alpha particle is the bare helium-4 nucleus — two protons and two neutrons bound together — produced by nuclear reactions, carrying 3.5 MeV of kinetic energy when born in deuterium-tritium fusion.
Ernest Rutherford named alpha rays in 1899 while sorting the radiation from uranium into types, and later demonstrated that the particles are helium nuclei. An alpha particle differs from a helium atom only in its missing electrons, but the difference is decisive: with a net charge of +2, it is gripped by electric and magnetic fields. It is also stopped by a sheet of paper or a few centimeters of air, which is why alpha radiation is harmless outside the body even though alpha-emitting materials are hazardous if inhaled.
In 📝fusion, the alpha particle is one of the two products of the 📝D-T reaction. Conservation of momentum splits the reaction's 17.6 MeV unevenly: the neutron, four times lighter, carries away 14.1 MeV, while the alpha particle keeps 3.5 MeV. That split organizes power plant design. The neutral neutron ignores the magnetic field and deposits its energy in the surrounding 📝blanket, where it becomes useful heat; the charged alpha stays confined in the 📝plasma, colliding with fuel ions and electrons until its energy is absorbed — the mechanism that lets a fusion plasma heat itself.