The primary auxiliary heating method for tokamaks, in which high-energy neutral atoms are injected into the plasma, ionized, and transfer their energy through collisions — also driving plasma current and rotation.
Neutral beam injection (NBI) works by accelerating ions (typically deuterium) to high energies (40–1000 keV), then neutralizing them by passing through a gas or plasma cell. The resulting fast neutral atoms cross the magnetic field unimpeded and enter the plasma, where they are ionized by collisions and become trapped. The resulting fast ions slow down by colliding with plasma particles, transferring their energy and heating the plasma.[1]
NBI also provides: (1) current drive — the directed fast ions carry current, reducing reliance on the central solenoid; (2) plasma rotation — tangential injection spins the plasma, stabilizing MHD modes; (3) fueling — injected deuterium atoms add particles to the plasma.[2]
NBI systems are large, complex, and have limited wall-plug efficiency (typically 30–40%). At high plasma densities, beam penetration to the plasma core can be insufficient. RF-based heating methods (ECRH, ICRH) offer complementary advantages.[3]