In a typical fusion device the neutral atom density is small. Therefore, the amount of radiated emission that results from charge exchange between impurity ions and neutrals is also small. On MST, the neutral density is enhanced by injection of fast hydrogen atoms via a diagnostic neutral beam (DNB). As a result, the radiated emission is greatly increased, though primarily along the beam injection path (the DNB is located below the deck, and cannot be seen from here; the injection path is from right to left across the plasma).