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 transportable. 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.

All operational designs use a fission device as the source of energy, although laser fusion is being explored, at least for power fusion and possibly for weapons.

Teller-Ulam design

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 Stanislaw 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. Early Secondaries were built around a plutonium core informally called — the "spark plug" — which underwent fission and provided additional neutrons and compression from the inside. While Secondary design remains classified, however, it is now widely accepted that more efficient compression systems have eliminated the spark plug, allowing further miniaturization.

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.

Laser fusion

In laser fusion, also called inertial confinement fusion (ICF), deuterium and tritium are again compressed, but using non-ionizing radiation from multiple lasers, rather than X-rays from a fission explosion. Direct drive ICF directs the laser against the surface of the fusion fuel.

Indirect drive wraps a Hohlraum around the fuel pellet, which, when heated, generates soft X-rays that actually compress the fuel. Indirect drive systems do not require the lasers to be as precisely aimed as in direct drive.

A third approach, dynamic Hohlraum, uses concentric cylinders of tungsten wires, which, when heated by the lasers, form a plasma that impacts a low-opacity target made of plastic foam. "The shock wave produced by this collision transfers a portion of the kinetic energy of the pinch implosion to internal and radiative energy within the foam. As the tungsten wires ablate, merge, and implode, the resulting high-atomic number plasma functions as a hohlraum, trapping radiation within."