Alpha heating

The heating of a fusion plasma by the helium nuclei, called alpha particles, produced by the fusion reactions themselves, as distinct from heating supplied from outside by lasers, microwaves or particle beams. A D-T reaction releases 17.6 MeV, split between a neutron carrying 14.1 MeV, which escapes the plasma and does not reheat it, and an alpha particle carrying 3.5 MeV, which is charged, stays in the plasma and deposits its energy back into the fuel. So only about a fifth of D-T fusion energy is available to keep the plasma hot. When alpha heating is the largest source of heating, the plasma is called a burning plasma; ignition is the point where alpha heating alone outweighs every way the plasma is losing energy.

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