An investigation of the effects of system parameters on the production of hollow hydrogen droplets

Many inertial confinement target designs have the fuel as a frozen spherical shell of hydrogen isotopes. One method of manufacturing these targets would be to produce the spherical shell first. In this paper we report on an experimental study on the production of spherical shells of liquid and solid hydrogen. These shells are made by acoustically breaking up a jet of superheated liquid hydrogen into drops and at the same time cavitating a bubble in the center of each drop. The resulting growth of the bubbles by evaporation produces the spherical shells. The size and the aspect ratio of the spherical shells are found to be affected by several parameters. The mass of the drop depends on the diameter of the nozzle from which the jet emerges. Also, varying the frequency of the acoustic excitation gives some control of the droplet size. The aspect ratio depends most strongly on the liquid temperature and the droplet-chamber pressure. Increasing the temperature or lowering the pressure increases the aspect ratio of the shell. If the pressure is lowered below the triplet-point pressure of hydrogen, the shells freeze forming a spherical shell of solid hydrogen.