Fusion device

IVY MIKE, the first thermonuclear test

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, IVY MIKE, 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.

• The most fundamental challenge in fusion devices is to direct adequate neutrons into a mass of hydrogen isotopes receptive to fusion upon the absorption of neutrons

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

Materials

Ted Taylor observed the "H-bomb secret" was a matter of "design, not materials", although, of special nuclear materials, ₂₃₅uranium or ₂₃₉plutonium, ₃tritium, and ₆lithium or ₂deuterium are clearly needed. Context suggests he was thinking of the Teller-Ulam design. In that approach, ₂₃₈uranium is probably used in the neutron shield, and in a Tertiary if one is used.

U.S. declassification guidance mentions that boron, beryllium, bismuth, tungsten, thallium and mercury are used in some unspecified nuclear weapons, but no detail is given.

Almost certainly, dense plastic foams are a key component and a very sensitive technology.

Early designs

Quite early in the Manhattan Project (summer 1942) and later in the Soviet program, long before anyone had built a fission device, attention was paid to a device that could use the much more energetic fusion reaction. It can be argued there were three early generations in the 1940s: Teller's "Classical Super", the more complex but more feasible multiple-concentric-sphere designs, and the eventual Teller-Ulam design, which is the basis of all operational fusion weapons.

The first two generations shared the property that they had a succession of coupled "layers", each of which amplified some aspect of the previous one, and in some cases established feedback. There was no clear separation between the fission Primary and the fusion Secondary. In contrast, the eventual "multistage" Teller-Ulam design sharply distinguishes between Primary, Secondary, and optional Tertiary.

Classical Super

Edward Teller was among the first theoreticians to approach fusion devices, which he called the "Super", with an early approach that detailed analysis showed would simply not work.

• The basic idea of the super was that a direct flow of neutrons from a fission device would be significant to begin self-sustaining fusion in an arbitrary mass of deuterium

Teller's basic Super idea; fusion propagates through deuterium

In Teller's basic idea, neutrons from a fission device would strike a cylinder of deuterium and fuse, which would produce more neutrons. The neutrons would continue down the cylinder, with more and more neutrons increasing the reaction.

It was a simple deuterium cylinder inside a simple mechanical case. There was no radiation or mechanical confinement around the fusion fuel.

Flaws in the Classical Super concept

The next illustration, still simplified in that the tamper and internal voids are not shown, depicts flaws in the Super concept. The first problem was that the fission device was not homogeneous as in the previous illustration. The fissioning pit, which generated the neutrons, was compressed by explosives, composed of low-Z (low atomic number) elements that are good neutron absorbers. Note the neutron absorbed in the explosive layer.

Remember that fission itself needed a high neutron density. For this reason, the pit and explosives were surrounded by a neutron reflector. Some neutrons would be reflected back into the pit, and, while they eventually might reach the deuterium, they might do so after the fuel at the nearest end was partially used, damping a chain reaction.

For those neutrons that do reach the deuterium and cause fusion, there is nothing to direct the fusion-generated neutrons. They may radiate out the sides of the cylinder and cause little or no additional fusion.

Alarm Clock

A variety of interim designs, which could produce greater yield than a pure fission device, were variously called the Alarm Clock (nuclear weapon) by Teller in the U.S., ("because it would wake up the world"), Layer Cake (nuclear weapon) or sloika in the Soviet Union, emphasizing its multiple concentric shells, and tamper boosting in the U.K.

• The basic idea of the Alarm Clock/Layer Cake/Tamper Boosting was that limited fusion could be produced in a shell of light elements concentric around a fusion device and backed by a reflector, with interactions among the three basic layers

• The next idea of the Alarm Clock/Layer Cake/Tamper Boosting was that multiple concentric fission-fusion layers might be possible, again within limits

Versions were tested but all proved inferior to the subsequent "multistage" or Teller-Ulam design. Indeed, active contingency work continued in increasing the yield of pure fission devices should none of the fusion approaches work; this resulted in the Mark 6 (nuclear weapon) designed by Ted Taylor, better known as the Super Oralloy Bomb ("SOB").

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. Andrei Sakharov, in the Soviet Union, appears to have independently come up with a similar concept, which he called the "third idea".

• The first and critical realization for Teller-Ulam was that X-rays travel at the speed of light, and are the main source of energy produced by a fission device.
• The Teller-Ulam principle is counterintuitive: one of the key elements of design is, for a brief period of time, keeping the Primary neutrons away from the Secondary. They are kept away until the fusion Secondary is prepared to fuse by treatment with X-rays.
• As opposed to the Alarm Clock, the interaction between the Primary and Secondary, and Secondary and Tertiary if there is a third stage, are distinct one-way phases.

The geometry of the bomb is critical. Its case, or at least the inner surface of it, is essential to controlling the reaction.

The original design was cylindrical, with a roughly spherical Primary at one end. The Secondary is a cylinder, concentric with the central axis that runs through the case and the Primary. A neutron shield is placed between the Primary and the Secondary.

These shapes change in recent designs such as the W88, in which the Primary is oblate — think of a watermelon or American football — and the Secondary is spherical. Such shapes fit much more efficiently into a conical missile warhead.

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.

RDS-220, known to its designers as Big Ivan, and nicknamed Tsar Bomba, was the highest-yielding nuclear weapon ever developed. Its ₂₃₈U Tertiary was replaced by lead to reduce its designed yield of 100 MT to 50-58 MT. Some reports of its design suggest that it used multiple parallel secondaries.

There has been little work on three-stage devices, because it has been realized that while there may be no theoretical limit on the size of a bomb, there is a practical limit to what is militarily useful. Somewhere in the 50-100MT range, the effects would begin to vent into space; a greater number of smaller and more accurate weapons is far more effective. Neither the Soviet Union nor U.S. ever deployed a weapon with more than 25MT yield.

Conceivably, if appropriate ground penetration technology could be developed, a very large bomb might have a role against a very deep underground target. Even in discussions of deflecting asteroids on collision course with Earth, extremely large single explosions do not seem to have a role.

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."