Describe the role of lithium deuteride in a thermonuclear weapon's secondary stage.

In the secondary (fusion) stage of a thermonuclear weapon, lithium deuteride (LiD) serves as the primary fusion fuel. It plays a dual role in producing tritium and deuterium, which are essential for the fusion reaction. When high-energy neutrons, produced by the primary fission stage, bombard the lithium deuteride, the lithium-6 isotope (one of the two naturally occurring isotopes of lithium) reacts with a neutron in a nuclear reaction called neutron capture, to produce tritium (Hydrogen-3) and helium. The deuterium component of lithium deuteride is already present, and does not need to be created. Then, under extremely high temperatures and pressures created by the primary fission explosion, the tritium and deuterium fuse together, releasing immense energy in the form of more high-energy neutrons and helium. This fusion reaction is the main energy-producing process in the secondary stage. The newly produced high-energy neutrons can then cause further reactions with additional lithium-6, creating more tritium and sustaining the fusion process. Lithium deuteride is advantageous because it is a solid at room temperature, making it easier to handle and package compared to gaseous deuterium and tritium. The use of lithium deuteride enables the efficient and scalable fusion reactions characteristic of modern thermonuclear weapons.