The challenge, however, in scaling up nuclear fusion has been containing high-energy particles within the walls of the reactor. For fusion to occur, isotopes of hydrogen are heated to temperatures greater than those on the surface of the Sun and exist in a fourth state of matter called plasma, before fusing to form helium molecules and releasing energy.
However, at these temperatures and high energy states, alpha particles leak from the fusion reactor, causing the plasma to become less dense. As the plasma loses heat, it can no longer sustain the fusion reaction, resulting in a loss of energy production. To prevent this, scientists and engineers design systems to prevent the escape of alpha particles.