IPPEX Glossary of Fusion Terms

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A heavy isotope of hydrogen whose nucleus contains both a neutron and a proton. The simplest fusion reaction to produce in a fusion reactor consists of fusing deuterium with tritium (see "tritium" below).

  Central Electron Density

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

  Energy Confinement Time

Characteristic time in which 1/2 of a system's energy is lost to its surroundings. In a plasma device, the energy loss time (or the energy confinement time) is one of three critical parameters determining whether enough fusion will occur to sustain a reaction. (See Lawson criterion)

  L-mode or Low mode

The "normal" behavior of a tokamak plasma, characterized by poor confinement of the plasma by the magnetic fields.

  Lawson Criterion

Scientific breakeven criterion when the fusion output power is large enough to offset energy losses. It is based on the product of energy confinement time and particle density. Together with plasma temperature, the Lawson value of a plasma indicates how close it is to self-sustained (ignited) fusion reactor.

  Neutral Beam Power/Neutral Beam Heating

Input power used to heat the plasma. One of the fundamental plasma heating methods. Ions of deuterium or tritium are accelerated to high energies and then neutralized. The neutralized beam is then injected into the magnetically confined plasma. The neutral atoms are unaffected (not confined) by the magnetic field, but ionize as they penetrate into the plasma. The high-energy ions then transfer their energy to the plasma particles in repeated collisions, and heat the plasma.

  Ohmic Heating

Heating that results from the flow of current through a medium with electrical resistance (such as the toaster in your kitchen). In a tokamak, the plasma current provides the ohmic heating. In a tokamak, ions are ohmically heated almost entirely by transfer of energy from the hotter, more mobile electrons.

  Output Fusion Power

The total amount of power produced by the fusion reactions. The maximum amount of output fusion power produced by TFTR so far is 10.7 megawatts.


A "Fourth State of Matter" in which many of the atoms or molecules are ionized. Plasmas have unique properties compared to solids, liquids, and gases. Most plasmas can be thought of at first as extremely hot gases, but their properties are generally quite different. Some (but not all!) Examples: the sun, fluorescent light bulbs and other gas-discharge tubes, very hot flames, much of interplanetary, interstellar, and intergalactic space, the earth's ionosphere, parts of the atmosphere around lightning discharges, and of course fusion plasmas.

  Plasma Current

All tokamak plasmas have a large electric current running "the long way" around the torus. For TFTR, this current is typically about 1-2 million amperes. Normally, most of this current is concentrated near the center of the torus (far from the walls), because this is where the plasma is the best conductor of electricity. The plasma current also generates one of the secondary magnetic fields needed to confine the plasma.

  Radio Frequency (RF) Heating

Process for heating the plasma by transferring energy to ions or electrons using electromagnetic waves at an appropriate frequency. (This is similar to how a microwave oven heats food.)

  Stored Energy

Is the amount of energy stored within the plasma. This must be a sufficient amount so that the deuterium and tritium ions can overcome their natural repulsion and fuse together.


Is a very clean plasma with a superior confinement time compared to low mode plasmas.

  Toroidal Magnetic Field

The principal magnetic field in a tokamak. The field lines travel around the axis of the tokamak. This magnetic field, however, is not sufficient by itself to contain the plasma. The magnetic field produced by the plasma current itself and other smaller stabilizing magnetic fields are all needed for confining the plasma.

  Total Input Power

Is the sum of the 3 input heating powers in a typical tokamak experiment: Neutral Beam power, Ohmic heating power, and RF power.


A radioactive isotope of hydrogen with one proton and two neutrons in its nucleus and one orbiting electron. The deuterium/tritium fusion reaction is the simplest to produce in a fusion reactor.


Non-Fusion-Specific terms


A word formed from the first (or first few) letters of a word. For example, DOS stands for Disk Operating System. Radar stands for radio detecting and ranging. Tokamak is an acronym derived from the Russian words toroid-kamera-magnit-katushka, meaning "the toroidal chamber and magnetic coil."


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


An atom that has lost or gained one or more electrons from a neutral atom (in which there were equal numbers of electrons and protons). An ion that has lost electrons will have a positive charge. A plasma is composed of ions and free electrons.


A form of an atomic element that has a different mass from the normal element, because of a different number of neutrons in the atomic nucleus. Deuterium and and Tritium are considered heavy isotopes of hydrogen because they have one electron and one proton, but 1 and 2 neutrons, respectively.

  Mega joule

A unit used for measuring large amounts of energy. "Mega" means 1 million, so a megajoule is 1,000,000 joules. 1 megajoule is approximately equal to 238,846 calories.

  Tesla (T)

The SI unit for magnetic field. 1 tesla = 1 newton/(coulomb *meter/second). 1 tesla = 10,000 gauss. The strength of a typical small bar magnet is about 0.01 T. The Earth's magnetic field near the planet's surface is about 0.0001 T.

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