How does a thermonuclear weapon use a fission primary to ignite a fusion secondary?

In a thermonuclear weapon, the ignition of the fusion secondary is achieved through a process known as radiation implosion, a key component of the Teller-Ulam configuration.12

The process functions as follows:

• Primary Detonation: The weapon’s primary stage, a fission bomb, detonates first. This creates an intense burst of radiation, including X-rays, and extreme heat.32
• Radiation Confinement: This radiation is trapped within an outer metal casing, typically lined with high-density material like uranium. The radiation heats the inner surface of the casing, creating a hot plasma that confines the energy.2
• Compression: The trapped radiation energy is channeled toward the secondary stage. It exerts immense pressure on the secondary's outer shell, known as a pusher, compressing the fusion fuel (such as lithium-6 deuteride) to a very high density.12
• Ignition: Inside the compressed secondary is a "spark plug" of fissionable material. The extreme pressure triggers this spark plug to fission, generating the temperatures of millions of degrees necessary to ignite the fusion reaction in the surrounding fuel.2