Neutral Beam Injection (steam-locomotive-sized box on the right) connected to vacuum vessel (left) of the tokamak ITER. (Credit © ITER Organization, www.iter.org)

A method of heating a magnetically confined plasma into which neutral particles are injected and transfer their energy to the plasma through collisions. NBI is a particle accelerator that accelerates charged particles, ions, by an electric field. Because the charged particles would not reach the centre of the plasma where their energy is most needed due to the magnetic field, they are neutralised before entering the plasma. Hydrogen or its isotope deuterium is most often used. The majority of NBIs accelerate positive ions, which are easy to generate, but the ITER tokamak will use negative ions, which are in turn easier to neutralise. The neutral beam injection method is a proven way of external plasma heating used by almost all tokamaks and stellarators.