The helium-4 nucleus produced in fusion reactions, whose energy is the key to sustaining a burning plasma.
An alpha particle is a fully ionized helium-4 nucleus consisting of two protons and two neutrons. In magnetic confinement fusion, alpha particles are the charged products of the deuterium–tritium (D–T) reaction and carry 3.5 MeV of the 17.6 MeV released per fusion event.1
Because alpha particles are charged, they are confined by the magnetic field and deposit their energy in the surrounding plasma through Coulomb collisions. When alpha heating alone sustains the plasma temperature without external power input, the plasma is said to have ignited. The scientific breakeven milestone — a fusion energy gain factor Q > 1 — requires that alpha heating plus any residual external heating exceed all energy losses.2
For effective self-heating, alpha particles must remain confined long enough to thermalise — typically a fraction of a second in a tokamak. Loss channels include toroidal-field ripple, Alfvén eigenmodes driven unstable by the fast-ion population, and orbit losses near the plasma edge. Controlling these losses is a central design requirement for burning-plasma experiments such as ITER.3
Diagnosing confined and escaping alpha particles is experimentally challenging. Techniques include collective Thomson scattering, lost-alpha detectors mounted on the vessel wall, and gamma-ray spectroscopy from alpha-driven nuclear reactions with impurity ions.4
Alpha particles also appear as products (or intermediates) in advanced-fuel cycles. In the D–3He reaction, the alpha particle carries 3.6 MeV, and in p–11B, three alpha particles share 8.7 MeV. These aneutronic or low-neutron reactions make alpha confinement even more critical, because almost all the fusion energy must be captured in the plasma rather than in a neutron blanket.