Like a wood fire, fusion fire does not combust on its own either. First the proper ignition conditions have to be met.
In a burning plasma a sufficient number of particles must collide with one another with sufficient frequency and intensity. The magnetic field cage must thus confine a sufficient number of particles whose thermal energy must not be transferred too quickly to the surroundings. This imposes requirements on the density, temperature, and thermal insulation of the plasma. These are:
- a plasma temperature of at least 100 million degrees
- an energy confinement time of about two seconds. This measure for the thermal insulation gives the time that elapses till the thermal energy pumped into the plasma by heating facilities is again lost to the outside.
- a plasma density of about 1014 particles per cubic centimetre – 250,000 times thinner than the earth's air mantle. This extremely low density means that, despite its high temperature, a burning fusion plasma affords a power density scarcely larger than an ordinary light bulb.
The closest to a burning plasma has been achieved by the European Community's JET device, which is the largest fusion experiment in the world. The product of temperature, density, and thermal insulation attained is only a factor of 5 short of the ignition criterion.