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All contemporary efforts to perform nuclear fusion as a source of power have focused on reactions between isotopes (often deuterium, tritium, or He3) and/or light elements (often boron or lithium isotopes). These have vastly larger cross sections / rates of reaction than the p-p or CNO-catalysis reactions that power main-sequence stars, that have power densities of only a few hundred watts per cubic meter.

One sees so little discussion of the possibility of artificial p-p fusion reactors that it's sometimes hard to even find detailed explanation of why nobody tries.

What I would like to ask is: has there ever been serious consideration of the potential for such a device? For example, were they envisioned in the early days of fusion research or by some of the more speculative Bussard projects?

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    $\begingroup$ It's not exactly a duplicate, but this is addressed by the answers to How much faster is the fusion we make on earth compared to the fusion that happens in the sun?. Specifically note the comment in my answer: The p-p fusion is about $10^{26}$ times slower than the D-T fusion. $\endgroup$ – John Rennie Jun 26 at 5:49
  • $\begingroup$ It is definitely worthwhile background information, but that doesn't cover artificial equipment at all, which seems relevant given that our research reactors exceed the sun in some parameters. $\endgroup$ – ikrase Jun 26 at 5:52
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    $\begingroup$ The p-p fusion has $10^{26}$ times smaller cross section than the d-t fusion. That's why it has never been seriously considered for a artificial reactor. Any such reactor would yield a ridiculously small amount of energy. $\endgroup$ – John Rennie Jun 26 at 5:58
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    $\begingroup$ @ikrase how can you go around a crossection? the sun does it with a lot of protons and a lot of energy in the plasma space.com/26956-proton-fusion-sun-power-source-infographic.html $\endgroup$ – anna v Jun 26 at 8:07
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    $\begingroup$ I would take the position that noting the difference in cross-sections is exactly what "serious consideration" means. That is how you winnow design possibilities and decide where to spend you effort. Some options are so obviously hard that they are not worth your precious time. If you think it should mean something else perhaps you could elaborate on exactly what that is. Are you asking if anyone has ever poured years of effort and hundreds of pages into that rat hole? $\endgroup$ – dmckee Jun 26 at 13:42
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One sees so little discussion of the possibility of artificial p-p fusion reactors that it's sometimes hard to even find detailed explanation of why nobody tries.

As others have noted, it's really just a matter of practicality. The ignition temperatures and energy density in p-p is just way harder than D-T, 10^26 times. That's a lot of times.

It should be noted that in the early days, before about 1957, there was widespread belief that fusion would be relatively easy. At that time it was believed that reactors would run D-D and then progress to other fuels. It was only during "the doldrums" from 1958 to 1968 (when the tokamak was introduced) that attention moved to D-T as it was somewhat easier than D-D and thus more likely to work.

It is possible to overcome many of the limitations of the thermalized approaches using beam-beam fusion systems. However, relatively simple calculations show that these systems are unlikely to ever work. This has not stopped TAE from trying, although with little to show for it to date.

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