 While neutral beam injection is used primarily for plasma heating, it can also be used as a diagnostic tool and in feedback control by making a pulsed beam consisting of a string of brief 2–10 ms beam blips.Deuterium is a primary fuel for neutral beam heating systems and hydrogen and helium are sometimes used for selected experiments.High-frequency electromagnetic waves are generated by oscillators (often by gyrotrons or klystrons) outside the torus.