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What is the theoretical limit of the amount of energy that can be extracted from a fusion reaction? I am not talking about the practical efficiency of a reactor, but rather what fraction of the mass-energy can be released.

Of the theoretically possible fusion reactions, combining 56 free nucleons into $^{56}$Fe would release 9.1538 MeV per nucleon. Combining 28 free protons and 34 neutrons into $^{62}$Ni would give a slightly tighter binding per nucleon. This seems to represent an empirical limit to fusion, converting 0.00935605478 (iron) and 0.0096783439 (nickel) of the nuclear mass into energy. So this limit seems to be $\approx$0.97%. By comparison, the proton-proton chain leading to $^4$He is 0.7% efficient.

But there could in principle exist other, unknown forms of fusion. The recent quark fusion discovery is about eight times more energetic than normal fusion, although it is a rather nonstandard form of fusion since it involves heavy quarks. We can generalise fusion as the process of reorganising existing matter to release energy by increasing the binding energy due to the strong force (this will leave out gravitational accretion). Were zero-pressure stable strange matter to exist it would presumably represent an even more efficient fusion target. It seems likely that fusion involving the full strong force rather than the residual strong force will be more efficient.

So, given these considerations, are there any nontrivial theoretical upper bounds on how efficient a fusion reaction can be?

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