through its soft X-ray emission.
📝SPARC — Commonwealth Fusion Systems' compact, high-field tokamak built to demonstrate net fusion energy for the first time.
📝Spectroscopy — diagnosing plasma composition, temperature, and rotation from the light it emits.
📝Spherical tokamak — a low-aspect-ratio tokamak shaped more like a cored apple than a doughnut, offering high beta.
📝Steady-state operation — running a plasma continuously rather than in pulses, the operating goal for practical power plants.
📝Stellarator — a magnetic confinement device that twists its field entirely with shaped external coils, needing no plasma current.
📝Strike-point sweep — moving the strike point across the divertor target during a pulse to cut the time-averaged, though not the instantaneous, heat load.
📝SULTAN facility — the Swiss split-coil superconductor test rig at Villigen, the reference bench where ITER conductors and VIPER cable were qualified.
📝Superconductor — a material that below a critical temperature carries current with zero electrical resistance.
📝Synchrotron radiation — microwave emission from electrons gyrating in a magnetic field, an energy loss that grows with field strength.
📝Synthetic diagnostic — a forward model of a real instrument applied to simulation output, producing the signal a detector would actually record — the mechanism that makes a prediction falsifiable before the machine exists.

T

📝TCV — the Tokamak à Configuration Variable at EPFL, built to produce a wider range of plasma shapes than any other machine without hardware changes.
📝Tearing mode — a resistive instability that reconnects field lines and forms magnetic islands.
📝Tesla — the SI unit of magnetic field strength; fusion-grade HTS magnets reach roughly 20 tesla.
📝TFTR — the Tokamak Fusion Test Reactor at Princeton, which performed the first magnetically confined D-T experiments in 1993.
📝Thomson scattering — the laser diagnostic that measures electron temperature and density point by point, the gold standard of plasma measurement.
📝Tokamak — the doughnut-shaped magnetic confinement device that is the leading configuration for fusion energy.
📝Toroidal field — the magnetic field component running the long way around the torus, supplied by external coils.
📝Toroidal field coil — one of the D-shaped magnets that together produce a tokamak's main confining field.
📝Toroidal Field Model Coil (TFMC) — the full-scale HTS magnet that reached 20 tesla in 2021, demonstrating the field strength SPARC is built on.
📝Transport barrier — a plasma region where turbulence is suppressed and gradients steepen, the mechanism behind H-mode.
📝Triangularity — how far a plasma cross-section's corners are displaced from its widest point, which sets pedestal stability and carries a meaningful negative branch.
📝Triple product — the product of density, temperature, and confinement time, the single figure of merit for fusion plasma performance.
📝tritium — hydrogen's radioactive isotope and the scarce half of D-T fuel, bred from lithium in fusion blankets.
📝Tritium breeding ratio (TBR) — the ratio of tritium bred to tritium burned, which must exceed one for a self-sufficient plant.
📝Tritium handling — the containment, accounting, and processing systems that manage tritium safely through the fuel cycle.
📝Tritium inventory — the total tritium a facility holds, minimized by design because the isotope is scarce and regulated.
📝Tungsten — the refractory metal of choice for divertor surfaces because of its unmatched melting point and low fuel retention.

V

📝Vacuum vessel — the sealed toroidal chamber that holds the ultra-high vacuum in which the plasma lives.
📝Vertical displacement event (VDE) — a loss of vertical position control that drives the plasma into the vessel wall.
📝VIPER cable — the high-current HTS cable developed by MIT and Commonwealth Fusion Systems for fusion magnet windings.

W

📝Wendelstein 7-X — the German stellarator that is the world's largest and most advanced, proving optimized stellarator physics.
📝WEST — CEA Cadarache's tungsten-environment steady-state tokamak, rebuilt from Tore Supra, which held a plasma for 1,337 seconds in February 2025.

X

📝X-point — the point where the separatrix crosses itself, steering exhaust plasma into the divertor.
📝X-ray crystal spectrometer — an instrument dispersing a plasma's X-ray impurity line emission by Bragg reflection, reading ion temperature from line width and rotation from line shift.

Z

📝Z-pinch — a confinement concept that squeezes plasma with the magnetic field of a current driven through it.
📝Zeff (effective charge) — the average ion charge of a plasma, a measure of its impurity contamination.

Glossary

A

B

C

D

E

F

G

H

I

J

K

L

M

N

O

P

Q

R

S

T

V

W

X

Z

Contexts