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How do alpha particles help keep an ignited fusion plasma hot?

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3D illustration of the interior reaction chamber of a tokamak fusion reactor, showing a large, cylindrical, metallic structure with curved walls and bright light panels. — fusion reactor A tokamak is a device used in nuclear-fusion research for magnetic confinement of plasma.

In a deuterium-tritium (D-T) fusion reactor, alpha particles play a critical role in maintaining the high temperatures required for a self-sustaining, or "ignited," plasma.1

When deuterium and tritium nuclei fuse, they produce a neutron and an alpha particle (the nucleus of a helium atom) at a high energy level of 3.5 million electron volts (MeV). Because alpha particles carry a positive electric charge, they are subject to the influence of the reactor's magnetic field, which allows them to be confined within the plasma.1

As these energetic alpha particles move through the plasma, they collide with the surrounding deuterons and tritons through electrical interactions. During these collisions, the alpha particles transfer their kinetic energy to the reacting fuel nuclei, effectively heating the plasma. When the energy redeposited into the plasma by these alpha particles exceeds the power lost through processes like radiation and conduction, the plasma becomes self-sustaining and reaches the state of ignition.1