neutral beam injection (NBI) is heating a 📝plasma by firing beams of fast neutral atoms into it.
The neutrality is the whole trick. A magnetic field deflects any charged particle, so a beam of ions would bend away before reaching the plasma core; a neutral atom crosses the field in a straight line. An NBI system therefore accelerates ions — typically 📝deuterium — to high energy, neutralizes them by passing the beam through gas, and injects the resulting fast atoms into the 📝Tokamak. Inside the plasma the atoms are promptly re-ionized, and the magnetic field captures them as energetic ions that share their energy with the bulk plasma through collisions. The scale is industrial: each of ITER's two injectors is designed to deliver 16.5 megawatts.
NBI is one of the two great families of auxiliary heating, distinguished from radio-frequency methods by its currency — particles rather than waves. It heats ions directly, drives current when aimed tangentially, fuels the plasma with the atoms it injects, and spins the plasma with its momentum, which is why it has been the workhorse heating system of most large tokamaks. Its main engineering constraint is penetration: the denser and larger the plasma, the higher the beam energy needed to reach the core rather than depositing power at the edge.