alpha heating is the self-heating of a fusion plasma by its own fusion-born alpha particles, which stay confined by the magnetic field and deposit their energy in the fuel before escaping.
Alpha heating stands against external heating — the neutral beams, radio-frequency waves, and ohmic current that raise a plasma to fusion temperatures from outside. Every external method carries a power bill; alpha heating is the only mechanism by which the fusion reaction pays for itself. Each 📝D-T reaction leaves one fifth of its 17.6 MeV in the charged 📝alpha particle, which the magnetic field holds inside the 📝plasma while collisions bleed its energy away — passing it first mostly to electrons, then through them to the fuel ions that sustain the reaction rate.
The fraction of total heating that alphas supply is a yardstick of progress toward practical fusion energy. As the fusion gain Q rises, alpha power grows relative to external power: at Q of 5 the two are equal — the threshold of a burning plasma — and as Q climbs further the plasma approaches 📝ignition, where external heating can be switched off entirely. Making alpha heating work in practice — confining the fast alphas long enough to slow down, without letting them drive instabilities of their own — is one of the defining physics questions for the next generation of fusion machines.