Neutral beam injection
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To initiate a sustained fusion reaction, it is usually necessary to use many methods to heat the plasma, including RF heating, electron cyclotron resonance heating (ECRH), ion cyclotron resonance heating (ICRH), and neutral beam injection.
Neutral Beam Injection (NBI) involves injecting a high-energy beam of neutral atoms, typically a hydrogen isotope such as deuterium, into the core of the plasma. These energetic atoms transfer their energy to the plasma, raising the overall temperature.
Although the atoms are injected with no net electrostatic charge, as the beam passes through the plasma, the atoms are ionized as they bounce off the ions already in the plasma. Because the magnetic field inside the torus is circular, these fast ions are confined to the background plasma.
The confined fast ions mentioned above are slowed down by the background plasma, in a similar way to how air resistance slows down a baseball. The energy transfer from the fast ions to the plasma increases the overall plasma temperature.
It is very important that the fast ions are confined within the plasma long enough for them to deposit their energy. Magnetic fluctuations are a big problem for plasma confinement in this type of device (see plasma stability) by scrambling what were initially well-ordered magnetic fields. If the fast ions are susceptible to this type of behavior they can escape very quickly. But there is evidence to suggest that they aren't.