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Plasma ignition occurs when the power released from fusion reactions is enough to sustain those reactions.

In a D-T plasma (i.e. one containing deuterium and tritium) fusion produces helium ions and neutrons. In a magnetically confined plasma, the energetic neutrons will escape since they have no charge. However, the helium ions (known as alpha particles) may be trapped in the magnetic field of the plasma containment device (e.g. tokamak, stellerator). Through collisions with other particles they could maintain the temperature in the plasma. This is known as α-particle heating.

The condition for the α-particle heating balancing the power losses at a temperature of 30 keV is presented in terms of the plasma density n and the energy confinement time τE

When this condition is met plasma ignition will occur and any external heating required to start the reaction can be switched off. However, the required temperature, density and energy confinement time have never been attained simultaneously in a plasma. Reaching it is the holy grail of fusion physics.

See also Lawson Criterion.