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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?

ikrase
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    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. – John Rennie Jun 26 '19 at 05:49
  • 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. – ikrase Jun 26 '19 at 05:52
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    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. – John Rennie Jun 26 '19 at 05:58
  • @JohnRennie I'm perfectly aware that it has a minuscule cross section, but that doesn't mean nobody has ever seriously considered it or thought that there might be a way around the cross section problems. – ikrase Jun 26 '19 at 06:07
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    @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 https://www.space.com/26956-proton-fusion-sun-power-source-infographic.html – anna v Jun 26 '19 at 08:07
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    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? – dmckee --- ex-moderator kitten Jun 26 '19 at 13:42
  • I can do D-T fusion all day (or all week) in the lab with fairly modest equipment. To quote Adelberger (Solar fusion cross sections, Rev. Mod. Phys. 70(4) 1265-1291 (1998)) - "... the rate for the fundamental $p+p$ -> $^{2}D + e^{+} + \nu_{e}$ reaction is too small to be measured in the laboratory. Instead, the cross section for the $p-p$ reaction must be calculated from standard weak-interaction theory." That is, in the last 100 years of nuclear physics experiments, we haven't been able to experimentally see $p-p$ reactions on Earth. – Jon Custer Jun 26 '19 at 15:56

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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.

Maury Markowitz
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  • What happened in 1957? And what calculations make non-thermalized systems unviable? – ikrase Dec 20 '19 at 04:20
  • 1958 was the first multinational public release of fusion data, where everyone realized nothing was working and any hopes of a quick road to fusion evaporated. 68 was the T-3 release (well, no one believed it until 1969) when it once again looked like there was a fast route. The calculations in question are twofold, one by Todd Rider in the 90s suggested that non-thermal fuels will leak energy at rates way beyond what their fusion events could possibly make up. There's a more recent result as well,but I can't seem to find it now. – Maury Markowitz Dec 20 '19 at 14:59