Auxiliary heating:

Power applied to tokamaks to raise the internal temperature when the contribution from ohmic heating is relatively small. Auxiliary heating usually uses neutral beams or radio-frequency waves.


β = p/(B2/2μo). The ratio of plasma gas pressure (p) to magnetic field pressure (B2/2μo) in a tokamak; p is the gas pressure in pascals (newtons per square meter), B is the magnetic field strength in teslas, and μo = 4π × 10–7 henrys per meter.

Beta limit:

Maximum beta attainable, usually resulting from a deterioration in the confinement.


The physical system surrounding the hot plasma. It provides shielding and absorbs fast neutrons, converts the energy into heat, and produces tritium. Blanket technology for the practical application of harnessing fusion energy is still under development. The ultimate design may include a liquid metal such as molten lithium, which produces tritium when it captures neutrons.

Bootstrap current:

In 1970, theorists predicted that a toroidal electric current will flow in a tokamak that is fueled 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 the Joint European Torus and the Tokamak Fusion Test Reactor; it now plays a role in the design of experiments and power plants (especially advanced tokamaks).

Burning plasma:

A fusion plasma in which alpha particles from the fusion reactions provide the dominant heating of the plasma.


The containment of plasma particles and energy within a container for some extended period of time. A fusion reactor must confine the fuel plasma long enough at high enough density and temperature in order to be economically feasible.

Confinement, magnetic:

A method of containing a plasma or charged particles in a finite region using magnetic fields. Charged particles travel in helical paths around the magnetic field lines, which confine their motion to the local vicinity of the magnetic field. A properly shaped magnetic field prevents particles from escaping the confining field. A tokamak is one example of a magnetic-confinement device.

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