How does the Hohlraum function in a thermonuclear weapon?

In a thermonuclear weapon, the Hohlraum (German for 'hollow space') serves as a radiation case that efficiently transfers energy from the primary fission stage to the secondary fusion stage. After the primary fission explosion, a tremendous amount of energy is released, primarily in the form of X-rays. The Hohlraum is a closed cavity surrounding both the primary and secondary stages. Its walls are made of a dense material, such as uranium or gold. When the X-rays from the primary explosion strike the inner walls of the Hohlraum, they are absorbed and re-emitted as a uniform, intense bath of X-ray radiation. This uniform radiation distribution is critical for evenly illuminating and compressing the secondary stage, which contains the fusion fuel (lithium deuteride). The Hohlraum ensures that the secondary stage is subjected to intense and uniform pressure and heat, causing it to implode. This implosion compresses the fusion fuel to extreme densities and temperatures necessary to initiate and sustain the thermonuclear fusion reaction. Without the Hohlraum, the energy transfer from the primary to the secondary would be inefficient and uneven, preventing the secondary stage from achieving the necessary conditions for fusion. Therefore, the Hohlraum acts as a critical component for efficient energy transfer and implosion of the fusion fuel, enabling the much larger yields characteristic of thermonuclear weapons.