Refractive Index Measurements of Solid Hydrogen Isotopologues

Fuel pellets made of a solid deuterium-tritium (D-T) mixture are supplied for inertial confinement fusion. Characterization of the D-T mixture is fundamental for the design and production of high-quality fuel pellets. However, during the phase transition, isotopologue fractionation may lead to fractional crystallization in the solid phase of the hydrogen isotopologue mixture. If this phenomenon occurs in solid D-T fuel, it will reduce the reaction efficiency of nuclear fusion. Currently, there is no effective observation method for fractional crystallization. This study aims to quantify the degree of fractional crystallization of the hydrogen isotopologues mixture in the solid phase using the refractive index measurement. For this method, refractive index information on the hydrogen isotopologues is necessary, therefore the temperature and wavelength dependences of the refractive index of hydrogen isotopologues need to be measured. Then, using the refractive index distribution of the solid D-T will show the composition distribution of isotopologues for assessing the fractional crystallization. Particularly, as far as we know, this is the first time that the measured values of the refractive index versus wavelength of solid D₂ have been obtained. Understanding the wavelength dependence of the refractive index for the dispersion compensation allows for a wider application of the fractionated crystallographic observation method.