Forgive me by posing this somewhat scary question, but it's just out of curiosity I'm asking it.

In this video we're witnessing the beautiful and at the same time horrific blast of the tsar bomba, the greatest man-made explosion ever.

The question arises: Is there a limit to the energy released in a fusion bomb? The energy released by the tsar bomba had an equivalent of about 57 megatons of dynamite, which is quite a pile!

So, is it possible to increase the released energy? Maybe by increasing the power of the fission bombs to detonate it (!), increasing the amount of deuterium and tritium, or by some other means (for example, using the hydrogen bomb as a detonation device for a high density form of deuterium and tritium; or will this device already be blown to smithereens before the subsequent fission of the large part of the deuterium and tritium takes of?), or is there a theoretical limit (which luckily can't be tested in reality, but maybe in supercomputers).

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    $\begingroup$ this is a nuclear engineering question. There is no limit for the energy that can be released by fusion, but how to sustain it for larger and larger volumes of fusible matter is an engineering problem, not physics. When we have the sun and large stars as an example, there are gravitational induced limits, on the size of a star sustained by fusion , but that is another question $\endgroup$ – anna v Apr 30 at 9:44
  • $\begingroup$ I agree with Anna. The Tsar Bomba was originally supposed to be a 100 megaton weapon, but it was reduced so the crew of the delivery plane and the crews of the filming planes could survive. Interestingly enough, most weapons today are no longer of these types (Teller Ulam designs). They now actually use 1 atomic device to set off another very similar device boosted with fussionable material. $\endgroup$ – Rick Apr 30 at 11:18
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    $\begingroup$ We can read here: "As powerful as the trigger is, there is a limit to how large a capsule it can compress in the brief time available. If a still bigger bomb is desired, then the explosion of the fusion secondary can be used to compress and explode a larger third stage. Each stage can be 10-100 times the size of the previous stage." $\endgroup$ – Count Iblis Apr 30 at 11:29

There is some debate about the theoretical size of a multi-stage fusion device. Some say it is unlimited, while others debate this.

The basic action of the Teller-Ulam design is to use the energy from one explosion to set off another. There is an important requirement however, that energy must be deposited very rapidly so the second reaction has largely completed before the mechanical effects of the original explosion reach the next stage. In the case of a two-stage device, a small plutonium bomb (typically) is used as the trigger and it's x-rays are the energy source for the next stage. The x-rays fill the inside of the case at the speed of light, while the explosion is travelling orders of magnitude slower. This gives the secondary time to heat, compress and explode before the debris from the first stage reach it even across the space of the bomb casing.

Three-stage designs are more difficult because you can't simply place the third stage next to the second, or the x-rays from the primary will hit it too. You need to arrange the third stage in a way that the initial explosion is not visible to it, or use some other energy from the secondary. It is not clear which the Tsar Bomba used, and there is speculation that there were several third stages, not one large one.

It remains debated whether a truly unlimited size bomb is possible. It is known that the US was working on the theoretical design of a 1 GT weapon and even a 10 GT one. However, such weapons would be so large as to be almost fixed. I do know a 150 MT bomber-carried design was seriously considered, but disappeared during the move to ICBMs.

Even if there is no theoretical limit, there is a very practical limit, however, which has resulted in the reduction in yield over time. Since the energy of the bomb is distributed spherically, as the bomb yield grows more and more of its energy is wasted travelling up into the atmosphere. Four small bombs falling in a square pattern will cause more damage than one large one of four times the energy. Although the single bomb is likely to have a better power-to-weight ratio than four small ones (due to both physics and mundane issues like minimum case thickness) the geometrical improvement far offsets this. So in the early 1960s ICBMs carried a single large ~5 MT warhead, whereas by the 1970s they would instead carry multiple smaller ones around 250 to 500 kT.

  • $\begingroup$ I've also heard the rumor that a 500+ megaton air burst would theoretically blow away some of the earth's atmosphere. Obviously, this would be a non-desirable result. $\endgroup$ – David White Jun 20 at 5:16

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