Fusion device
A fusion device is any assembly of components that can produce an explosion from nuclear fusion. A fission bomb can be dropped from an airplane, or at least transported to a real target. In contrast, the first thermonuclear device, CASTLE BRAVO, detonated on Bikini Island, was not transportabe. A fusion warhead is a device that is sufficiently small and rugged to be used, operationally, as the warhead of a guided missile, artillery shell, or unguided rocket.
This article describes the general principles of a fusion device, also called a thermonuclear weapon. Rather than generating energy by splitting nuclei, it generates even greater amounts of energy by joining the nuclei of lighter elements (e.g., hydrogen isotopes) into a heavier one (e.g.,helium). Thermonuclear fusion generates vastly more energy, per unit of weight of the physical device, than does a pure fission bomb.
Many of the details remain classified, but there is reasonable confidence that all current fusion weapons use the Teller-Ulam design, created by Edward Teller and Stanislaus Ulam, in which a fission bomb (i.e., the Primary) is used to generate the radiation that compresses and heats the fusion fuel. The Teller-Ulam principle is counterintuitive: one of the key elements of design is, for a brief period of time, keeping the Primary energy away from the Secondary. That energy is kept away until it is redirected into forms optimal for producing fusion.
The geometry of the bomb is critical. Its case, or at least the inner surface of it, is essential to controlling the reaction.
The overall device, in principle, is cylindrical, with a roughly spherical Primary at one end. The Secondary is a smaller cylinder, concentric with the central axis that runs through the case and the Primary. A radiation shield is placed between the Primary and the Secondary.
Primary
When the Primary detonates, it produces large volumes of X-rays. These are blocked by the shield, and are reflected along the inner lining of the case, (i.e., called the Hohlraum), surrounding the Secondary, so that the X-rays symmetrically hit the Secondary along its entire length.
It is public information that the basic mechanism caused by the Primary's X-rays is radiation pressure, which compresses and heats the Secondary until it reaches the extreme temperature and pressure needed for fusion. The exact mechanism by which radiation pressure couples to the Secondary, however, remains classified.
Secondary
The first detailed suppositions suggested that the X-rays caused a dense plastic foam, filling the gap between the case and the Secondary, was converted to a plasma, and the plasma heated and compressed the Secondary. Other accounts argued that the X-rays themselves, without an intermediate mechanism, caused the compression. Teller said that the X-ray wavelengths are in the "soft" rather than "hard" energy range. Since the X-rays generated by the Primary tend to be hard, there is speculation that the bomb case is the source of the soft X-rays; it generates them when ionized by the hard X-rays from the trigger.
The most generally accepted explanation is that the X-rays vaporize the surface of a "tamper" or "pusher" surrounding the Secondary, and the ablation (evaporation) of the tamper surface, completely symmetrical with respect to the Secondary, drives the pusher against the next layer of fusion fuel. Essentially, the tamper becomes a rocket, with its exhaust being its surface being vaporized.
There has never been official confirmation if one or more of the mechanisms are responsible for fusion bombs. The idea of ablation, however, makes more sense, since experiments with laser fusion do not use a Primary, but compress a small bead of thermonuclear fuel with laser energy hitting its tamper from all sides, compressing the fuel.
As the tamper compresses the thermonuclear fuel, probably lithium deuteride, it heats it. The innermost part of the Secondary, however, is a rod of fissionable material — the "spark plug" — which is compressed enough to itself undergo fission.
Once the spark plug triggers, the fusion fuel is both bombarded by neutrons from the spark plug, and also by the tamper. The hydrogen isotope to helium fusion is now possible, with temperatures and pressures comparable to the interior of a star.
Additional stages
The combination of a Primary and Secondary is called a "two-stage" design, assuming the Hohlraum is radioactively inert. In a "three-stage" design, such as the B41 bomb, the Hohlraum is made of Uranium-238, which is not normally fissionable. Bombarded with neutrons from the Secondary, the Hohlraum converts to a fissionable isotope, and then undergoes fission. In principle, there can be more than three stages, and there is no theoretical limit to the power of a thermonuclear device.