In the sun and other fusion plasmas, atoms of hydrogen and its isotopes are the fuel. Plasmas are gases that are so hot that electrons are knocked free of the atom, making the atoms electrically charged ions. The un-ionized atoms are called neutrals. On earth, accurately measuring neutral hydrogen concentration in plasmas could offer insights into future fusion experiments and impact the design of a future fusion-based energy source. To measure the hydrogen density, scientists need to use a calibrated measurement method. They use krypton gas, which absorbs two chunks of light energy at the same time (photons) and in turn emits another photon. The problem is that the light emitted is not at the right wavelength for accurate hydrogen density measurements. In this study, scientists discovered that xenon atoms emit light at a wavelength that calibrates well with hydrogen and improves the measurements of neutral hydrogen density. Read more.