User:Howard C. Berkowitz/Layer

In the development of thermonuclear weapons, the first technique considered was surrounding a fission device at the center with a supply of light isotopes for fusion. It was believed that the expanding shock wave would heat and compress the fusion fuel, but the only plausible nuclear reaction that would undergo fusion under these conditions was deuterium-tritium. This turns out to have significant limitations, but has been used at least experimentally by several countries, on the way to the Teller-Ulam design.

This design was reinvented independently several times:
 * In the United States by Edward Teller, called “Alarm Clock”
 * In the Soviet Union by Andrei Sakharov and Vitalii Ginzberg, termed "Layer Cake”
 * In Great Britain by Keith Roberts, called “tamper boosting”

The earliest and most obvious idea for using fusion reactions in weapons is to surround the fission core with a fusion fuel. The radiation dominated shock wave from the expanding fission core would compress the fusion fuel 7- 16 fold, and heat it nearly to the same temperature as the bomb core. In this compressed and heated state a significant amount of fusion fuel might burn.

Calculations quickly showed that only one reaction ignited with sufficient ease to make this useful - the deuterium-tritium reaction. The cost of manufacturing tritium relative to the energy produced from the fusion reaction made this unattractive.

Two ideas were later added to this concept to make a practical weapon design: The first: use lithium-6 deuteride as the fuel. The excess neutrons released by the fission bomb will breed tritium directly in the fuel blanket through the Li-6 + n -> T + He-4 + 4.78 MeV reaction. A layer at least 12 cm thick is necessary to catch most of emitted neutrons. This reaction also helps heat the fuel to fusion temperatures. The capture of all of the neutrons escaping ahead of the shock wave generates about 2.5% as much energy as the entire fission trigger release, all of it deposited directly in the fusion fuel.

The second: encase the fusion fuel blanket in a fusion tamper made of uranium. This tamper helps confine the high temperatures in the fusion blanket. Without this tamper the low-Z fusion fuel, which readily becomes completely ionized and transparent when heated, would not be heated efficiently, and would permit much of the energy of the fission trigger to escape. The opaque fusion tamper absorbs this energy, and radiates it back into the fuel blanket. The high density of the fusion tamper also enhances the compression of the fuel by resisting the expansion and escape of the fusion fuel.

In addition the uranium undergoes fast fission from the fusion neutrons. This fast fission process releases far more energy than the fusion reactions themselves and is essential for making the whole scheme practical.

This idea predates the invention of staged radiation implosion designs, and was apparently invented independently at least three times. There is room for significant variation in how this overall scheme is used however.

One approach is to opt for a "once-through" design. In this scheme the escaping fission neutrons breed tritium, the tritium fuses, and the fusion neutrons fission the fusion tamper, thus completing the process. Since each fission in the trigger releases about one excess neutron (it produces two and a fraction, but consumes one), which can breed one tritium atom, which fuses and release one fusion neutron, which causes one fast fission, the overall gain is to approximately double the trigger yield (perhaps a bit more).

The gain can be considerably enhanced though (presumably through a thicker lithium deuteride blanket, and a thicker fusion tamper). In this design enough of the secondary neutrons produced by fast fission in the fusion tamper get scattered back into the fusion blanket to breed a second generation of tritium. A coupled fission-fusion-fission chain reaction thus becomes established (or more precisely a fast fission -> tritium breeding -> fusion -> fast fission chain reaction). In a sense, the fusion part of the process acts as a neutron accelerator to permit a fast fission chain reaction to be sustained in the uranium tamper. The process terminates when the fusion tamper has expanded sufficiently to permit too many neutrons to escape.

The advantage of the once-through approach is that a much lighter bomb can be constructed. The disadvantage is that a much larger amount of expensive fissile material is required for a given yield. Yields exceeding a megaton are possible, if a correspondingly large fission trigger is used. This design was developed by the British. The Orange Herald device employed this concept and was tested in Grapple 2 (31 May 1957). A U-235 fission trigger with a yield in the 300 kt range was used, for a total yield of 720 kt - a boost in the order of 2.5-fold. A variant design was apparently deployed for a while in the fifties under the name Violet Club.

The second approach was adopted by the Soviets and proven in the test known as Joe-4 to the West (actually the fifth Soviet test) on 12 August 1953 at Semipalatinsk in Kazakhstan. This resulted in a very massive, but much cheaper bomb since only a small amount of fissile material is required.

Since there is an actual multiplication effect between the fusion reaction and the tamper fast fission, an improved yield can be obtained at reasonable cost by spiking the fusion layer with tritium prior to detonation.

The Joe-4 device used a 40 kt U-235 fission bomb acted as the trigger and produced a total yield of 400 kt for a 10-fold enhancement, although tritium spiking was partly responsible. 15-20% of the energy was released by fusion (60-80 kt), and the balance (280-300 kt) was from U-238 fast fission. A later test without tritium spiking produced only 215 kt.

This design has a maximum achievable yield of perhaps 1 Mt (if that) before becoming prohibitively heavy. The USSR may never have actually deployed any weapons using this design.