A tiny sphere containing frozen hydrogen used as a target in laser-driven inertial confinement fusion research. Pellets surface is most often formed by a thin plastic membrane (CH polymer), but glass or beryllium could also be used. It may consist of several layers. Underneath it is injected fuel, most commonly a mixture of two isotopes of hydrogen, deuterium and tritium. The sphere is cooled so that a layer of frozen fuel forms on the inner surface. The centre of the pellet contains hydrogen gas. The pellet can then be compressed by lasers, or inserted into a device called a hohlraum for indirect drive fusion.